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 switch the relay from normally-closed (NC) to normally-open (NO), 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 to go from NC to NO. 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.
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