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Monday, October 14, 2019

Leakage of SSR driving AC Solenoid

message: I have a 115vac ssr powering a solenoid only pulling a 160ma. upon activation 115vac is applied, but upon deactivation 78vac is still present. I'm told I need a bleeder resistor, but no one knows exactly what size? Can you help?

Mail from GH USA

When a solenoid draws 160mA, you dont need a bleeder. That itself is a bleeder. The leakage of the ssr is very less, it is mainly the current thru the snubber caps.

The SSR cannot leak so much, Is the solenoid a simple coil solenoid or has circuitry? A coil solenoid is also a bleeder for the 1 or 2mA max AC current from caps.

If the solenoid has some control circuit then you may need a bleeder, put a 100K 250V 1W. It also could be a defective SSR or a NC (normally closed) SSR with improper control signal.

Read More - Solid State Relays SSR.

SSR driving AC Solenoid - delabs

Check if control and load circuits are isolated too, make careful measurements across the ssr. Tell me if you still are in doubt.

See the connection diagram

Connection Diagrams of SSR


Battery Level Indicator

Do you have a battery level indicator for a 24V Ni-Mh recharegeable battery source ? Prefer the display as a LED or LCD traveling bar graph.

Mail from EY

have a look at  BVM1 - 12 Volt Battery Voltage Monitor then let me know better, how i could help you.


Thanks for the effort and your effort.

I think the is good. and the instrument looks good and simple to implement. Now I am studying the circuit of solorb

Reply from EY

Battery Level Indicator - delabs

Battery Level Indicator

This circuit uses a LM339, a quad comparator. LM339 can work on single or dual supplies, it has a open collector output that can drive 15mA, low power consumption. The circuit is an untested design but it should work. I did it as many searches were made in my webpages with these keywords.

Saturday, September 14, 2019

Digital Interface Part Two

Continuation of Digital Interface Discussion, only my part of the communication here. This will help you understand some common methods and issues.

Use a IO port to drive low (not pulse) the base of this PNP with a resistor 10K, emitter is +V and collector is the output to power the Sensor V+. You can use a pull up resistor on the port like 47K to keep noise low.

InfraRed LED Flasher for Optical Switch

Try things practically before you connect to the Embedded System. Make test codes to check your interface circuits, watch the LEDs flash on off on the sensor. Test the sensor with obstacle and see if a clean High Low pulse is seen with a scope or DMM on a Port input.

There has to be a 100 uF 16V Electrolytic Cap in the Breadboard across +/- (supply terminals) coming from Battery. This is the first decoupling cap to prevent oscillations. It acts as a mini battery.  It should be on the board and not soldered across the battery.

Optical Retro-Reflective Proximity Switch

It may be a good idea to use this pololu sensor which can be easily enabled and disabled. This will resolve most of the issues of interfacing basic IR circuits. This post is about other raw IR sensors which need more circuit to interface well.

pololu IR sensor

Pololu 38 kHz IR Proximity Sensor, Fixed Gain, High Brightness

Yo may need a bleeder resistor 10K across the supply terminals of both the proximity switches. This ensures the cap in the proximity switch discharges after you turn off the supply within a few mS determined by the 104 decoupling cap in the proximity switch and the bleeder a 10K will do.

Digital Interface Conclusion

Here i am listing the last few parts in this Interface discussion. Only Points to remember are listed.

Resistors attenuate voltage or reduce. Current flow has to be limited. The adc may get damaged when voltage above Vcc is applied

Let us assume 4v is applied to a voltage divider of 30k first 10k second. 3v drops across 30k and 1v drops across 10k - This 1v sample can be measured

Values like 10k and 30k are non standard and expensive. Use 1% MFR resistors only

You can go upto 1 mega - Beyond that leakage currents cause errors. A DMM has 10 meg input resistance For the same reason. Not to save battery but to avoid loading the point. Being tested

DMM Amps mode protect

A transistor heats up if it is not fully on or saturated. Ensure it is fully on. - If Vce sat is 0.5 v it is fully conducting -- If Ic is 1A and Vce is 0.5 V then heat dissipated is 1 x 0.5 watts or 500mW

Try out and see the current through motor using DMM carefully with a 10 ohm 5w in series. 10 ohm is to protect DMM at 2A range, In case of short circuit, dmm fuse won't blow. Put dmm, motor and 10 ohm  in series.

Knowing Electrical Basics both theory and practical is important to be a good embedded systems professional. Just dont jump into coding and expect to master the Art of Electronics. 

Friday, September 13, 2019

Digital Interface Third Part

Only my Messages are combined here to create paragraphs. Related to Interfacing an Embedded System to external circuits.

Pull up means a resistor connected from a point to +V. Pull Down is Resistor connected from a Point to GND.  The point can be a Gate Input, Opamp Input or a Port Output. We use these when open collector output is used,  this way we can get a clear High Or Low or avoid a point to float(no potential)

Industrial Process Control Circuits

When uC is sleeping outputs should be written high. Which is also high impedance  like a insulator- The pull up of 100 k is put to keep the point high

Diode and Zener Clamp Protection

Here are two examples of IO protection from my older circuits. The diode or Schottky protection is for circuits measuring low voltage inputs like mV. Schottky forward drop on low currents is as low as 200mV. General Purpose diodes like 1N4148 have a drop of 400mV at low currents. Use low leakage zeners if you are protecting digital inputs. you can also diode clamp to VCC and GND or VDD too. Then inputs can swing betwwen VCC and VDD.

Diode and Zener Clamp Protection

When a low is written to the port a current Ib flows from vcc thru emmiter to base via a cuurent limit resistor. Beta times Ib which may be like 200 times ib flows from emitter to collector. This is the amplified Ic - Transistor switches on and saturates like a switch. - Vce may go as low as 0.5 volt on proper load

Monday, August 19, 2019

Digital Interface Basics

After a long time a thread of discussion. It is about digital interfacing of embedded system. Only Education Points listed. Initials and country of origin also will not be mentioned henceforth.

When this Circuits FAQ started decades back around 2000, It was a PHP mail form called "Problems and Solutions". The mail was coming like 3-4 every day.


Hello Sir. I am looking for someone that can help me with simple challenges from time to time, deciding on best electrical components and designing wired circuit diagrams to my specifications. Thank you!"

Only the delabs Answers are Reproduced Here

The output logic of your embedded system port bit (one pin) goes high  it can drive a drive a ULN2003A. - The output of this is open collector and can operate relays or lamps.

Using the ULN2003A

The Proximity Sensor can be turned on with an external PNP power transistor if the current consumption of that is above 300mA. It is not a good idea to heat The ULN2003A chip too much. Keep the return paths of these to a separate but linked ground as they carry more current.

If you are running short of ports in the Embedded System. Latch the Output by using 74HCT373. There may be logic level comparability problems check your Embedded System Supply. There may be ground loop problems.

Use ULN to drive PNP Transistor and things like a Relay, Lamp or your Proximity Switch.

It is important to learn and do things, this is the real electronics.  Do not overload any device, study current and voltage ratings. Keep Mains HV and High Current circuits that are dangerous in another card with optoisolaters.

Isolation Resistance and Insulation Resistance

message: what is insulation and isolation resistance, why and how it is measured? what are the consequences if it is not measured? 2008

Isolation Resistance and Insulation Resistance are both related to each other. Isolation Resistance could be in Giga Ohms and Tera Ohms and includes Surface Resistivity.

Insulation Resistance will be in the Range of Mega Ohms and Giga Ohms. You could say Isolation Resistance is a Tera Ohm and Giga Ohm Measurement Test.

Isolation Resistance and Insulation Resistance

Les us assume a 5KV Isolation Opto-Coupler on a Glass Epoxy PCB. You could Measure its Isolation or Insulation Resistance and it will be in the range of 1 Tera Ohms in a good PCB.

A good PCB will have a Slot Routed below the Opto Coupler for High Isolation. This will ensure Dust and Grime along with Moisture will not form a Leakage path below opto-coupler.

Use a Megohmmeter for Insulation Resistance of Transformers, Coils and Relays. Use a Teraohm Meter to measure Isolation Resistance in Opto Couplers, PCB, Ceramics and Capacitors.

Megohmmeter is a special type of ohmmeter used to measure the electrical resistance of insulators. Insulating components, for example cable jackets, must be tested for their insulation strength at the time of commissioning and as part of maintenance of high voltage electrical equipment and installations.

6520 Programmable Teraohmmeter makes resistance measurements up to 10 PΩ. Whether used in automated mode via the Guildline developed TeraCal Software or as a stand-alone meter,

DC Small Motor Control using PWM

message: Greetings, I have a kit motor controller. I works just fine. It uses the NE556. I read some where ( on a NE555 )that I can change one of the capacitors with a variable capacitor to get the full range of frequencies out of the NE555.

 Is it possible to get adjustable frequencies out of the NE556? I have the circuit drawing. Is there any one I can send the diagram to so they could tell me which capacitor I can change? Thank you,


PWM DC Motor Speed Control - This is a circuit for controlling the speed of small DC motors, it works nicely as a speed controller for an HO or N gauge model railroad.

DC Motor Control for Low Power

Timing 555 Motor Speed - It uses a variable frequency 555 astable. The astable triggers a monostable that produces pulses of a fixed width. At lower frequencies, the monostable pulses are infrequent and the output is high about 20% of the time.


Tuesday, July 23, 2019

An Oscilloscope on your TV

Over the last few days I've been playing around with an old small black and white tv....I did the infamous "Oscilloscope on your TV" thing that's all over youtube and the rest of the web...grew bored with that, so I've decided I want to make it into a fully-functional scope that I can use in my lab. I was pleasantly suprised when I came across the one built in '86, it suits all my needs, it was very generous of them to make the design public domain.

What I was wondering is if any improvements to the design was made. They mentioned the possibility of ramping up the bandwidth to 100 Mhz, and I'm looking at high-speed alternatives to the FETS and drivers and so forth. I am also looking into switching regulators to replace the transformers.

poormans OscilloScope

Do you have any suggestions as far as what the maximum current (and any other important ratings) should be for these replacement devices? Also, a few of the components in the schematics don't have a part number...

1) Resistors and variable capacitors on the rotary switches
2) Transformers
3) FETs and PNP transistors on amp

If you could let me know the values on those components I'd be extremely grateful.

AT USA 2006

I am just leading you to some resources in the web. A Scope can never perform well on TV. It would be better you go for a USB PC Scope that are not only practical but professional too.

Miniature TV converted to oscilloscope - Inside the cabinet of a TV or computer monitor you will find at the back of the picture tube, the yoke coil assembly. Once you locate it, note the clamp that secures it to the picture tube.

The µSCOPE - A poormans OscilloScope A (good) oscilloscope was then a very expensive instrument available only to a happy few. As a result many electronics hobbyist made their own scope. The popular electronics magazines in the sixties and seventies were full of scope designs almost always based on vacuum tubes.

Sunday, July 21, 2019

Audio Triggers a Relay Switch

message: Hello there i am just wondering how i would go about trying to get a circuit designed?

The situation i have is i have got a sound signal peak voltage at approx 0.3v. I require this to trigger a switching device such as a relay. There is a 12 volt DC supply available to power up any components.

If you could either send me in the right direction or if possible quote price on having this circuit designed and possibly made it will i will be most grateful.

Thank you CD from UK in 2008

Here are some circuits in the web - Voice operated relay or switch, Clap switch, Sound operated light switch ... all these mean the same. A condenser microphone, pre-amplifier, rectifier, comparator and relay in that sequence. If is the first hobby project for learning home automation and the idea as old as the 70s. It was used in Toys and Lights extensively, when the fad was still on. It is still an excellent learning project that will make an new enthusiast more familiar with components. Avoid using ICs so that your fundamentals become strong.

Using ICs are like ready to cook and serve food packets. Cooking your own meal is similar to ... Using Transistors and discrete components like diodes, zeners, resistors and capacitors. This will strengthen your foundation. Winding your own coils is another thing you have to learn, if you want to be a ace Ham Radio Operator.

Audio Triggers a Relay Switch

Audio Signal Detector Switch - This circuit will activate a transistor switch when it detects at least 50mv peak to peak of an audio signal. It could be used to turn on a relay, routing the signal to were it is needed.

Design of a Clap Activated Switch - This involves the design of various sages consisting of the pickup transducer, low frequency, audio low power and low noise amplifier, timer, bistable and switches. It also consists of special network components to prevent false triggering and ensure desired performance objectives.


Monday, July 08, 2019

Hall Effect Water Meter System

I have 36 water meters below ground level which are exposed to intermittent flooding and located throughout a four acre property. I would like to relocate the meter heads to a central location for ease of reading and protection from flooding. The heads can be removed and are coupled to the base by a magnetic pulse generated by a rotating disk in the base.

A Hall effect sensor would pickup the pulse, amplified as required to drive a coil ( off the shelf choke ? ) that would pulse the meter head at the central location. A conduit runs from the central location to each meter for wires to be run, as few as necessary please. the nearest meter is 60 feet away the farthest is 700 feet away. Please email me for additional info and terms.

Thanks! EH USA  - Email got 2007

This mail was not resolved by me due to a break in communications. I also feel this is a interesting system that can be solved by Wireless Device Networking like WSN - Wireless Sensor networks.

Here is a very old website which has shown home automation interfaces.

Water Meter Monitor - Revisited

Water Meter Monitor - Revisited

"After I received the sensor, some experimentation showed that it would certainly detect the spinning magnetic field of the water meter. See the plot below. It shows actually two 'dips' per revolution of the magnetic drive.

This implies that there are two spinning magnets, each negative dip being a separate magnet. The peak-to-peak amplitude is about 0.1V. Based on the sensitivity of the meter (1V=1.6 Gausss), the amplitude of the magnetic field at the perimeter of the water meter is 0.16 Gauss."

- Edward Cheung

Tuesday, June 25, 2019

Sequential Timer Design

I’m working on a small electronics project but don’t know too much about designing circuits so I figure I would ask someone who knows and this seems like the place. Ok,

What I’m looking for is the following: I need a circuit that when the switch is pressed it will close the circuit and in an adjustable amount of time via adjustment knob: I’m not sure what the name of the adjustment component is called but its: (A small box shaped component that can be adjusted using a small screw driver. In a circular clockwise motion to speed up the time and counter-clockwise to slow it down) a solenoid will actuate and move in the forwards direction....

 See entire thread here Two Stage Sequential Timer - del90004

Two Stage Sequential Timer
Mail from RH  USA 2004

This is an example of a cascaded or sequential timer, here two CD4541 are forming a two stage timer. You can add more in a chain, but better to use a microprocessor or Microcontroller for such a purpose.

The circuit is a two stage sequential timer with auto reset. It can turn a Solenoid On/Off after a Sequence of delays that are preset. This is also a basic example of timer usage in automation.

All Timers are 1 - 30 Secs Analog with Presets to adjust with a screwdriver. The Switch, Solenoids and Power supply has to be via terminal blocks or plug-jacks. The PCB may be upto six Square Inches for standard DIP components but with SMD it may be smaller.


Monday, June 24, 2019

Timer and Counter for RTD System

Thank you for your assistance. My answers are by your questions.

Hope this helps. This unit is more for troubleshooting a air flow problem. If the counter shows many 130 Degrees detected or alarm is going off several times a higher priced system for a forced air flow will be needed. Currently no way to detect heat increases going around vent so client doesn't want to purchase forced air flow system. This is a safety device so client can decide on this cheaper system to monitor or a more expensive permanent fix. I will need a PCB setup after I can test the circuit in test environment.

Mail from NV 2004

The temperature in a pipe is monitored with four RTD Pt-100 Sensors, The type of probe could be Extruder RTD probes with Bayonet or Compression Fitting, Get it from Omega. The temperature at four points are measured by Temperature Module and compared with one setpoint 0-200 deg C, an alarm output LED out-a goes ON when the measured temperature exceeds the common setpoint.

Timer and Counter Modules - del90002

Timer and Counter Modules
Pan and Zoom the Circuit for detailed Study here - Timer Counter for RTD

a. do you need individual setpoints for all the four points you measure ? - NO

b. is the temperature deg C or deg F, centigrade or Fahrenheit ? - 130 F

c. do you need a relay output when the LED goes ON to turn off the heater ? NO

d. the time taken to measure 4 points will be 4 seconds is that fast enough ? YES

e. do you need a display of the measured temperature as shown above ? NO

The Analog Timer Module can be set from 0- 30 minutes. If out-a remains high even after set time a out-b alarm goes on with a relay output, the relay contacts can be used to turn off a heater or sound a buzzer.

a. do you want to use a digital timer instead ? - I'm open. Trying to keep costs down

b. do you need a solid state relay output ? - Used like a smoke detector. Alarm to notify client to check air flow. This is in case the air flow stops.

Mail from NV 2004

Saturday, June 22, 2019

Temperature with Timer Triggers Counter

I'm looking at an idea for a circuit design; The idea is to sense a temp of 130 degrees and then light an LED and start a timer delay. The delay would be 3 min, after the delay if the temp is still above 130 sound alarm.

With the LED I would like to drive a counter display to show how many times the temp was sensed. I would like powered by 120vac and a 9 volt battery backup for counter circuit.

If you can assist please let me know.

Mail from NV USA 2004

The circuit you require is possible, what is the time frame within which you require it ? You also need to tell the resolution you require is it 130 or 130.0 and counter upto 9999 ? what are the controls you want and the outputs required if any.

I can design above with simple analog-digital based circuits, microcontroller based designs will
take much more time but can be upgraded easily.

Four Channel Temperature Alarm

Reply from delabs

Temperature with Timer Triggers Counter
Pan and Zoom the Circuit for detailed Study here - Pt-100 RTD Four Point Alarm

130 1%, at least 0-999...

a reset for counter and I would like the temp sensor to have 4 sensors so I could surrond a pipe and any sensor would trigger circuit. The unit would be mounted about 6-8 inches away from sensors. Thinks a probe type sensor so it can protrude from a plastic tube that wil be sheped to meet pipe or vent design.

I might not need AC if the circuit can be powered from 9 VDC battery like a Smoke detector.

Mail from NV

Tuesday, May 28, 2019

CJC or Cold Juction Compensation

About a month agoo I asked a few explanation on blind controller circuit design, up now i did focused on mechanical parts of my rubber press and now these are ready, i turned myself to the controller :

Qestions came up as I tried to duplicate part of your circuit design on a bread board and could not get a significant reading on DMM.

Why value of TH1 is 5 ohm in this circuit other circuits you design it is mostly 12 ohm ?

I used document DACT0006.PDF to make TH1 and re-calculate values of resistors involved around OP07 for gain and R26 using formula you gives. R26 : 330K ? to not induce anyone error i won't replicate here these calcul, but could you confirm me this value ? (sorry for this one I know it is unpleasant : think I'm lost... )

Mail From SY

Another Method using a Copper Resistor

If wires of two dissimilar metals are joined at both the ends and the junction formed at one of the ends, is heated more than the other junction, a current flows in the circuit due to Seebeck thermal emf. This effect is used in thermocouple temperature sensors.

Thermocouple Temperature DPM - DMM

Thermocouple Temperature DPM - DMM

The Peltier effect is the converse of above Seebeck effect, which means that if a current is forced through junctions of dissimilar metals, the junction will generate heat or absorb heat (cooling) depending on direction of the applied emf. This effect is used to make portable and small refrigerators.


What is the range i should use for DMM for this blind proportional circuit ? what should be the reading ?

Iideal for my purpose will be 1mV per degre Celcius as cheap panel meters range 0- 200mV

One of LM324 operational amplifier output is connected to ground... which is separate from VDD, could you explain me more?

I 'm making now the box that will contain the circuit and support the press itself. When I get over this stage i will make pictures, is 100Kb suitable size to send you ?

Mail From SY

Radar using Laser - Detectors

Radar using laser and microcontroller will u pls help me and provide me the necessary info.

Mail from  VV

Here are some pages in the web that may be useful in your study ...

Integrated electronic and optoelectronic circuits and devices -  for pulsed time-of-flight laser rangefinding

The main focus of this work concerned with the development of integrated electronic and optoelectronic circuits and devices for pulsed time-of-flight laser rangefinding is on the construction of the receiver channel, system level integration aimed at realisation of the laser radar module and in integration of all the receiver functions of laser radar on one chip.

Unusual Radar Detector

This unique anchient design uses a chopper amplifier - The microwave diodes are chopper-gated by a multivibrator so the RF signal level can be detected by an audio frequency amplifier.

 Laser Radar for Vehicle Speed

Laser Radar for Vehicle Speed

Abstract In this research work, new exploration of a pulse laser radar system is presented with two splitted parallel laser beams for the measurement of precise velocity of moving vehicles along with their contours and ranging distances. The emission of optical pulses is eye-safe, and hence the radar can be classified as a class-1 laser system.

Thursday, May 23, 2019

2.5 GHz Frequency counter

"Imagine you have a radio receiver and you wish to display the receiving frequency. As you know the oscillator in a radio receiver works 455 kHz or 10.7 MHz above the receiving frequency. The 455 kHz and 10.7 MHz difference is called IF (Intermediate Frequency)."

2.5 GHz Frequency counter

A Google talk around 2006 with another electronic enthusiast

davidhvm (3:36:51 PM): Hello, i assembled de prototype and work fine
davidhvm (3:37:24 PM): anyway i add a push button to Set/Reset to 00 the counter
davidhvm (3:37:39 PM): thanks a lot again
davidhvm (3:38:13 PM): check this 2.5 GHz Frequency counter
davidhvm (3:39:07 PM): i'm looking for some apps for counters and i
found a lot o pages with very usefull
davidhvm (3:39:44 PM): circuits of course with PICs (Microchip or Atmel)
davidhvm (3:39:47 PM): Bye
delabs (3:44:58 PM): hello david
davidhvm (3:45:37 PM): hi all work fine
delabs (3:45:48 PM): great
davidhvm (3:46:40 PM): are amazing these projects
davidhvm (3:47:15 PM): ok thanks may be for long time
delabs (3:47:30 PM): fine
davidhvm (3:47:41 PM): i'm at middle of my carrer

Monday, April 29, 2019

Battery Brightness and a Blinking LED

Many many thanks for this quick reply again.

We are also experimenting with stobing the LEDs very fast using digital out of the MCU's I/O to kind of PWM the LED to calibrate it properly for our application .

I plan to use 3 x AA batteries. If I add a 4th battery, The solution has to be really cheap since there will probably be 4 constant current circuits -

The 74HC14 is cost effective, so it's a possible solution if the circuit works well. Yes - I need to be 100% sure that the LED brightness is the same for full battery to say 80% battery

Mail from AB

You could look at this TPS61043: Constant Current LED Driver

The TPS61043 is a high-frequency boost converter with constant current output that drives white LEDs or similar. The LED current is set with the external sense resistor (RS) and is directly regulated by the feedback pin (FB) that regulates the voltage across the sense resistor RS to 252 mV (typical).

The 74HC14 can be used to drive-light 4-6 LEDs. It can be directly connected to MCU.

The LEDs should be flashed at around 5 Khz. Then a constant current source is not required.

Why do you wish to use constant current. Is it for a steady light.

If the problem is dropping voltage. Another battery or voltage doubler is required

An voltage doubler may not be cost effective. Another battery may be a simpler solution.

By using a 74HC14 you can avoid the PNP transistors. This is an inverter, you can use a buffer chip too.

A series resistor with LED or 10Kz flash is to limit current.

The 74HC14 just replaces your PNP transistor saving 1/2 a Volt. The Microcontroller PWM can be used to control brightness. Then you can skip even the series resistor. You can even use a 555 chip for pwm.

Flashing must be above 1 KHz. The persistence of Vision is 20mS. Very high rate will make it dim, something optimum.

A better Idea is to use LM317 to get 3.3V regulated supply from 5-6V of batteries. Use this regulated supply to drive LEDs. This also needs to be tried practically. As voltage is constant the current in leds will be constant. for 20% drop.


Sunday, April 28, 2019

The Return of Bright LED

From my Thunderbird archives. it may be interesting to read, maybe you may learn a thing or two.

Many thanks for your quick reply - much appreciated. The FET solution is new to me, but I have tried most of the others.

I think the problems are:

1) As voltage for these LEDs drops from 4.65v, fairly quickly, the constant current starts to move.

2) The 'constant' I can acheive would be only 4% while the current is stable.

Do you think my problem can be solved? Many thanks again,

Mail from AB 2006

NSSBC : the status indicator

On the right we see the status indicator drivers. One 74HC14 gate buffers (and inverts) the signal. The other gate re-inverts the signal. The trick now, is to mount the LED and resistor in parallel to the second buffer gate.

The Return of Bright LED

The example above may be the solution you need...

The LED you use needs 20mA at say 3.3V as per datasheet you sent. 3 AA batteries are 4.5V right, and say 20% drop is 3.8V ok.

Of the 3.8V 3.3 is needed by LED we have only 3.8-3.3=0.5 left With 0.5V dropped across Control Transistor PNP, you have nothing left.

A simple idea is to use a cmos chip 74HC14 to drive LED. Instead of constant current you flash LED at High speed. This way Battery and LED will last longer and work at lower voltage.

1.5 Volt LED Flashers

The LED flasher circuits below operate on a single 1.5 volt battery. The circuit on the upper right uses the popular LM3909 LED flasher IC and requires only a timing capacitor and LED.

1.5 volt dual LED flasher

This 1.5 volt led fasher runs more than a year on a single 'd" cell and alternately flashes 2 LEDs at about a 1 second rate. The circuit employs a 74HC14 CMOS hex inverter that will operate at very low voltages (less than 1 volt).

delabs - 2006

Saturday, April 27, 2019

Driving a LED from MCU

I just read your blog and found it very helpful - thank you.

I am working on a new toy project and have a problem I think you can help with.

I am turning an LED (see attached) on and off from a small taiwanese MCU (like a PIC chip). I am using a PNP transistor for this.

My application needs constant current to the LED from 3xAA batteries down to about 3.6v-3.8v. Since it's a toy, the circuit has to be very very cheap! So far I have only managed 'constant' to be about a 4% drop, but I need to do better.

I need a circuit which includes the PNP from the I/O pin of th MCU. Help!

Mail from AB USA in 2006

I am trying to understand your need. All you need is a PNP transistor driving a LED. A series FET constant current element has to be added.

Current Source for LED

The circuits below can show parts of the circuits. If you need something special, i can design for you.

 Constant Current Source LED Drive

A current source based on a JFET

In Lecture 1 we introduced the idea of a current source, and mentioned that these were slightly more difficult to implement than the perhaps more familiar voltage source.

White LED Stroboscope

With Constant Duty Cycle and, Constant Current Drive. The circuit is composed of an oscillator, a pulse slicer, and a gated constant current source. All of the circuits except the anode of the LED receives power from an LM78L05 5 volt regulator.

delabs - 2006

Wednesday, February 27, 2019

24V and 25V Power Supply with 2A Current

I am looking for a 25V Power Supply with 2A current for my Application. A regulated supply.


Here is a simple to build regulated power supply. The two diodes are to boost the 24V to 25V DC. It uses regulator 7824 and TIP2955 PNP Transistor to boost current.

24V and 25V Power Supply with 2A Current


Thanks alot! Do you know of a good transformer manufacturer that has the one you specify? Also, is that "100V 3A" at the bridge the rating on the diodes? Also does the transistor circuit increase the voltage to the regulator so that it is 3 or more volts above the regulating voltage? Lastly, is your ground reference on the gnd of the regulator?

Wait, one more....this is + or - 25 volts correct?
Thanks alot!


1. choose a transformer with a close match of what is available.

2."100V 3A" at the bridge is the minimum diode rating also(or use 4 x 1N5408). "100V 3A" is also minimum bridge rating use a close match. make a bridge with 4 diodes refer my other circuits, there are many.

4. 22V AC leads to around 30V unrectified DC which is good, if the 30V becomes more, the heatsink needs to be very big.

5. the ground of regulator is lifted by (0.7*2) by 2 diodes to provide 1 V above 24V, as 25V regulator does not exist. the power supply 'neg' can be grounded or earthed,

6. the supply is a+25 V from ground or negative, but things are only relative.

7.if you need a dual supply 3 terminal , that is ground +25 and -25 then for -25 one more circuit using 7924 and tip3055(npn) has to be made.

8. use any high current pnp transistor (5A) you have put a heatsink with heatsink compound.


Monday, February 25, 2019

UPS and Inverter Schematics

Do you perhaps have an inverter schematic for a sinewave inverter or stepped squarewave using the SG3525 chip.

I'm also interested in connecting the inverter to the grid via a solar panel. I hope to reduce my electricty bill by pumping power into my 220v system. Do you perhaps have circuits to synchronise the inverter to the 50hz main.


Nice to get your email. .. Have you seen the circuits in my page....

Power and Energy Schematics

Switch fast from mains to inverter. The inverter is always on with the battery charged by the mains.

UPS and Inverter Schematics

The mains load is of two types

lighting low current 5A ??  and  heating high current 20A??

You should isolate both of them, the UPS is only for lighting. For heating you need a very big UPS, solar will not help, oil might.


I want to build a home UPS System. Uninterrupted power supply for computers and with Solar Cell Integration. I need some Schematics..


You have to design and develop these equipment, just a circuit or instructions will help you learn but you cannot make a robust product. Anyway have a look at these applications to learn ..

Sine wave Inverter 800VA

Learn about UPS here  UPS Power Inverter and DC to AC


Thursday, February 21, 2019

Making Coils and Transformers

Message - Can we make Coils and small transformers at home. I have copper wire and some lamination in my stores. - 2005


Making Transformers with good isolation and safety is a task for a firm and a home user must learn those skills by working in a winding company first.

As this is a general question many ask i am posting some educational pages. here...Coils and Transformers - Magnetics, Inductors, Chokes, Ferrites - Iron and Copper, Electromagnetism, Alloys, Insulators

Making Coils and Transformers

We have low frequency and high frequency transformers and coils in power electronics and electrical engineering. Then we have signal coils and transformers used in communications.

Low frequency transformers are made of a core of iron alloys in the form of insulated laminates to reduce hysteresis and eddy current losses. The copper wire insulated by a thin insulating enamel coat is used to wind the primary and secondary coils.

Coils and Transformers for SMPS, In Switching Power supplies and RF Circuits, the coils and transformers have many types of cores. Ferrites, Powder Iron, Air and others.

Producing wound components

This page provides practical guidance for students, staff and researchers at this University who need to wind their own inductors, transformers or solenoids. The types of wound components available, and their applications, are so varied that only general guidelines can be provided.

Compact Fluorescent Lamp (CFL)

Compact fluorescent lamps have some benefits in comparison with classic light bulbs. It is lower power consumption (to 80%) and much longer lifetime (5 to 15 times). Disadvantages are longer starts mainly at more expensive types, impossibility to use darker and price.

Practical transformer winding

In the good old times it was a matter of fact that every electronic hobbyist or technician would wind himself any power transformers he needed, and rewind any that burned out. Unfortunately, nowadays transformer winding is fast becoming a lost art, and I have seen many people despair about where to find some very specific transformer, or pull their hair out about the cost of having one professionally wound to specifications.

Winding Coils - Ham Radio

I used to view the inductors of RF circuits in books and magazines as problematic, often shunning any radio circuit having coils which are difficult to find . RF inductors of the correct value, unlike standard components, are not easy to buy, if at all. However, there are some great radio circuits around and it's a pity not to be able to reproduce them just because you normally can't acquire their coils off-the shelf.


Tuesday, January 29, 2019

Mixed Signal Grounding Method

How do we interface Analog and Digital Blocks. The earlier post explains ground connections that are radiating from supply but does not show Analog and Digital Blocks Interface.

Analog Blocks have Opamps, Analog Mux, A/D Convertors and D/A Convertors. Some microcontrollers have built in A/D and D/A then you have to use the separate ground pins they provide. The implementation remains the same.

Mixed Signal Grounding Method

The diagram above is self explanatory. There are mainly types of return ground paths... to the power supply ground

Pulsing Ground - From Digital circuits that are switching in MHz speeds.
High Current Ground - From Interface or Power, LED, Motor Circuits.
Clean Signal Ground - From Analog Signal Conditioning and Mixed Signal.

Use proper decoupling caps at every chip and every functional circuit block. Have a ground plane that doubles up as a shield in on or more layers of PCB. In RF circuits you could use two ground planes on two layers of pcb and signal tracks in between.

Wednesday, January 16, 2019

LM3914 Circuit Ground Connections

I am about to build the circuit that you have here: - Millivolt Meter - Bargraph

I am pretty sure that I understand all of it, except for the symbols that you have at the grounds which are little triangles with the letter F next to them. Please excuse my ignorance, but what does that mean? Is that the ground from the tested millivolt source?


One Ground carries the LED Currents and that track has to be thick and go directly to the Voltage Regulator 7805 which provides 5V supply. If this track is looped with the signal ground, millivolts of error may occur in reading and if tracks are thin it may lead to oscillations.

LM3914 Circuit Ground Connections

We have a High Gain Amp  in this circuit that will amplify even these small jumps in mV due to track Resistance in ground loops.

Analog Ground, Digital Ground, Supply Ground, RF Ground and Earthing have to be carefully designed to get accurate performance. PCB Autorouting may ignore these subtle requirements.

Monday, January 14, 2019

ICL7135 PCB Layout Method

I have design a PCB layout for a 4.5 digit voltmeter using the ICL7135 chip, and i have problems with the analog and digital ground. Do you know if somebody have make a PCB for this chip?

I have seen in your schematic a superb circuit- del2003.sch, ICL7135 based auto ranging DVM. Do you know if the PCB layout is avalaible?

I will appreciate to have some help or if someone can send me the PCB layout for the del2003.sch.
Thanking you in advance,

Mail from RL

I am not able to locate my PCB Layouts, but to study about analog and digital grounds for this chip and pcb layout see this Service Manual i found online.

delabs Notes -  Discuss Topic here EE Forum - Analog Ground

Digital Grounds have Switching Noise or Pulses, Interface Grounds have higher currents. Sensor  Low inputs that are not isolated may have potentials and may lift the system ground and also pose hazard. Electrical Earthing and System Grounding are also different things.

Analog Ground and Digital Ground

Keep the analog and digital current return paths separated.  Remember all tracks and wires are resistors and conductors too. connectors have contact resistance. soldered joints (and connectors) have thermometric EMF. Looping Tracks is wrong Radiating tracks is right.

Read this PDF too Grounding in mixed-signal systems demystified, Part 1
Undoubtedly, grounding is one of the most discussed subjects in system design. Though the basic concepts are relatively simple, the implementation is difficult. For linear systems, the ground is the reference against which the signal is based; and, unfortunately,