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A Simple Continuity Tester

1cm69ltd

Hi all,

 

I am an amateur hobbyist currently messing around with Arduino/ESP8266 boards & have made a number of simple data loggers for around my house.

 

I found that my current DMM, a Bryman BM235 has a relatively high continuity cut-off, 30 to 480 Ohms & this has caused me an issue lately as I suspected a short in one of my loggers but this particular logger had a 150 Ohms resistor which my DMM did not even see.

 

Searching the net for a simple alternative I came across a straight forward schematic: Continuity Tester

 

Initially I constructed this circuit on a breadboard & it did not function, no sound when touching the probes together. I tried the various troubleshooting steps laid out on the page with no success. I even swapped transistors, checked all the actual resistor & capacitor values butr it still did not function.

 

I decided that it could possibly be an issue with the breadboard in some way & then constructed the circuit on some protoboard.

 

Still the same result or should I say no result, I get nothing from the tester.

 

Through fault finding I found that shorting between B & E of Q1 gives an audible tone but that's it so I know the buzzer is working.

 

I took a few readings from the circuit:

 

Input Voltage: 3.278v

Across Buzzer: 2.699v

Across 100R: 0.446v

On Probe: 0.758v

Base Q3: ~ 2.7v

Across Diodes: 2.055v

Probe leads resistance: 7 ohms

 

Can anyone see a possible glaring error with this circuit that I am unable to see.

 

Regards..,

 

Kirk

  • What's the DC collector-emittor voltage over Q1 with the probes open and closed?

    Did you use a PNP transistor for Q3?

    Voltage accross the buzzer should be AC.

    Removing Q1 should turn on the buzzer all the time. If it doesn't, the issue is with the Q2/Q3 circuit.

  • Hi Kirk,

     

    Could it be that the buzzer you're using expects DC? The circuit calls for a piezo element which as Jan says, requires a changing voltage.

    Also, could the multimeter be set to 'diode check' instead of continuity by accident? 480 ohms seems very excessive for continuity to be registered.

    You mention probe resistance is 7 ohms, but that seems strange too. Ordinarily it would be less than 1 ohm. You can short your multimeter leads together, and then either choose a null option if it exists on the meter, or mentally subtract that value. If it really is 7 ohms after such nulling/subtraction, then there are poor connections somewhere.

  • in reply to shabaz

      wrote:

     

    ....

    Also, could the multimeter be set to 'diode check' instead of continuity by accident? 480 ohms seems very excessive for continuity to be registered.

    ....

    I can check that here too. I have the same Brymen BM235...

     

    edit:

    Brymen BM235 210Ω (spec : 30 - 480Ω)

    RS Pro IDM 66RT 110Ω (spec: 30 - 200Ω)

  • in reply to Jan Cumps

    I see.. maybe it's possible to just put a resistor in series to check continuity (for example, if 210 ohm is the limit, then a 150 ohm resistor would make the limit 60 ohm).