Here's a control circuit for low cost plastic molding machine. A control circuit is a sort of circuit that is used to regulate the operation of a separate power circuit. Consider a huge industrial engine with 1,000 horsepower running a water pump. The motor is powered by a 2,400-volt high-voltage electrical source. The thermostat for a house's air conditioner is an example of a common control circuit. The thermostat is part of a low-voltage control circuit that operates a relay that energizes and de-energizes the compressor's power circuit.