This tester is for tracing wiring on Printed Circuit Boards. Resistors below 50 ohms act as a short circuit; above 100 ohms as an open circuit. The circuit is a simple multivibrator switched on by transistor T3. The components in the base of T3 are Dl, Rl, R2, and the test resistance.
With a 1,5 volt supply, there is insufficient voltage to turn on a semiconductor connected to the test terminals.