After our Monkey Couch Guardian project went live last week, I realized there was some wonderment about how the Single Pole Double Throw (SPDT) relay functioned. Not satisfied with simply verbally explaining the switching going on inside this mysterious black box, I wanted to visually show the relay in action. Aha, “A GIF!,” I thought to myself.

Relays come in many package configurations, but I specifically wanted to use the 5V DC micro relay we used to trigger the toy monkey. I carefully cut away the case around the relay using my utility knife, by slowly shaving down the corner edges until I could snap the lid and then the sides off, revealing the relay. Since I don’t have a PIR sensor available, and simply wanted to actuate the relay, I needed a novel circuit. Low and behold, a Flickr user posted this simple circuit to the MAKE Flickr pool in 2009 for driving a relay with the Arduino’s 5V power supply. For R1 I used a 1KΩ resistor; for TR1 I used a 2N2222 transistor; and for D1, I used the 1N4004 rectifier diode.

Now you can see below, when current passes through the coil it produces a magnetic field and triggers the springless armature. In our project, this switches the monkey from a silent toy into a cymbal-banging guardian, but you could easily use this to drive many types of switching circuits.

Slowly shave off the edges of the relay’s plastic case to reveal its innards. First, shave and remove the top, then shave and remove the sides, leaving the base intact.

After removing the coil’s sheath, the wire and power leads are seen here.

The armature pulled back, exposing the relay’s core.

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