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
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
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.
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