Audible TTL Probe (CB5907E)

This circuit was found in a 1980s documentation. The IC is the 7400 and should be powered with 5 V. The transducer is a ceramic or crystal capsule.

Light Activated Relay (CB948E)
This circuit uses a photocell as sensor and a sensitive relay according the power supply voltage....
Current Sensor with Optical Isolator (CB5229E)
This 2005 circuit from a Linear Technology manual makes use of the LTC610HV and an optical coupler that...
Touch Triggered Flip Flop (CB143)
The 4093 or any other NAND gate can be used for the very sensitive configuration of a R-S...