This circuit was obtained from a 1970s RCA. The circuit uses LDRs as sensors. The resistors depend on the A / D converter used. The circuit can be used as a shield.
This National Semiconductor circuit appears elsewhere on this site. The circuit must be powered by...
In this circuit the frequency is determined by the coil and the variable capacitor. Transistors like the...
This circuit is from old American documentation, but the transistor used is still very common...