Build Your Key Finder: A 3-4kHz Circuit

General:

Picture a scenario where you're just waking up, anticipating the bus for work, yet you’ve misplaced your keys. If this kind of situation frequently occurs for you, this circuit represents a perfect solution. All that is required is a small activation. The overall circuit is compact enough to attach to your keys easily.

Description:

The acoustic output of the circuit is a piezoelectric buzzer. Generating a sound wave at a frequency of 3...4 KHz, the piezoelectric buzzer converts it into an oscillation. This oscillation passes through C1 and is processed by inverters N1 through N3. The inverters, alongside R1 and R2, constitute an amplifier with a gain of 400 !. C8 and C9 serve to stabilize and reduce the signal's magnitude. Subsequently, the signal is rectified by diodes D2 and D3. The integrated circuit n4, alongside C3 and R5, ensures that the circuit is not activated by extraneous noises but solely by the intended trigger. When the output voltage increases at N4, T1 shuts off, and the input of n4 moves to a high logical level. Its output proceeds to <0> and C5 is charged, via R9, until N5 changes its operational state.

Circuit diagram

Next, C5 discharges, and the process begins anew. The alternating current flows through the buffering circuit of T2, which initiates the buzzing sound. A potential issue exists here – the buzzer will continue to buzz indefinitely because the output signal triggering the circuit again passes through C1. This is addressed by implementing the following: when the buzzer is producing sound (until C3 discharges through R5), the input of n1 is maintained at a high logical state via D4, C7, and D3. This prevents the amplifier from receiving and amplifying any external signals.

Parts

PCB