This sophisticated circuit with two LF411 operational amplifiers appeared in the November 1984 issue of Radio Electronics magazine. The circuit uses a Mallory buzzer as an indicator.
This Philips commercial circuit is from the 90's documentation. The integrated circuit must be...
We found this circuit in the September 1971 issue of Radio Electronics magazine. The sensor is an...
This circuit is from a Hands-On Electronics from 1987, but it can still be easily assembled, as...