Darlington Pair Touch Circuit

Updated 02 August 2018

Ever seen a touch lamp in action and wondered how on earth we can make that using basic components? This project is a basic circuit with 2 transistors to amplify a signal from your touch.

Extra: The LED doesn't actually blink like that, just a camera glitch on my phone. This is a great project for grasping the basics of transistors.

Parts you'll need:

Circuit Diagram:

darlington-pair-touch-circuit-schematic

Instructions:

1. Attach the red wire of your power source to the left Ground (negative sign) column of your breadboard.

darlington-pair-touch-circuit-step-1-image

2. Attach the black wire (GND) of your power source to the plus column of your breadboard.

darlington-pair-touch-circuit-step-2-image

3. Take a jumper wire and connect it from the red wire's column to A1 on your breadboard.

darlington-pair-touch-circuit-step-3-image

4. Take 1 of your transistors and place its collector on E1, base on E2 and emitter on E3. (The left pin on the curvy side is the collector)

darlington-pair-touch-circuit-step-4-image

5. Attach a jumper wire to A2 (leave the other side hanging, this is the part you touch).

darlington-pair-touch-circuit-step-5-image

6. Place the other transistor on the breadboard and put its collector on E4, make it face the same side as the other transistor.

darlington-pair-touch-circuit-step-6-image

7. Take another jumper wire and connect A3 to A5.

darlington-pair-touch-circuit-step-7-image

8. Use another jumper wire to connect the red wire's column to A4.

darlington-pair-touch-circuit-step-8-image

9. Take your blue LED and attach its long leg to A6 and its short leg to A7.

darlington-pair-touch-circuit-step-9-image

10. Use a jumper wire to connect E7 to the black wire's column.

darlington-pair-touch-circuit-step-10-image

11. Touch the jumper wire hanging out (the pin bit). If it lights up, you made it right.

darlington-pair-touch-circuit-step-11-image

How it works:

A transistor has 3 pins, one is the collector, one is the base and one is the emitter. The base is like a switch activated by either a negative or positive charge depending on what type of transistor you are using. Once the base is activated, it allows current to flow from the collector to the emitter, amplifying the current. With this concept, since our fingers store charge, we can use that charge to activate our transistor with its collector connected to a power source. Since the emitted charge is still extremely low, we can use this new charge to switch on another transistor with its collector connected to the same power source as previously. The new emitted current is large enough to make a LED glow bright before it returns to the ground.

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