As a general electronic module with large power equipment under control of a small current signal Relay Shield (5V 4 Relay Shield) can ensure the single chip to control large power devices and is widely used in intelligent household projects made by single-chip microcomputer system; moreover it can directly be inserted to varieties of main boards compatible with Arduino without connecting the flat cable. See below features of the module: 4 relay shield standard Arduino shield interface and shape; other Arduino extension board is available; three M3 positioning screw holes convenient for installation; high level driver (5V or 3.3V); normally open contact closure; with onborad relay work indicator light(red). Special attention: when it is connected by Arduino UNO main board the green terminal should be avoided from short circuit by its shell. Parameters: Voltage Min. 4.75V;Typical 5V; Max. 5.5V;Power consumption (@5V) 180 mA; On and off times per minute is 30; Max. switching voltage per way is 125 VAC / 24 VDC; Max. switching current is 3A; relay: sky wave 5V direct-current relay with model HJR-4102-L-5Vamong which Relay 1 represents No. 1 relay and so on. Control interface: totally four pins with the four control signal pins sequent pull-down resistor comes with the circuit and the high voltage level can drive the corresponding relay (x) to absorb the armature so as to connect NOx and COMx; power interface: 5V direct power supply interface. Indicator light: four red indicator light and the corresponding indicator light will be on when the relay is driven by high voltage level. Relay terminal: connection terminal with large power can connect equipment with large current and voltage. No. 1 pin (NOx) is a normally open pin NCx is a usually closed pin and COMx is a common terminal i.e. if the relay is not driven NOX and COMx will open and NCx and COMx will connect; when the relay (x) control signal pin is in high levelNOx and COMx will connect and MCx and COMx will open. Pin resources that occupy the main control board compatible with Arduino / Catduino: four data pin in total. D4: control Relay 4; D5: control Relay 3; D6: control Realy 2; D7: control relay 1; therefore the other Arduino shields that will not occupy the four pins can directly be inserted to the extension board i.e. they can use the two extension board at the same time.