Keys Finder

Source: www.electronics-lab.com

 

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 Н4, 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, Ф1 cuts off  and the input of Н4 goes to high logical level. It's output goes to <0> and C5 is charged, through R9 until N5 changes operation state.

Click on the image for the full version.

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 Н1 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 = 10м/16V Tantalum

C5 = 1n5

C6 = 100n

C8 = 56n

C9 = 15p

D1...D4 = 1N4148

T1, T2 = BC547B

IC1 = HCF4069UBE

 

PCB