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

This Opto Isolator circuit allows you to pass signals between two circuits without having to connect them electrically. They are great for eliminating noise problems, or to connect circuits that have different voltages.source