Circuits FAQ

This is the Archive of Electronics & Technologies related Question & Answers. Ask Questions at EE BBS FAQ. To get a project designed by delabs visit Design Service.

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Friday, February 21, 2020

What is a Relay - Jim Asks

(A text chat in Google talk long ago from my email records - 2007)

Jim: hello,
delabs: hello

Jim: i have a Small problem,it's about would u pls give me an information about relays,pls give different types of relays and their differences.

delabs: for what do you want to use the relay for.

What is a Relay - Jim Asks

Jim: Two door alarm settable for close or open door

delabs: there is a control signal and an output for relay, if you are using just a buzzer for alarm, no relay needed if you want to operate a big hooter or 230V lamp a relay is needed- or even a motor or solenoid to move some thing  Even a Solid State Relay can be used.

Bigger loads like large motors may need drives and containers. Some even have speed control. Big doors are more difficult to open and close. safety features needed.

The picture above shows also reed relays that are used for small signals - analog and telecom. In Solid State switching also we have small signal and power switches, using mosfet or triac.

Soldermans Basic Electronics: Relays and Contactors

Jim: let me check dis addr nd i'll get back 2 u just now.
Jim: ok but if i have a problem i'll get back 2 u,bye

Thursday, February 13, 2020

Soldering Iron Tip

Can I use stainless steel to replace a defective soldering iron tip?

mail from EA

Soldering iron tips are mostly alloys of copper. the thermal conductivity of steel is poor. buy a soldering iron tip available in the local market.

This image has a Tip for Soldering Iron and also has Soldering Iron Tips.

Soldering Iron Tip

Change Weller ET Series Soldering Tips

Changing the soldering tips on a PES51 pencil is very simple. You just have to make sure to take your time and don't force anything. Damaging your iron is not only possible, but easy when replacing the tips, so proceed with care.

Weller tips are magnetic and only original weller tips to be used.

The Basic Electronics Soldering & Desoldering Guide

The Basic Soldering Guide contains everything the beginner needs to solder electronics successfully, with guidance on choosing the right soldering iron, details of various solders and fluxes,


Thursday, February 06, 2020

Passive volume control

please if you don't mind I need to help to find a control circuit for volume of speaker 4 ohm or 8 ohm located in the another room far away of cassette or CD player by using only passive components with potentiometer without transformer.

mail from AD

i have attached to this mail a simple circuit you can build at home at low cost. This is the only alternative for the need that you have specified. it uses just a rotary switch and wire wound resistors.

Home Made Circuits

Volume control circuit for speaker 4 ohm or 8 ohm located in the another room far away. by using only passive components. This gives limited control and helps reduce volume when a conversation starts. The resistors are very low value.

Passive volume control with Potentiometer

A simple circuit you can build at home at low cost it uses just a rotary switch and wire wound resistors. Add more resistors and a rotary switch with more contacts for finer control.


Monday, January 13, 2020

Seven Segment LED display

how is the circuit for driving a seven segment led

mail from EA

A 7 segment LED display is used to display a number 0-9 using 7 LEDs, The circuit you design must have a BCD output, BCD means Binary coded decimal for number 0 it is 0000 and for number 9 it is 1001 see this chart i put in my site....  Electrical Tables

the BCD is fed to a BCD to 7 segment decoder like CD4511 or 7447 which has the logic to take care to drive your display, the displays are two types common anode and common cathode you have choose a driver to match your display, you will need to use current limiting resistors for every LED to limit current to 15mA or less. there are many circuits in my website which has examples of these.

Seven Segment LED display

you can also drive such displays with 7107 or 7135 to display analog signals, or with a microcontroller like 8051 using software decoding, even such examples and code is available at my website.Digital-Circuits. and Logic-Circuits.


I made an online tutorial for seven segment LED. Hope it helps you learn more.

Digital up down BCD counter "Simple Digital up down counter CD4029, with interactive javascript simulation." You can run the circuit online and try out without wiring it up.


Winding Transformers and Coils

i would need your help in designing transformers for rechargeable lamps. i really don't understand what the dots in front a coil stands for. all help (including pictures) would be really appreciated. my own just packed a few days ago and i would love to recoil the transformer.

mail from LA

You will have to do a lot of study in the internet, and experimentation too. Transformer winding is a specialized profession not a hobby at all. You can wind some coils or small signal transformers, that is ok. Please do not try to wind mains or 230/110 V transformers at home.

Winding Transformers

I can only give some tips, precautions and techniques to a winder. study these links....

Electrical Formulas, Calculations, Basic Electronics Engineering
I have put the link of a page but see all the pages of the website. The Dot polarity in a transformer stands for the direction of winding relative to each other. The Direction of winding could be clockwise or anti-clockwise.


NE612 - Balanced mixer with Local Oscillator

I am designing a circuit that implements an AM radio circuit that ultilises the IC Double mixer and Oscillator NE612. I was just wondering whether there is any standard symbol for this kind of IC.

mail from CM

There is no standard symbol for NE612, just a square box. You have to make a custom part in the CAD software or see if someone has done that. check these links on info of NE612 circuit and symbol....

GM47: Simple 30m Band QRP CW-Transceive 

The Rx mixer is followed by a IF stage featuring an NE592. The mixer's balanced output is directly connected to the amplifier's differential input, minimizing component count at this point.

Direct Conversion Receiver using NE612 for HF Bands

The NE612 is an integrated circuit which contains a balanced mixer with its own on-board local oscillator and voltage regulator. The mixer can provide up to 18dB of gain at 45 Mhz, and the local oscillator will operate up to 200 Mhz.

NE612 - Balanced mixer with Local Oscillator

HF Transceiver using NE612

 I would be working on the Transverter first, as with this, I could try out the 9Mhz SSB Module that I already have. I would be building the Transverter for the 14 Mhz band depending on the values from my DC Receiver project using the NE612 IC

    Sunday, November 17, 2019

    Guard rings in strain gauge measurement

    Hi... Can you please tell me as to what is a guard circuit used in bridges?

    Mail from SN

    The very high impedance amps for bridge circuits have inputs in tera ohms and bias in pico amps. The sensitive bridge circuits must be able to detect a 1 uV difference when a 100 gm weight is placed on the cantilever. So it is very sensitive.

    The power lines and other logic lines close to chip high impedance pins may leak pico amps to nano amps of currents, humid-air or PCB contamination or power pins being too close, the leakage currents may show as full scale reading without input.

    Read also driven gaurd - a better method

    " The basic principle is to protect the sensitive wire with a surrounding guard that is held at the same voltage as the wire, if they are at the same voltage then there will be no current flow."

    Read also driven gaurd - a better method

    The guard rings around these sensitive pins or shielding will act as return paths for the leaking nano amps.Even ionic discharge from a sharp pin with high voltage can imbalance bridge. As it is like an Electrometer.

    Read more here. guard rings in strain gauge measurement

    The Strain Gage

    Strain gage transducers usually employ four strain gage elements electrically connected to form a Wheatstone bridge circuit

    Electronic Electroscope
    This simple circuit can detect the invisible fields of voltage which surround all electrified objects. It acts as an electronic "electroscope."

    Guarded resistance bridge
    By using a guarded scanner and two sources to form a guarded resistance bridge, measurements from 100KW to 10GW can be made with excellent accuracy. This system provides a simple yet effective way to activate both the high and low guard circuits.

    Compact DWDM laser Temperature Control with the MAX8521

    The guard ring functions as follows. Because the DAC output voltage is similar to the op amp's inverting input voltage there is essentially no potential difference between the guard ring and summing junction and its associated components inside the ring. The ring intercepts outside surface leakage currents where they are shunted by the low impedance of the DAC.


    Saturday, November 16, 2019

    Lighting a LED String - Serial Drive

    Hi there, my name is steve. I would like to ask you about the best way or the most efficient way to supply LEDs. there are 36 small LEDs or 72 small LEDs (5mm).

    could I just put then in series the 36 LEDs and a diode, resistor and plug in to AC power line (110)

    (An email i got some time back)

    Answer - Lighting a LED String - Serial Drive

    Remember that the LED for lighting or otherwise needs to be protected from over current on Forward Bias and High Voltage connection in Reverse Bias.

    That means you could use a 3 A Diode like 1N5408 to prevent wrong polarity connection. Then all you need a constant current source with a voltage output a little above the product of the Number of LEDs and Forward Drop of a LED.

    5 LEDs in Series with a forward drop of 3 each means 5 x 3 = 12. You need a 15 Volts Regulated Supply with a current limit of around 30mA or more. See the LED datasheet. Go beyond limits you lose your LEDs the same day or months later.

    Use SMPS designs for smaller sizes and it has to be Robust to Mains Power Supply Variations. Use a Surge Protection Assembly from a Reputed and Quality Supplier. (this surge part spoils the reputation of your Lighting Product and affects the entire LED lighting Business)

    Read these pages to learn more....

    Lighting a LED String - Serial Drive

    The Perfect LED Light -  by Garry Petrie

    "The perfect LED light is designed for cavers who want an efficient, uniform and constant bright light for caving. This article contains some very specific technical information and it is assumed the reader has a basic understanding of electronics. To build the light, the reader also must have some experience soldering, drilling, wiring and cutting small objects."

    A linear Constant Current LED Lamp Dimmer

    "This simple Linear circuit provides continuously variable regulated current (~25-400mA) from a 4-6 Volt source.  I chose a linear design for simplicity, reliability, ease of repair, and to avoid switching EMI in my Cave Radios.  The circuit requires only 0.2V headroom above the parallel LED Array voltage to provide regulation at maximum current. "

    Extra Extra !