This audio quartz crystal tester circuit is designed to test quartz crystals using audio signal. Using this This is possible if the output frequency band is brought to the audio band audio band using demultiplication with a circuit like the diagram below.
This continuity tester is built using an operational amplifier 741, used as a comparator which command of it output a buzzer. Inverting input (-) is at a potential equal to half the supply voltage, formed by R3 and R4. When potential of noninverting input (+) is lower than the negative input, which is the case if the resistance between two terminals is small enough, buzzer is activated.
With specified values for R1, R2 and P1, sensitivity of tester is about 1 k.
Its sensitivity can be increased to 100 ohms decreasing ten times the values of R1, R2 and P1.
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