Jerry O'Keefe
The circuit tracer in Figure 1 is a handy tool for finding connectivity paths on a pc board. Because the sense voltage you use to measure the path is lower than a transistor's VBE voltage, you can use the design in circuits containing semiconductor elements without affecting the measurement. The tracer's output takes the form of audio tones. An open circuit produces ticks at the rate of approximately one per second, and a short circuit results in a 2-kHz tone. An audible sensing device is ideal for a circuit tracer, because your eyes can focus on the circuit paths you're tracing and not on a meter movement. If you want to find the connections to a circuit point, a useful technique is to attach a lead to that point and just scan the other lead over the other sections of the circuit. When you hear a high-pitched tone, then you know that you have a pc-board connection.
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| Figure 1. |
With practice, you can quickly determine the quality of the circuit path by discerning the wide dynamic range of ticks to tones. You can also detect the presence of capacitors, which produce a sweeping tone as they charge. The circuit in Figure 1 is sensitive enough to produce a noticeable audio change if you make contact with a circuit with wet fingertips. R1 produces a 0.4-mA current to bias the current mirror comprising Q1 and Q2. Q1, the resistance-sense transistor, is the heart of the circuit. The resistance between its emitter and VCC determines C2's charge current. Because C2 receives current from a constant-current source, the waveform on the capacitor is a linear ramp. When C2 charges and passes IC1's threshold, IC1 generates an output pulse. R2 determines the discharge rate for C2. IC2, a 74C74, converts the NE555 pulses to symmetrical square waves to differentially drive the piezoelectric speaker. With normal, day-to-day use, the 9V battery should last approximately a year.
