The continuity detector in Figure 1 is based on W Dijkstra’s “Fleapower circuit detects short circuits” (Reference 1). The buzzer indicator allows you to devote full attention to making the connection without having to observe an LED. The circuit also consumes less power than Dijkstra’s circuit. Power comes from two AA or AAA cells, which last for a period equal to their shelf life. Current consumption is less than 2.5 mA when the circuit detects continuity and less than 1.7 mA for an open circuit. Open-circuit voltage is less than 100 mV, and short-circuit current is less than 1 mA. You can use a number of op amps for IC1, provided that the specs indicate rail-to-rail operation with a low-voltage single supply.
 |
|
| Figure 1. | This audible-signal continuity tester consumes little power and allows you to detect opens and shorts without observing an LED. |
The piezo oscillator driver uses only 700 µA when operating and consumes only Q1’s leakage current when it is not operating. This type of piezo transducer is a passive, resonant-feedback type, which provides high power efficiency and low-voltage operation. With RX values greater than approximately 12 Ω, the inverting input of the op amp is at a higher potential than that of the noninverting input. The resulting output is 0 V plus the saturation voltage of the output stage. This output provides no bias current through R5 and thus keeps Q1 cut off. With RX values lower than approximately 12 Ω, the inverting input of the op amp has a potential lower than that of the noninverting input. The resulting output is 3 V minus the saturation voltage of the output stage. The approximately 3 V output biases Q1 into the linear region. Q1 and the piezo transducer, with their associated feedback, oscillate at their resonant frequency. Most transducers and the listed op amps operate with supply voltages as low as 2 V.
