This tiny conductivity tester works with LM 3909. The tester makes a beeping sound if the resistance between the test probes between 0 and 100 O lies. Due to the volume of the beep, the resistance between the test probes can be determined. Audio Lm 3909 IC-conduction tester Circuit Diagram Parts List R1 = 1 k C1 = 10 uF C2 = 100 nF LS = Loudspeaker 12 to 16 Ω IC1 = LM 3909