This low power circuit was obtained from an American 80's documentation. The circuit indicates the voltage of a battery being trimmed by the trigger point. The transistors can be the BC549.
These days the unijunction transistor (UJT) is not common, but IF you have one you can mount this...
The coil L2 has 64 turns of wire 28 or thinner wire in a 1 cm form coreless and L1 10 turns on L2....
We show only one of the channels of an excellent audio preamplifier with the LM382 integrated...