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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.