by pebe » Sat May 14, 2011 10:51 am
Here is the circuit as promised. It tests the cable connections sequentially and continuously by energising each one in turn from the outputs of a decade counter.
The 555 timer IC1 clocks the two 4017 decade counters, IC2 and IC3, at about 250Hz. Only one Q output of each counter can be high at any time, so they step a high from Q0 up to Q3 while the remaining outputs stay low. Each time IC3 steps on to Q4, the high output there resets both counters so they count together through Q0, Q1, Q2, Q3, and repeat Q0…etc. The reset time is increased with R5 and C1 to ensure IC2 resets. R1, R2, and R3 are the same values at each Q position. The components shown inside the red outline connected to D need to be duplicated for the other 3 lines, A, B, and C.
To understand the circuit operation, look at the components between the Q3 pin of IC1 and Q3 of IC2. When any of the other outputs is high, then the two Q3s will be low and there will be no drive to TR and the LED will be off.
When the 555 clocks both Q3s high, the test circuit for that line will be enabled. The diode at D will be reverse biased because its cathode is high, but the voltage on D will stay low if no cable connection is present because R3 will pull it down to 0V, so TR will stay off. But if the cable is correctly wired, point D will be on potential divider comprising R1, R2 and R3, and will be at about 10V. That will be enough to turn on TR and the LED. Only a correctly connected cable wire will light the LED.
You can leave the circuit running as you work on the cable connections, because the resistors R1 and R2 protect the ICs from static. An added bonus is that a short circuit between adjacent pins will cause the LEDs for those wires to decrease in brightness.
Best of luck and I hope you find it useful.
(20/05/11 Edited penultimate para re. LED brightness )
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Last edited by
pebe on Thu May 19, 2011 8:06 am, edited 1 time in total.