A very simple test probe can be built using this electronic circuit diagram. Probe test is build with transistors and allows rapid measurement of voltage levels on digital gates, fuses, diodes, batteries etc. Tester does not provide absolute values, but a correct indication of good or poor functioning.
Measurements are made on pins A and B. If the potential difference between A (pin reference) and B is <1.4 V higher than the potential of A, D3 lights. Finally, if the potential of B
is 1.9 + 2.0 V D2 lights. If the potential of B is> 11 V compared to that of A, D1 will light.
The probe allows measurement of alternating voltage.
Maximum input voltage is highly dependent on the dissipation allowed for R1. For example, if the resistor has a nominal power of 0.5 W, input voltage can reach 200 VEF