This circuit was obtained from the July 1970 issue of Radio Electronics magazine. Zeners are power types that can provide a good current to the circuit. Selection is made by rotary switch.
We found this circuit in a 1990 documentation. The operational amplifiers are type 741 and the LED is...
We found this circuit on an Elektor from the 90s. The frequency is given by capacitor C1. The...
Found in National Semiconductor publication from the 1980s. This gain-100 circuit can be...