#170 ClapSwitchKit

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

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

Notes

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

Construction

Credits and References

• LEAP: ClapSwitchKit2 - another clap-switch kit project
• Clap Switch DIY Kit - from a seller on aliexpress, USD1.70 at time of purchase
• Wikipedia: Flip-flop
• RSFlipFlop project demonstrating the basic set-reset flip flop circuit implemented with BJTs