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Project Notes

#170 ClapSwitchKit

Build and analyse a simple clap switch circuit available as a kit

Here’s a quick video of the kit in action:

ClapSwitchKit Demo


Here’s a simple circuit that’s available as a cheap kit - “shopping-cart stuffing”. There are no unusual components so aside from the PCB, everything you need is probably already in your parts bins.


The core of the circuit is a set-reset flip flop implemented with BJTs - similar to the RSFlipFlop project.

A momentary pulse from the electret is amplified with Q1/Q2 and triggers the Q3/Q3 flip:

  • the cap C5/C6 on the side of the flip-flop that is “off” will charge momentarily
  • when Q2 shuts-off, the charged cap voltage offset will reverse-bias the corresponding D1/D2
  • causing the corresponding BJT to be enabled and trigger the flip/flop

The output LEDs are driven with a push/pull configuration of Q6 and Q7. Q6 and Q7 are driven with complementary base “inputs” (Q5 prividing the inverted signal for Q7).

So when the right-hand output of the flip-flip (tapped at the Q4 collector) is “low”:

  • Q6 will be on (D3 conducting via Q4)
  • Q7 is off (Q5 base pulled low via D4/R12/Q4)
  • therefore LED1 (green) off, and LED2 (red) on

And when the right-hand output of the flip-flip is “high”:

  • Q6 will be off (D3 reverse biased)
  • Q7 on (D4 reverse biased, so Q7 based pulled high by R13)
  • therefore LED1 (green) on, and LED2 (red) off



The Schematic

The Build

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.