This is Coaxial cable tester circuit.It is useful to check the cables inside and outside. Check the continuity of the cable, check cable short-circuited. And detection of the high resistance of the cable. Check that all lamp displays with LED.
Operation of the circuit is when I press the test button to S1 current flows through R6 and R7. D1 is a conductor outside of the lines R4 and LED3. If conducted outside of the coaxial cable to the LED3 will light up at the same time flows. Electricity is flowing through R6, R7, R2, and LED1. conductors inside the cable D2. The LED1 will light showing the continuity of the cable inner conductor (LED1 and LED3 is green).
If the lines outside and inside the same circuit. Electricity is flowing through R6, R7 and D1 from the outside to the inside of the cable short-circuited, and D2. The LED1 and LED3 off both, which flows increased more than normal from 10 mA to about 80 mA, voltage drop across R6 will increase, until the Q3 work, so a current flows through LED4 make it light.