This circuit is useful for removing ground loop hum on a remote line level audio signal line. It can be used to to connect a computer sound card to a stereo amplifier’s line input. Other uses include tapping into a line level signal for powering a remote amplifier, and removing common mode ground interference on 12 Volt audio equipment such as a car stereo. The circuit can be used in mono applications by simply ignoring the second channel. source

This circuit allows you to hook your Audio Board up to your synthesizer. source

This circuit is great for battery-powered systems that use slotted type optical interrupters. It draws only 10uA from a 3v battery that should allow up to 5 years of operation from a lithium battery.source

This is a simple light detector circuit board that has 8 detectors that can be used with visible or infrared light systems. The detectors use LM339 voltage comparators as the active element. Phototransistors or CdS photocells can be used as sensors for this circuit. If phototransistors are used the circuits can also be used as infrared light detectors.source

This is a circuit to use a standard, low quality opto isolator to transfer an analogue signal with reasonable linearity and without complicated feedback loops to monitor and linearise it.The circuit was designed to interface a mains driven thryristor motor controller to low voltage analogue circuitry.source