This circuit was obtained from a 90s Radio Electronics. The value of R1 determines the scale according to the table next to the diagram. The power suppley must be symmetrical.
This circuit uses two relays that switch when the voltage exceeds the value given by zener D2. The...
This circuit is from the 80's integrated circuit manual itself. The component is not as common today...
This circuit was found in a French publication called Generation Electronique, It can be assembled...