Instructables
Picture of Breadboarding: The Darlington Pair
Recently, iv found myself surfing Ibles, and noticed alot of simple circuits, but few show, or even better, explain the fundamental circuit of  the Darlington Pair. I plan, by the time that you have flipped through this article, to have shed a great deal of light on this great circuit for the beginner and the veteran builder alike. Enjoy, and if i get anything wrong, or neglect to clarify a process  feel free to comment or to private message me!
 
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Step 1: A little insight....

As we know, transistors are in just about everything. With that being said, its obvious that they have a HUGE range of functions, from amplifiers and switches, to voltage regulators, and mass storage devices, like memory cards and RAM.

The Darlington transistor, or more commonly, the Darlington pair, consists of two separate bipolar transistors sharing a common collector, and when combined, offer an exponentially higher gain than both transistors separate. 

The idea was created by engineer Sidney Darlington of Bell Labs in 1953. He also patented the idea of having 2 or 3 transistors on a single chip,sharing a collector pin.

In  a way, the pair could be seen as operating in 2 ways. The 1st, and most commonly accepted, is where both transistors act like amplifiers, the 2nd stage further strengthening the output of the 1st stage. I have a slightly different view of it, where the 1st stage acts as an amplifier, beefing up the input, so that it saturates the base of the 2nd stage, turning it on, like a switch.

With all that being said.... LETS BUILD SOMETHING!!!!!!!!!



I was wondering how to increase the limit of the current running through transistors and luckily found this well-written 'ible of yours.Thank you very much for taking such an initiative.
KJ4ZVQ (author)  Adarsh_tronix1 year ago
Your welcome. Glad it came in handy for you!
asesingle1 year ago
Can you explain this to me. How and Why the touch-switch here works? what kind of touch switch; capacitance or resistance? I have a defense on Monday please answer. This circuit troubles me.
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KJ4ZVQ (author)  asesingle1 year ago
I see that the "switch" in this diagram is just like the one above.Transistors TR1 & TR2 form the Darlington pair. It works by taking the very small electrical potential your body has, and amplifies it to eventually saturate or "Turn on" the switch. I believe, don't take it for fact though, that this is a resistance switch. Your body conducts more energy between the plate and ground than just the air around it when its not being touched, so the change from the relatively high resistance from the air, to the relatively low of your finger is what turns the circuit on. If this isn't what your looking for, I could go more in depth into this circuit, but I believe this is what you needed. Good luck!
vricsi941 year ago
Nice instructable :D However, I can't understand something: what could cause the LED to sometimes stay turned on? I want to use this circuit with an optocoupler as a turn on "button" for my computer, but if it gives a too long signal, the computer will turn offxD
Do you have any suggestions?
btw I'm using BC 517 transistors
KJ4ZVQ (author)  vricsi941 year ago
The only reason I could see that the LED would remain lit is either stray capacitance in the circuit, or strong electrical/magnetic interference. If you have it built on a breadboard, both of those causes are very likely, and building a final circuit on perf board would probably eliminate this. All transistors have different values of gain, and different amounts of sensitivity. I have never used the BC 517's in a circuit, but those are the only reason I could think of. Best of luck :D
Yeah I built it on a breadboard, will try your solution, thank you :D
KJ4ZVQ (author) 1 year ago
Questions? Comments? Suggestions?
nice tutorial! I actually have a darlington array instructable in the works, but I hadn't gone to this level of detail. great info here! what do you normally use these for?
KJ4ZVQ (author)  amandaghassaei1 year ago
Why,thank you:) Just the other day I made a liquid level sensor out of 5 pairs. (Instructable coming soon) Other than that, im still playing with options and lay outs!

i made it with 3 transistor.. "super gain "

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KJ4ZVQ (author)  mohammadsalem944 months ago

nice work!

Cheerfultrout7 months ago

The potential for this bit of circuitry are overwhelming, I'll definitely make this next time I'm breadboarding. Thank you for making this great instructable!

KJ4ZVQ (author) 7 months ago
MPW ; I cant really tell by the pictures but it sounds like either there is a break in the circuit between the two transistors or a path of high resistance. either that or your using low gain transistors. sorry for the late reply , and hope it helps!!
MPW1 year ago
can you please help me, I've tried over and over to replicate what you have done but no matter what I try it will not work. if however , I touch the legs of the LED whilst holding the wire you are supposed to touch then the LED will light up, I have tried different transistors and double checked the ECB but it just will not work. The last one I tried were the "can" ones but I get the same results no matter what I try. They are all NPN types. If I touch the top of the right hand metal transistor the LED will also light up. I have checked them with the multimeter to make sure they are OK and they all test OK I am baffled please help
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