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!

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!!!!!!!!!

Step 2: What You'll Need

Being a basic, project, you will only need basic skills, and parts.
In this rendering of the circuit,there are only 3 actual components, or 4 if you decide to run 9 volts instead of 5.
All parts can be had from Radio-shack, or any hobby shop for around 3-5 bucks, assuming you already have a breadboard.

2 NPN transistors (i used MPS2222's , but any NPNs should work well)
Short jumpers, preferably some small solid core wire you could cut to fit
LED capable of handling 5 volts, 5 mm whites work well
560 ohm resistor (only if using 9 volts, value not critical)
5 or 9 volt supply, your choice, i just had a USB supply handy.
A pair of old nail clippers works well for the cutting and stripping of wires as well. 

Step 3: Placeing the Transistors

1st off, you will want to place the 2 transistors as shown in the photo and in the drawing.

Step 4: Adding the V+ and Ground Jumpers

Next would be adding the ground jumper to the left most emiter, and add the V+ jumper to the strip just next to the right most collector.             **************Make sure you do not connect V+ to the collector!!!**************

Step 5: Insert the Collector-collector & Emitter-base Jumper


This step involves jumpering the emitter of the right transistor to the base of the left transistor, and jumpering both collectors together.

Step 6: Installing the LED and Touch Wire

Almost there!!! You've made it this far,bare with me for a few more seconds, and you'll have your very own Darlington pair touch switch!! 

The next step is installing the LED and touch wire. KEEP IN MIND the led is polarized, meaning it will only light when it has -voltage to - and +voltage to +. I will attach a drawing of how to tell the difference,just in case you don't know. 

As you will see,the touch sensitive wire attaches to the base of the right transistor.

Step 7: Juice It Up, and Start Touching!!!

Congratulations! You built it! Throw some power toit,and everything should work. If there are any problems, go back over your circuit, and my schematics,and if all else fails, drop a comment, or PM me. Have Fun!!!!!

Step 8: But Wait!!! the 9 Volt Schems.

The entire Ible' was built with running the circuit on 5 volts in mind. its so simple to convert it to 9 volts,il let the picture and tag explain that! Enjoy!!



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    22 Discussions


    6 years ago on Step 8

    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.

    1 reply

    6 years ago on Introduction

    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.

    1 reply

    Reply 6 years ago on Introduction

    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!


    6 years ago on Introduction

    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?

    3 replies

    Reply 6 years ago on Introduction

    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

    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?

    1 reply

    Reply 6 years ago on Introduction

    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!


    2 years ago

    When I constructed this on the breadboard the LED stayed lit even if I didn't touch the touch cable how can I make the LED turn on only if I touch the cable???


    2 years ago

    Not to nit pick, but shouldn't you have a resistor in series with the LED to limit the current in the output stage?


    5 years ago on Introduction

    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

    1 reply

    Reply 3 years ago

    i do realize that this is 3 years old.

    but, my guess would be that since you are battery powered your finger has no real potensial in respect to the circuit. if connected to to usb charger or similar connectet to the mains, you have a potential in respect to ground. when you toutch the led pin this also applies


    3 years ago

    Hey i am making a project of producing electricity by walking by using piezos

    The voltage i am getting is 20 V

    But current is too low about 12-14 mA

    I want to increase it to 1 A (0.5 A will also do.

    Can i increase current using darlington transistors to this extent.

    I founded that using uln 2003 ic having 7 darlington transistors array)

    Can u pls help me out of this.

    If possible can u tell me how to make the circuit for increasing current.

    PLs reply fast..............


    3 years ago on Introduction

    Hi, thanks for this tutorial. I'm trying to increase the power output coming from an H Bridge going to a DC motor. At present its a tad lacklustre. Do you think this circuit would work in that respect? I have used an Arduino Uno board to send the motor forward and then reverse for fixed periods, but as I say its not sending enough power to the motor. I'm a bit of a novice as I'm sure you can tell!! If you can give me any advice I'd be really appreciative.

    The Expert Noob

    4 years ago

    There is a microchip out there tjat houses 7sets of darlington pairs. Usefull if you need a lot. I use 2N3904 transistors.