We have always used the continuity function of a multimeter to find out continuity in PCB,wires,circuit traces,fault detection,etc. When continuity is found the Buzzer inside the meter rings and when there is no continuity it does not ring.
We will here make the same thing by using just a Basic Comparator and some resistors..
This will enable us to make such circuit ourselves and will also give us a practical example of working of OPAMP as Comparator,,
Step 1: Parts Required
1. LM358 IC
2. 3 1K resistor
3. 10K pot
7. Male headers
8. Perfboard and Soldering kits
Step 2: Theory
We just use an OPAMP as comparator to build up this..
See that the 10K pot is used to set a reference voltage of 3V and is fed into Non Inverting pin of OPAMP...we denote this 3V as V+...
the resistor divider with 3 1K resistor is used. Let voltage at inverting input be V_ . Now when there is continuity the voltage at inverting pin of OPAMP will go to 2.5V ( because the middle 1K is now shorted and the voltage at the middle of divider is now 5*1/(1+1) = 2.5V) ..... When there is no continuity, it will be 3.33V ( 5*(2/3)=3.33)....
So when there is continuity V_<V+ and hence output is HIGH and LED or Buzzer will ring....and when there is no continuity , V_>V+ and hence output is LOW and LED or Buzzer does not ring..
LM358 is used here and resistor divider with 1K value is used, in theory any 3 resistor of same value would yield the same result..
Step 3: Testing the Circuit on Breadboard
gather your components and build up in breadboard...circuit diagram is in the previous step...this is for testing purposes...take out two wires as continuity probes
after building up give 5V supply and short the wires..the LED should glow now, otherwise it should be Off.
You can now make it up in Veroboard
Step 4: Soldering
Now to use it, you must solder it on a piece of Perfboard..see my structure of the circuit in the Pics...keep the circuit diagram beside you while soldering...attach a battery terminal for it
Now test it again...if not working as before....try to see carefully the circuit traces under light and magnifying glass...
Now you are all set and ready to go with your continuity tester..
(Note that I've not used my Buzzer in the PCB since I don't have an extra one...You can use it too in parallel with LED)