Very nice small relay module yet handles 10A. Also very useful since accepts a range of input voltages 3.3 – 5V so can work with Raspberry Pi and Arduino or similar microprocessors. Only has NO (normally open) contacts so contacts act as SPST switch. There are four wires but you do not connect the white wire. Red connects to VCC (+ power), black connects to ground, yellow (signal) connects to the microprocessor pin to control the relay. High turns it on.