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Project Notes

#130 Darlington Touch Switch

Test a Darlington-pair touch switch.

The Build


A Darlington pair refers to a couple of bipolar transistors connected so that the amplified emitter signal of one is used to drive the second. This results a multiplication of the gain, so the circuit is hyper-sensitive to any input signal.

An ideal applications is a touch switch - the classic demonstration case.

The load in this circuit is just an LED with a current-limiting resistor. Two NPN transistors form the Darlington-pair. The circuit works over a wide-range of voltages - basically constrained by the specific transistors used and the size of the current-limiting resistor (to avoid blowing the LED).

The touch switch control comprises two leads:

  • to the base of the Darlington-pair
  • to positive power

A 100kΩ resistor is included to protect against bad things happening if the touch switch is completely shorted.

It doesn’t take much to activate the switch:

  • touch the base lead only will turn on the Darlington-pair to a degree
  • a single finger pressing across the base and power lead will turn the switch hard on
  • touch one lead with one hand, and the other lead with the other hand (using your body as the conductor) will also turn the switch hard on



The Schematic

Credits and References

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This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.