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.

See all of the Weekend Projects posts

Nick Normal

Nick Normal

I’m an artist & maker. A lifelong biblioholic, and advocate for all-things geekathon. Home is Long Island City, Queens, which I consider the greatest place on Earth. 5-year former Resident of Flux Factory, co-organizer for World Maker Faire (NYC), and blogger all over the net. Howdy!

  • adcurtin

    AHH! time to let my inner Shia shine: NO NO NO NO NO!

    You are not changing the relay from NC to NO. You are simply actuating the relay. You could say you are moving the switch from the NC contact to the NO contact, but that’s still a little confusing.

    NC and NO mean normally closed and normally open. With the wording you chose, it’s very confusing (even more confusing on a relay that only has 4 connections instead of 5, where it will be strictly NC or NO.

    If you change the wording to “actuating the relay” it’s not ambiguous or confusing at all.

    • Nick Normal

      Hi adcurtin, Can you clarify when you say “has 4 connections” are you referring to the letters in the image, and not the physical pins? Thanks for reading!

      • Sean Ragan

        Nick, I love all the cool stuff you do with animated GIFs, and I love the “lets open up the black box” approach, as well. This is a great post.

        I agree the “NC” / “NO” terminology is confusing. To say that you are switching it from NC to NO suggests that you are somehow changing the default (“normal”) condition of the relay, when in fact what you’re doing is really much simpler–you’re just actuating the relay.

        • adcurtin

          Yes, this is a great post, I didn’t mean to imply otherwise. It’s great to show how stuff works at this level, it can really clarify things. Just meant to suggest a clarification of terms.

      • adcurtin

        The relay you use has 5 connections: 2 for the coil, 1 for the switch contact (this is a very ambiguous term, sorry), and 1 each for the NC and NO connections, totalling 5.

        Many relays have only 4 terminals, 2 for the coil, and the other two are either NC or NO. For example, typical automotive relays have just 4 connections (and most are NO). Here’s a link to on from autozone:

        • Nick Normal

          Thanks – great insight.

  • Chuck

    ‘Normally’ just means ‘powered off’ – with no juice to the coil, the NC contact conducts and the NO one does not. Power the coil and the NC contact becomes an open circuit, and the NO one now makes the monkey bang.

    — Olde Time electromechanical hacker

  • Gary

    The first thing I noticed is that it’s sending ‘CQ’ in Morse code. Nice!

    • Nick Normal

      cold… warm, warm!

      • MakeAlot

        M-A-K-E — .- -.- .

        • Nick Normal


  • http://lalala allaa

    the contact (or button as they are afectionally called) can be made of cadnium. best be careful and wash hands after touching

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