Micro-power continuity checker.

[davekni]

About 15 years ago I built a couple simple continuity checkers, one for home and one for work.  They apply low voltage to avoid forward diode junction conduction and light an LED when resistance is below about 10 ohms.  Power is from a single 18650 LiIon cell.  I find them very convenient - probing an ECB to see which side of a bypass capacitor is ground, checking for hidden solder bridges under parts, etc.  However, these draw enough quiescent current that an on/off switch is necessary.

Last weekend I made two new continuity checkers drawing only 10uA quiescent.  Also reduced threshold resistance to between 1 and 2 ohms.  No on/off switch to forget to switch and battery life of 5-10 years between charges.

Normal people would use a micro-power opamp for such a circuit.  Being abnormal and fond of discrete circuity, mine uses three transistors instead:



Dremel-tool cut circuit board:



Finished continuity checker.  Most of the volume is the 18650 LiIon cell.  Board and probe tip are taped to the cell.





The circuit can easily be made in reverse, using PNP transistors and an NFET.  (Actually, one of the two I made is reversed.)  Power supply is reversed too.  I prefer the version shown here, as one probe is the negative supply which is battery case.  Makes accidental shorts of the battery less likely.

[klugesmith]

Applause!  What's the open circuit voltage and short circuit current at the probe tips?

The first electronic project I remember making, more than 50 years ago, was a continuity checker that beeped.  Circuit, out of a magazine, had one or two transistors and a tiny transformer IIRC. Perhaps a blocking oscillator like that in Joule Thief circuit. I think it predated the invention of NE555.

[davekni]

Applause!  What's the open circuit voltage and short circuit current at the probe tips?

Thank you for the appreciation!  Voltage with a 10meg meter load is 127mV.  Voltage across a 1.5ohm resistor where the LED starts glowing is 25mV.  LED is fully on by 1.0 ohms and 20mV.