I const = 1.25 V / R1
to produce a current of 5 mA. This part of the circuit forms the transmitter end of the cable tester. The conductors of the cable under test can be connected to the transmitter in any order. The receiver consists of five LEDs whose connections are taken directly from terminal block X3. If the corresponding points in the two parts of the circuit are wired to one another using a working cable, all the LEDs on both receiver and transmitter sides will light. If there is a fault in the cable, the following situations are possible.
Two LEDs opposite one another fail to light: two conductors are crossed or shorted.
Only the LED on the transmitter side lights: one or both of the conductors in the pair is broken.
One of the even-numbered LEDs on the transmitter side (D2, D4, D6 or D8) fails to light: there is a short between the outer conductors of the neighbouring pairs.
Several neighbouring LEDs fail to light: the conductor corresponding to the first unlit LED is crossed with the one corresponding to the last unlit LED, or they are shorted.
If all LEDs light on both sides, there is still a chance that two pairs might be interchanged. Buttons S1 to S5 can be used to test this: the same LED should extinguish on each side when the button is pressed. If the wrong LED goes out on the receiver side, a pair must be swapped over. More complicated effects can result from combinations of these five faults.
Different colours of LED have different forward voltage drops, and so the same type of LED should be used throughout. The required current can be put into the formula to calculate R1, which can then be altered if necessary. Of course, these remarks do not apply to the power indicator LED (D15). The LM317 used for the constant current source can only deliver the calculated current if its input voltage is at least about 3 V higher than the voltage required at its output. The load voltage depends on the number of LEDs in the transmitter and on their forward voltage drop. For nine red LEDs at least 20 V is required.