This circuit is for 12 V batteries but can be used with batteries of other voltages by changing the zener diode. The trigger point is adjusted by R1. The operating system accepts equivalents.
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...