100mV Continuity Tester
A new take on the classic continuity tester in several ways. This tests with just 100mV across the probes, so it ignores semiconductor junctions and its low input impedance makes it immune to even very large capacitances. It displays continuity to 3 thresholds and generates 3 different tones. All transistor based (instead of 5 op-amps!). Easy to build all-in-one design.
· Test continuity in or out of circuit instantly.
Introduction
This design started as a “self challenge”. I wanted to design a good quality continuity tester that would not be fooled by semiconductors and large capacitances. I also wanted to make it with no ICs. Crazy? Yes, of course. The design would be much simpler with a couple of cheap quad Op-Amps, or even a little micro, but where’s the fun in that?
The design comprises of the following stages:
Input Front End - To create 100mV across the probes.
Linear Amplifier - To amplify the voltage across the probes.
Comparators - To switch on LEDs when the amplified voltage reaches certain thresholds.
Oscillator - To produce an audible tone which varies according to the number of LEDs lit.
Comparator - For low battery detection.
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