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!
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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.
reddynet1 month ago

Great tutorial. Thank you

didgitalpunk2 months ago

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)  didgitalpunk2 months 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!
kgwolf2 months 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)  kgwolf2 months 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
mrmerino6 months ago
Say I had a circuit that used a manual button to do something. How would I connect a transistor to the existing circuit, without removing the button, so that the transistor can activate the circuit in the same way the button can?
jpoopdog (author)  mrmerino6 months ago
well, the simplest way would be to use an NPN transistor. since it operates best when your working with positive voltages, or rather its easier to understand.

Now, find the directionality of current, find which terminal on the button is connected to positive voltage, and which is negative in comparison. Power needs to flow INTO your collector and OUT of your emitter, so find which button terminal is the IN and which is the OUT. if it leads directly to ground or negative then that terminal is your OUT.
When triggering the transistor make sure you use a lesser voltage than your transistor is driving, and ensure that if you are using a seperate power supply to trigger the transistor as what you are driving with the transistor, it wont work unless you connect your negatives together, because the base input needs to circulate through ground/- and back through your power supply to complete a circuit, you know how it goes.

Does this answer your question?
To reiterate, Collector takes current in and emitter emits current out.
mrmerino jpoopdog6 months ago
I got the gist of it, and that's certainly a more thorough explanation than I've th gotten from anywhere else. If I understand this right, I want to put the transistor circuit in parallel with the switch. That makes sense. Thank you!
astral_mage7 months ago
actually depending on . how old yr set up might be. it could be the electronic board in side could be screwed up.
urekaeee9 months ago
thank u really this notes useful for me
Alan Reyes11 months ago
thank you, I was very confused about relays and transistors, but after reading this everything is much more clearer
jethro11 year ago
Excellant tutorial.
Have you ever published any data to describe how a voltage regulator for an alternator works?
jpoopdog (author)  jethro11 year ago
sorry no, but i wouldnt imagine it to be that complex, it would likely be a typical regulator, the output, rectified to DC, and trimmed with a capacitor to reduce the voltage spikes and declines, rounding them off so to speak, so they are friendlier to the regulator.
The voltage regulator would then work by trimming off and dissipating excess voltage, while completely maintaining the current output.
It would either be super heavy duty, and dissipating allot of heat, or just heavy duty, but able to regulate the voltage within significantly less heat dissipation. Truth be told i dont know much about the workings of the alternator and its regulator, so dont quote me on this.
But in simple terms thats about it.
chithracj1 year ago
thanks for helping me to understand transistor working in a easy way
Question: I am a notice...I have a cordless lithium drill a little over a year old, so out of warranty. The battery works as the "tiny LED work light" works fine and the battery check shows full charge. I have checked the motor and it is good. But when i pull the trigger on this little 4V drill the drill will not work and the "work-light" cuts off. When I release the trigger the light comes back on, but with a micro-second of hesitation. What component would/could be the problem? I actually have 4 of these drills and 3 of the 4 don't work with the same identical symtoms...Any idea?

jpoopdog (author)  hollowijohn1 year ago
Sorry i didnt quite get that, you need to type more clearly.
What is the "work" light your speaking about?
does it indicate your battery is functioning or the drill is turned on? If its turning off when it is normally meant to be on, this indicates a short circuit occurring somewhere. meaning no power can go to the light.

if you can open the drill and post a photo of the circuit board i would have a better idea of whats going on. Chances are though that the transistor responsible for driving the motor is damaged. Its quite easy to perform a test also to see exactly whats wrong.
danny591 year ago
I don't know if your a teacher by profession, but you would make an excellent one. You have a way of explaining things that make it very easy to grasp, and your animations are also an excellent tool for learning. Great Job!
rippentorn1 year ago
Awesome job! I appreciate your thorough, simple, explanation!
jpoopdog (author)  rippentorn1 year ago
And i appreciate your telling me. Thankyou! :)
tyts71 year ago
Tku vm much clearer now.
jpoopdog (author)  liamm13201 year ago
the link was unnecesary, although i dont really care, you might want to remove it as it could be interpreted as spam, also, my instructable is awsome, it should have cleared things up 150%! so theres that.
feka1 year ago
The description of relay N/C and N/O workings is wrong.

"If input = on (power going through coil)
Else/otherwise (no power)
COM + N/O"

COM+N/C and COM+N/O should be swapped to tell the story correctly. I am a noob in this, but the swf and your description says this short description is wrong. Correct me if I am mistaken.
The Diagram shows the N/O (normally OPEN) and N/C (normally CLOSED) contact labels swapped. The one which is closed when the power is OFF should be labelled Normally CLOSED and so the other is the Normally OPEN one.
jpoopdog (author)  BasinStreetDesign1 year ago
i have just changed the swf files, all should be good now!
Soon i also plan on adding a demonstration video of a relay in action, my dad was kind enough to let me have the old relays for the fuel pump from his car after he replaced them, although not working in the car, they work fine off a 9v battery XD

the cases of them came off easily enough, and they might be more insightful than pictures might be.
jpoopdog (author)  BasinStreetDesign1 year ago
HAHAHAA, i hadnt realized this. ill change it right away.

When writing the instructable i kept thinking if the states of the relay being as they are when it is ON, not OFF
bwrussell1 year ago
"(As a general rule , avoid using more than 100mA from a 555 when powering any kind of coil including a relay)"

Can't you just place a diode between the 555 output and the coil (relay, motor, etc) to block the reverse current produced when an inductive load is shut off?
jpoopdog (author)  bwrussell1 year ago
reverse current isnt the problem, its forward current.
if you stall a small motor rated at 100ma for instance, it has the potential to draw a few amps, and similarly, with relay coils, you can get massive current spikes which can damage the 555.

even so, just because a 555 CAN source 200ma, doesnt mean its ok to run it non stop at 200ma, a 555 is a signal chip, and should only be used for just that, signals.

an electronics expert once told me that, you should never run ANYTHING from a 555, and this is a rule many follow. signal chips like the 555 and others also, are meant to drive transistors which then run things.
the exeption of course would be low current leds.
Do0dle1 year ago
Thank's so much. I have spent months trying to find out what a transistor does and all the books I read just didn't sum it up as well as you did. I totaly understand about the MOSFETs and why they were attatched to heat sinks.
fatboy071 year ago
well explained. :) thank you.
xtgirl1 year ago
I created an account ESPECIALLY so that I can say THANK YOU for this BRILLIANT instructable.

I am girl and always AVOIDED electronics because I always thought I will NEVER understand ANYTHING - but you have managed to explain it -- hope to find LOADS more from you!!

I can't wait to go and find some TVs and stuff to smash up and scavenge bits off!!

PS - you must make an ACE boyfriend, because you sure know how to communicate in a way that women can understand!
Noender2 years ago
Thank you Sir!

Even if I've been lurking around instructables for years, I finally created an account to thank you for posting this truly useful article. I've been trying to understand the proper use of this components for days and its finally clear now, even if I need to get more hands on experience on the transistors.
I can certainly understand the bother of explaining basic concepts, resulting even tedious chore for a knowledgeable individual.. But as the saying goes "Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime."

So then.. Thank you.
There's a lot of informatio0n here, and I'll probably have to read it through several times. Thank you for putting it here all in the one place.

Now maybe you could help clarify a point or two from a practical level. I get that a Transistor can work as a switch, but I'm not clear how I'd specify the particular device for a particular job, is there a code like on resistors or something?

Say I want to use a computer or microprocessor to control something in my house. I live in the UK so Mains Voltage is 240v at 50Hz and most things require a 13A safety fuse. If I can get 3 to 5 volts out of the controlling circuit I'm guessing I'm going to need some kind of MOSFET and a BIG heat-sink. How do I specify the right part either to extract from another device or purchase from somewhere. Isolating the controlling circuitry is outside the remit of your instructable so let's just assume for the sake of argument that I've got a nice fat Opto-Isolator in there to protect things.

I can find a few places that give me part numbers for a similar device to work on AMERICAN power but no real real advice on how to make the conversion to UK. What numbers or codes or whatever should I be looking for?
jpoopdog (author)  Dream Dragon2 years ago

For one you cant use a single MOSFET to drive AC as it contains a diode which will rectify the mains. two on the other hand i know will work, just fine but need to be set up in a special way which i cannot help with

There is no secret code or anything. Do what i do when looking at parts inside stuff, and just look up the code on the part, and find its datasheet.

I am asuming you are searching for a MOSFET pairr though which can handle AC at 240v correct? Just look up on google, 300v mosfet with the current you need it to switch, simple as that.

On all parts just look up the data sheet via the part number (first line of numbers and letters), it will state its maximum voltage and current, generally you want to keep under that. But really there is no way to tell how just by looking at it, not that i know of.

since your switching AC mains, i would recomend you buy a relay driver circuit, which takes as little as 5uA-5mA to switch (typical driver). For mains generally everyone goes with relays, transistors are easily destroyed by static, which is caused by the inductance of your homes electrical wiring.

Looking at costs here, a small relay will cost allot less that the huge MOSFETS you are going to need for this, plus for the MOSFETS your going to need big heatsinks, and probably fans.


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