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I don't understand how this circuit works. Its a clap switch circuit with transistors. I would appreciate if someone could explain how the current flows through the circuit and which transistor turns on or amplifies the current to turn on the LED at the end when the piezo detects sound. What i mean by this is the path of the current when the piezo detects sound.

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    \$\begingroup\$ Use LTSpice or something similar and simulate it. \$\endgroup\$ – GER_Moki Nov 9 '14 at 18:56
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    \$\begingroup\$ Put some reference designators in your schematic so that we can talk about it. \$\endgroup\$ – Dave Tweed Nov 10 '14 at 0:56
  • \$\begingroup\$ Not enough of a guru. But this is an analog circuit. The piezo resonates in response to sound impulse, and the AC current goes through the base of the first transistor (BC547). Then the collector current (amplified by beta of transistor) goes through the base of the second transistor (BC557). Then there is ANOTHER beta and the collector current mostly goes trough the base of the last transistor (BC547). One more beta. So that is three betas. And the last transistor makes the white LED turn on. I imagine with all those betas, it is going rail-to-rail at the end, or would be without feedback. \$\endgroup\$ – mkeith Nov 15 '14 at 6:12
  • \$\begingroup\$ mkeith: But doesn't the current flow out from the emitter of the first transistor (BC547), so how does the second transistor(BC557) get the current to its base? \$\endgroup\$ – Freakup2 Nov 17 '14 at 1:40
  • \$\begingroup\$ @Freakup2, base current is much smaller than the collector or emitter currents when the transistor is in its linear region. The collector current is therefore almost the same as the emitter current. Since the current going into the base is an AC current, it will create a much larger AC current at the collector. You can Google common emitter amplifier to read up on that basic configuration. This circuit is basically a cascade of 3 common emitter amplifiers, but there is feedback between the stages. \$\endgroup\$ – mkeith Nov 21 '14 at 6:12

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