The compact test probe is made from five transistors, three LEDs, a zener diode, and three resistors. It enables rapid ‘measurement’ of voltage levels at digital gates, fuses, diodes, batteries, and others. Of course, it does not provide absolute values, but rather a good indication of correct operation or otherwise.
Measurements are carried out with pins A and B. If the potential difference between A (the reference pin) and B is 1.9-2.0 V, D2 will light. If the voltage at B is <1.4 V higher than that at A, D3 will light. Finally, if the potential at B is ≥ 11 V with respect to that at A, D1 will light.
Transistor T5 is used as a zener diode, which keeps the total current drawn down (since transistors ‘break down’ in a stable manner at a lower current than zener diodes).
The probe allows measurement of alternating voltage. The maximum input voltage is highly dependent on the dissipation allowed in R1. For example, when this resistor is a 0.5 W type, the input voltage may be as high as 200V r.m.s. The current drawn by the circuit depends on the number of lighting LEDs: it is not more than 10 mA at a supply voltage 3 V. In quiescent operation, the current is so low (about 5 µA) that an on /off switch is not necessary.