Low battery indicator (CB9133E)

This circuit was found in Maxim's documentation from the 80's. The integrated circuit is from that time, uncommon today.

Sound Operated Switch (CB6315E)
This sound relay or Vox was found in 70s RCA documentation. SCRs can be common types today and...
Simple Timer (CIR865E)
This simple two transistor timer can be used to measure short time intervals. The power supply...
Wien Bridge Oscillator with Diode Regulator (CB25908E)
This circuit for 1 kHz sinusoidal signals uses a common operational amplifier circuit and is...