This circuit was found in a 90s Radio Electronics. The integrated circuit admits an equivalent. The power supply will depend on the integrated circuit used.
This bargraph-type circuit employs a not very common integrated circuit today, but can be adapted...
Suggested by Radio Electronics from the 90s, this circuit activates a buzzer and an LED when the...
This filter was found on a 90's Radio Electronics. With the components used the cutoff frequency...