General:
Imagine that is morning , you barely anticipate the bus for work and you can't find your keys. If you suffer from this situation frequently, this circuit is ideal for you. A fizzle is the only thing you have to do. The overall circuit is small enough to attach it to your keys.

Description:
The ear and mouth of the circuit is a piezoelectric buzzer. Fizzling in a frequency of 3...4 KHz produced sound waves that piezoelectric buzzer converts them to an oscillation. This oscillation passes thought C1 and thought inverters N1...N3.The inverters along with R1 and R2, compose an amplifier with gain 400 ! .The C8 and C9 stabilize and reduce the signal's level.
Next the signal rectified from D2 and D3. The integral circuit n4, C3 and R5 ensures that the circuit will not triggered from common noises but only from late fizzle. When the output voltage increases at N4, T1 cuts off and the input of n4 goes to high logical level. It's output goes to <0> and C5 is charged, through R9 until N5 changes operation state.

Circuit diagram

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Next C5 discharges and everything is starting from beginning .The AC voltage goes thought the buffer circuit of T2 to buzzer witch starts to fizzle. There is a small problem in this point, the buzzer will never stop fizzling because the output signal triggering the circuit again thought C1. This is solved in this way: when the buzzer sounds (until C3 discharges thought R5), the input of n1 goes through D4, C7 and D3, to high logical state. In this way until the buzzer stops the input of the amplifier can't receive and amplify any given signal.

Parts
R1 =10k
R2, R5, R8 =10M
R3, R4, R6, R7, R9 = 100k
R10 = 4k7
C1 = 22n
C2, C7 = 47n
C3 = 560n
C4 = 10u;/16V Tantalum
C5 = 1n5
C6 = 100n
C8 = 56n
C9 = 15p
D1...D4 = 1N4148
T1, T2 = BC547B
IC1 = HCF4069UBE

PCB