Picture of How Electronic Switches Work For Noobs: Relays and Transistors
This Instructable is targeted at those just stepping into the world of electronics.

In this guide I will explain how the two main types of electronic switches work, these being relays, and transistors.

Firstly, what is an electronic switch?

An electronic switch is essentially just a switch that uses an electrical current, to turn on, usually turning off when the current is turned off. Some applications of switches can be quite inconvenient for someone to go and press a button to turn on or off, such as for the starter motor in a car, or the "turn off nuclear meltdown" button inside a nuclear reactor, or in an electronics project, a small low power device such as a receiver, must somehow power a large energy guzzling component, like the motor in a garage door opener. And others just want to control their houses with their computer's, which could never possible supply the 240v/120v mains needed to run some appliances.

This guide will include a very noob friendly explanation of the internal workings of relays and transistors. First, we will begin with the simplest one to explain, the relay!

Step 1: What is a relay

For the beginner, a relay can be a very difficult application to first understand, it was for me, i spent 3-4 days researching for a simple explanation as to how a relay works, and quite recently found myself being asked for an explanation, by a clueless noob who didn't understand.

In simple terms, a relay is a device that uses an electromagnet to mechanically pull two connections together to complete a circuit, in the exact same way your finger mechanically pushes two contacts together in a toggle switch.

A relay is used wherever a small low power device or power supply needs to switch on a much larger one, usually completely isolated from the signals power source, or at a much higher voltage than the signal could provide.

However this is usually not enough to help someone really grasp the idea of how these mysterious boxes work.

Move over to the next step for a better explanation along with an flash animation to show you how it all works.
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DmitryU1 month ago

Thank you!
Finally i catch up with transistor's mechanics! ^_^

15-08-901 month ago

Great work.Would like to get more knowledge about relay. A brief history of relays and their applications in the electrical.

Manny891 month ago

Nicely done with excellent examples... I appreciate all the time you put into this...!

AnthonyT113 months ago

Is the description of N/C and N/O backwards in his description? : "N/C: Short for normally closed, this terminal is not connected to COM when there is no signal, but when there is a signal, the pin inside the relay is pulled down until it touches N/C, which would connect it to COM"

I thought N/C should mean that without input(relay off), it's closed in a circuit, i.e. connected to COM. ??,

rhetherington3 months ago

Great explanation! Thank you!

Does that mean that if 2.5mA was applied to the Base only 250mA would be allowed through the Collector creating a total of 252.5mA at the Emitter?

Also why do people say that transistors amplify? - is it just based on the fact they combine both the Collector and the Base?

MalikE24 months ago
chrissp15 months ago

Thanks for this tutorial man, this exactly what i was looking for. But I still need to ask you this. Can i use a regular transistor to replace a burnt out mosfet in a pioneer 760w car amplifier? Or would it cause it to get worse? The transistor i want to use is a ML7808A and the mosfet i want to replace is a P55NE06FP.

If not possible, can you suggest what other kind of transistors I could use to replace it? Just like you i took apart old electronic devices that i had in my garage and wouldn't like to drive an hour to the closest electronic store for such a small component. Thanks in advance.

To answer your question simply: no, you can not replace the MOSFET with the transistor. Now here comes the disclaimer/explanation: the circuit in that amp was designed to drive that MOSFET not a transistor. A transistor uses current at it's base to "control" the flow of current through the collector and emitter pins.

Think about a transistor as a garden hose with your thumb over the end (pretty sure we've all used that trick to squirt someone). The flow varies with the way you move your thumb, just like a transistor. The MOSFET, however, is like the super-soaker. As soon as you pull the trigger (apply voltage), the gun is shooting water. There is no half way off or half way on, you either are blasting someone with as much water as possible or you're not (and usually running cause you're out of water).

The most important thing to anything electronic is the datasheet. The datasheet will tell you EVERYTHING about the device you are working with... even the maximum limits... which... well... you can test if you want. Hence the name, Fusekiller.

Use websites like: datasheetcatalog.com or findchips.com to look up who sells the parts. Even the search engines can lead you to the datasheet. Most of the time they are PDFs.

Putting P55NE06FP into findships.com came up with a distributor offering it for $8.0880. Now, with the datasheet, you can find a cheaper, comparable MOSFET from Newark.com, digikey.com, mouser.com, etc. by using the specs in the datasheet!

Hope this helps,


TharangaW6 months ago

I've guessed how relays work since I've opened a few of my car but never understood the transistor switch. Helped me a great deal. Thank you very much.

live10526 months ago

Thanks very much for this. I understood relays and I thought I understood transistors but could never get them to work on my project board but I didn't know the base current had to be lower than the collector and how you explained that was perfect. Nice one!

kbapana6 months ago

Hey Can I use any transistor to Flash LED If yes then how........

If no then Explain............

Thanks man, that post really helps understand the relay.The one I'm wiring is in a motor control panel and has 8 "pins" though. I'm familiar with NO and NC, COM is a new term to me. I'm not sure how to see what's what with my meter while I'm putting it together inside the panel with no power on. here's a picture that points to it
2015-01-29 13.09.14.jpg
blueshadownet7 months ago

Great Instructable!

trinityx8 months ago

Thank you for taking the time to write this, it's explained very nicely.

Hi. Thanks for the write up, much appreciated and massively useful. I've been told I can use a power MOSFET insted place of a relay on an automotive circuit. The circuit is designed to switch in excess of 14 volts to the headlight bulbs. In the relay circuit, the existing feed to the bulbs is used to trigger the relay and heavier gauge wiring is used to supply the switch side with full power (via a fuse) to the bulbs.

At maximum, ie. for main beam, having both bulb filaments on (H4 bulb), the combined wattage would be 120 watts. A fully charged auto battery with a charging system in good order, supplies in the region of 14.4 volts. Therefore the current requirement for the relay is 8.33 amps.

Most auto relays are easily capable of this and are very resilient devices. For my circuit, would a power transistor suffice or do I need a MOSFET as I've been advised??

Many thanks :-)
AdrianC58 months ago

great introduction for someone who know nothing about the concept. thank you

sina elec8 months ago


brunofgm8 months ago

Thank you very much for this!

Franscois9 months ago

Thank you so much. Will certainly use some of the tips. Cheers.

ktow9 months ago

Fantastic overview. Thanks for all your work. It was well written and the animations were very helpful!

Good effort on how to explain about transistors. Keep it up!!

ajohnnapthali10 months ago

This document is nice..

vimalrkv11 months ago


what is the difference between current controlled switch and voltage controlled switch?

how it differs from each other?

jpoopdog (author)  vimalrkv11 months ago

a current controlled switch is controlled by current, a voltage controlled switch requires a set amount albeit tiny amount of current, but the resistance is determined by the voltage. voltage controlled transistors are FETs like MOSFETs

mufazin1 year ago

the gain proportion mentioned here is the transconductance (Gm) ??

jpoopdog (author)  mufazin1 year ago

i dont actually know. Ive only ever known it as "gain", look up a datasheet and it should hold them term if its not the common aplification gain.

Would I be able to use a FET transistor to turn on/off the signal to an ESC signal wire to control a brushless motor, or would it distort the signal? I want to be able to control whether the signal comes from an Arduino or a RC receiver by switching on or off a FET (one FET for each signal source). I would be controlling the "gate" voltage with the Arduino.

jpoopdog (author)  Hailstorm991 year ago

it wouldnt distort the signal, or at least, not enough to do anything if it did, the PWM signal varies in the milliseconds, so you could probably even do it with a relay.
however, a fet isnt really suited to the role, youde just want a regular transistor, Mosfets are sensitive to static and cost more than the same power rated transistor. since your just controlling a pwm signal, which i beleive is around 5v at a few mA, and most any transistor could turn on and off a thousand times between each cycle of the signal, a transistor is much more suitable.

in short, you dont need a mosfet, a high gain transistor will be fine.

mosfets are only really suited for fast repeated switching of extremely high currents. or super accuracy/precision waveforms.

Oh ok. I was talking to my electronics teacher, and he said that Field Effect Transistors would work better. Then again I don't think he understood my question.
jpoopdog (author)  Hailstorm991 year ago

well yes, it would work better, but its totally unnecesary, like in the same way a Ferrari is better than a station wagon, but that fact is irrelavent when you just need to drive down the road to the supermarket

cmusada1 year ago

Hi i would like to ask some help after reading your tutorial about transistors ive understande the use of transistors a bit more. but still not much but i would like to ask some help. can i use a transistor as a switch to control charging/discharging of a battery. what transistor should i use? thanks in advance :)

jpoopdog (author)  cmusada1 year ago
well this gets a bit tricky. no, you wont want a transistor any more than you would use a push button switch, its possible yes but its wasteful, inneficient and difficult.

for batteries like SLA nimh and nicad, all you need is to output 1.8-2.4v in a bistable circuit that turns off when you reach a certain voltage, that function though is done with transistors. for adjusting the voltage youll want a 5v variable voltage regulator, it uses 3 pins, in, adj/gnd and out, and the output voltage is determined by a resistor connected between two of the 3 pins. for lithium based batteries you need a special kind of variable charger specific to that kind of battery, or just a slow charger which takes days at a safe minimum

i just wanted to make a circuit to measure the capacity of a rechargeable li-ion battery with the use of a arduino. so i cant you use any transistor? how about a relay?

jpoopdog (author)  cmusada1 year ago

Im not sure exactly of the function you want to perform, if you for whatever reason just wanted to do a low voltage cutoff with the transistor then thats fine, for that ide recomend to use a relay though moreover, since its going to be always on, and also, for 3.7v lithium cells, try to find a 5v-12v transistor since they have lesser resistance.

reddynet1 year ago

Great tutorial. Thank you

what transistor(s) would you recommend for doing the setup you have described above? I'm trying to make a strobe light, but the LED I have is a high power Luxeon LED, wich obviously can't run off my 555 timer.

jpoopdog (author)  didgitalpunk1 year ago

well, this depends on the required current, if its an amp or more, just do a general search for transistors which are classified as "power" transistors. In general power transistors are never used for signal deliveries, but instead, switching heavy loads.
one in particular i know and use is called the sd882 NPN (sb772 is the pnp version), it can handle at least an amp but i think can go as high as 2, with a peak of 3 amps. Its a medium-low power transistor and is very cheap.
On the other hand, keep in mind that you can use several transistors in parallel if individually they arent up to the task, though its not recommended since the failure of one could damage the rest.

Thanks for the quick answer!
kgwolf1 year ago

This was a great tutorial; very informative. I am just getting into the world of building circuits, and this write-up was very clear and easy to follow. With regard to switches, is it correct to say that all they do is enable a secondary power source (switch it on)? So if, for example, I wanted to turn a lamp on and off with signals from a microcontroller, it would still need to be plugged into the wall, but with an open circuit, which the switch would then close? Thanks!

jpoopdog (author)  kgwolf1 year ago
yes, but keep in mind, its not so much the fact its a switch, as it is, its a low energy high speed switch which can offer variable currents between saturation and being completely off, plus it CAN be controlled by a microcontroller, wheras a relay or mechanical switch cannot.
A transistor for example, unlike a relay might be used at a super high frequency with PWM, to dim a lamp which which wont turn off between the ON-OFF-ON phase. SOmething like a incandescent bulb would do this.
But in essence yes, a transistor just enables a secondary power source, though, since the signal is encorperated into the current, its primarily thought of an an amplifier
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