Working with Reed Relays

One of the main benefits of Physical Computing is being able to control your environment, not just monitor it. Transistors, MOSFETs, Solid State Relays, and Mechanical Relays all allow you to turn devices on and off based on inputs and decision making processes.

One of my favorite Mechanical Relays is the Reed Relay. They come in a variety of coil voltages, and the one we use has a 5v coil, and is a perfect match for the Arduino. It has a resistance of 500 ohms, so it draws 10ma, right within the range of the Arduino output capabilities. Reed Relays come with or without snubber diodes. Snubber diodes are used to prevent the inductive kick of the coil from damaging the port on the Arduino. We will be using a Reed Relay with a built in snubber.

We will be using a digitalWrite command to output a 5v HIGH to the output pin that connects to the + pin on the reed relay. The Magnecraft W107DIP-5 is a fairly inexpensive Reed Relay, with Normally Open contacts. Normally Closed, and SPDT varieties are also available.

Connect Pin 3 of the reed relay to the output pin (13) on the Arduino. Connect Pin 5 of the reed relay to Gnd. Pins 1 & 7 are your switched contacts. Connect Pin 1 to power, Pin 7 to load, and the other pin of your load to the other power connector. The load and load power can be AC or DC, and up to .5 amp current draw.

int outputPin = 13;       // Relay connected to digital pin 13

void setup()
{
  pinMode(outputPin, OUTPUT);   // sets the digital pin as output
}

void loop()
{
  digitalWrite(outputPin, HIGH);   // sets the relay on
  delay(5000);                  // waits for 5 seconds
  digitalWrite(outputPin, LOW);    // sets the relay off
  delay(5000);                  // waits for 5 seconds
}