This car battery monitor was featured in the January 1988 issue of Radio Electronics magazine. The circuit uses common transistors and has 3 levels of indication made with common LEDs.
This Japanese radio is from the 60s, presenting excellent sensitivity and an output power of 100mW...
Found in a Popular Electronics from the 90s, this circuit uses components that are still current....
This circuit consists of a simple capacitance bridge. The handset can be high impedance from 500...