This Cable Tester circuit is simple and require few common components and can be used to verify in one time more conductors from cables . This device apply a different voltage to each conductor from the cable at one end . For indications this tester use light emitting diodes (led) which are powered from a constant current source given by a Lm317 voltage regulator .
The constant current is given by a simple formula I= 1,25/R2.
The reference voltage is given by 9 leds connected in series . The first and the last conductors are connected directly to the + and – from power supply.
The “receiver” it’s formatted from 5 leds.
If it’s connected a cable that it’s working the led from receiver and the other from transmitter will light .
The tester indicate this situations :
If two leds opposite other fail to light means that the conductors are crossed or shorted .If the led from transmitter lights means that one or both conductors from cable are broken.
If one of the leds (d3,d5,d7,d9) fails to light means that there it’s a short between the outer conductors from the neighbouring pairs .
If several neighbor leds fail to light means that the conductor from the firs unlit led it’s crossed with the last unlit led .If all leds from emitter and receiver lights means that is still a chance that two pairs are interchanged and we must use the switches to test. For nine leds we must use a power supply across 20 volts . If we want to use more leds we must use a corresponding power supply.