Posted on Dec 14, 2012

This circuit is designed around a 4049 hex inverter/buffer. Two inverters are used ina dual-frequency signal-injector circuit, another inverter is used as a logic probe, and the remaining three inverters are used as a sensitive dual-input, audio-signal tracer. The signal-injector portion gates are configured as a two-frequency, pulse-generator circuit. Under normal conditions, the generator`s output frequency is around 10kHz, but when 52 is closed, the output frequency drops to about 100 Hz.

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The logic-probe portion is made up of U1c, the output of the inverter decreases. The low output of U1c reverse biases LED2, so it remains off. That low output also forward biases LED1, causing it to light. But when a logic low presented Ulc"s input, the situation is reversed, so LED2 lights and LEDl darkens. The audio-signal tracer portion is made up of the three remaining inverters which are configured as a linear audio amplifier to increase the input signallevelby a factor of 10 or 100. The amplified output signal feeds a miniature piezo element of audible detection.

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