Simple continuity checker

  
The pitch of the tone is dependent upon the resistance under test. The tester will respond to resistance of hundreds of kilohms, yet it is possible to distinguish differences of just a few tens of ohms in low-resistance circuits. Ql and Q2 form a multivibrator, the frequency of which is influenced by the resistance between the test points.
Simple continuity checker - schematic

The output stage Q3 and Q4 will drive a small loudspeaker or a telephone earpiece.




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