Low battery indicator 555 (CB9551E)

This circuit can operate with different voltages, depending on the zener used. We found it in a 90s American documentation.

Field Strength Meter (CB13267E)
We found this circuit in an old American publication. The transistors can be BC558 and diode 1N34....
Continuity Tester (CB15938E)
This simple circuit operates with only 1.5 volts of a battery and detects low resistance making a...
Divider by 4 with 4013 (CB16056E)
The cascade connection of two flip-flops allows division by 4, as shown in the figure. Note that...