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| Continuity tester circuit |
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| OM222O:
I want to build a continuity tester that is not fooled by ICs, so it should preferably use less than 0.3V on the probes. I tried taking apart my DMM to see if I can reverse engineer how it's doing it, but no luck :P I think I can make it work by using a Wheatstone bridge and comparator, to make a "resistance comparator" but I'm not sure how good this circuit is or if there are better ones: here is my circuit By removing the 1\$\Omega\$ short between P+ and P- you can see the led turns off and the output voltage of the comparator goes to 0. this can be used to both visually detect the short and use the output of the comparator as a logic output to be used in some kind of automated test equipment. I tried looking online for some circuits but they just make something using a few transistors with no theory of operation / analysis of in what situations they might give a false reading. I think I can use a pot instead of the 500\$\Omega\$ resistor to have adjustable resistance threshold which would be nice. If there are better/simpler ways of doing this, please share the schematics, otherwise what do you think of my approach? |
| 2N3055:
http://elm-chan.org/works/cch/report_e.html |
| OM222O:
500mV seems a bit much :P the circuit diagram is both more complicated and less flexible in terms of having thresholds ??? using a micro controller for this task seems a bit of an overkill imho |
| IconicPCB:
would a toneohm like solution do You? Output variable pitch tone ( frequency goes up as You approach a short circuit) good for mm fault discrimination Probe voltage 100mV does not like capacitors since measurement signal is not DC |
| OM222O:
that would also be acceptable since I can use a frequency counter using an MCU if it provides better accuracy than my method. unfortunately the capacitor issue is less than desirable since in circuit operation would be nice. what is the frequency range? if it's not too high, I think it can work. P.S: 100mV probe voltage is perfect! |
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