I'd like as an end state of my project for sound to activate a relay and then either

  • a) the relay to stay on for a limited amount of time (~15 seconds) or
  • b) ideally - the relay to be triggered off with another sound

I understand that either of these can be achieved with a 555 timer chip, I am loathe to use option a) because I hear that they can be fairly inaccurate when it comes to lower time limits (e.g. 15 seconds)

Here is what I have currently

Microphone can be found here https://www.sparkfun.com/products/9964

The LED flickers pleasantly in time to music. This seems problematic for a bistable switch - because I can imagine that the sound activation (ideally a shout, or a knock) will have to many pauses and lilts for it to trigger the bistable switch only once - I think this is a phenomena called bounce.


Is this circuit dangerous? How could it be improved? How do I fulfil option a) or b)? Why if I put a capcitator (I've tried a range of capacitances and ceramic and electrolytic types) before the LED and after the resistor the LED will no longer turn on.

  • \$\begingroup\$ Why three transistors? \$\endgroup\$ – Phil Frost Jan 18 '13 at 19:31
  • \$\begingroup\$ You need "The Clapper"! \$\endgroup\$ – Dave Tweed Jan 18 '13 at 21:57
  • 1
    \$\begingroup\$ Assuming your circuit works at all, those can't be PNP transistors. If they're NPN, then you just have a "triple Darlington" that's boosting the current available from the microphone. If you're putting a capacitor in series with the LED, that's blocking the DC current it needs to operate. \$\endgroup\$ – Dave Tweed Jan 18 '13 at 22:01
  • \$\begingroup\$ 3 transistors - I assume that each one amplifies the signal from the microphone enough to power the LED. One on its own didn't work They're P2N2222A - which I thought were PNP, the internet has proved me wrong. Thanks! @DaveTweed - I'd rather build the clapper myself! \$\endgroup\$ – Joseph Jan 18 '13 at 23:13

you can design the circuit using IC555 and CD4033(D-Flip Flop).By using an SPDT switch you can select option a or b.

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  • \$\begingroup\$ may i know why you are using a capacitor in series with LED \$\endgroup\$ – yogece May 19 '13 at 4:45
  • \$\begingroup\$ That was a sort of general question. I understand why it doesn't work now. The capcitator blocks DC current. I ended up building it with a 555 timer only. I used the same idea as this. But with a transistor to let current through rather than a pushed switch. With the base of the transistor attached to the microphne \$\endgroup\$ – Joseph May 20 '13 at 23:01
  • \$\begingroup\$ can you post your schematic \$\endgroup\$ – yogece May 21 '13 at 16:50

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