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Project Notes

#135 555 Timer Touch Switch

Test a touch switch circuit based on a 555 timer


This is basically a 555 timer monostable, but triggered by touch pad on pin 2 (trigger). I’m using a 100kΩ/10µF configuration which gives an output pulse of 1100ms when triggering occurs. The R/C values can be adjusted to change the pulse duration.

The circuit takes advantage of the fact that the trigger input is quite sensitive since it is essentially the input to a PNP darlington-pair. Trigger occurs upon application of a negative trigger pulse of less than 1/3 VCC, which sets the internal flip-flip. The flip-flip is reset at the end of its cycle, ready to receive another trigger input.

In practice, a finger touch to the flying lead attached to pin 2 may be enough to trigger the 555. If the body is holding too much charge, it may be necessary to press across the two flying leads (pin 2, ground) in order to pull down pin 2 sufficiently.

Other circuits include a pull-up resistor of 10MΩ or so on pin 2. But doesn’t work for me at all.

To improve the sensitivity, pin 2 could be pulled down via a transistor, with the touch applied to the BJT base as described here.



The Schematic

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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.