Audible Logic Tester, Continuity Tester

Parts List:
          R1,R2 = 100K                     Q1 = 2N3904, NPN-transistor (NTE123AP)
R3,R4,R5,R9,R10 = 47K                     IC1 = LM339, Quad op-amp
             R6 = 10K                     IC2 = CD4011, (or 14011) Quad 2-input Nand
             R7 = 20K                   D1,D2 = 1N914, signal diode 
             R8 = 1K                      LS1 = Loudspeaker, 8-ohm
          C1,C2 = 0.1uF, ceramic

A logic tester with an audible indicator can be handy when troubleshooting or testing a crowded circuit board. It allows you to keep your eyes on the circuit, rather than on a LED on the tester. The project described in this article is just such a tester. It provides an audible indication of the logic level of the signal presented to its input.

Power for the circuit can be a 9V Alkaline battery, or any other power source from 4.5 to 15V.

D1/D2 can be also a regular 1N4148 diode but I have not tried that.

A logic 'high' is indicated by a high tone, a logic 'low' by a low tone, and oscillation is indicated by an alternating tone.

The input is high impedance, so it will not load down the circuit under test. It can be used to troubleshoot both TTL or CMOS logic.

The choice for Q1 is not critical. It is a NPN, general purpose BJT type.
Almost anything will do: 2N4401, 2N2222, BC548, TUN, ECG123AP, etc. Origianally the circuit called for 50K resistors for R3, R4, and R5, but 47K or 51K will do fine. It is just a voltage divider and either value will work.
For R7 use two 10K resistors in series if you can't find the 20K value.

Back to Circuits page