I'm building a robotic tea-maker/watchdog robot and have a power problem. I would like to be able to have the robot approach a socket and insert the power cord of a cheap immersion heater (120V, 300W, see links below) to turn the heater on. However, the force and precision required to plug it into the wall is beyond the capabilities of my stepper motors/Arduino.

My solution was a magnetic breakaway power cord like the charger on a Mac but at higher voltage. Deep fat fryers have suitable ones (120V, high power, see links below). However, the problem is I need both sides of the connector, and I can only find the magnetic breakaway power cord, not the opposite side, which would normally be built into the deep fat fryer. I don't fancy buying a whole fryer just to get one little part...

Any ideas? Alternatives to a breakaway cord? Anyone know of any (cheap) 120V induction chargers? I'll resort to a mechanical on/off switch and just leave the robot plugged in if I have to, but I was hoping for something a bit sleeker.


  • $\begingroup$ As you say, as a last resort have it plugged in, but for sleekness instead of a mechanical switch have an electro-mechanical or an electrically activated switch instead. $\endgroup$
    – Fred
    Aug 29 '15 at 5:53
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    $\begingroup$ A relay that can switch 120 V controllable by electronics is pretty easy to find (and easy to splice into a extension cord) $\endgroup$
    – ratchet freak
    Aug 29 '15 at 18:33

Since you specifically mention Macs, I'll throw this out there; you can find replacement ports for the actual computer online and it probably wouldn't be too hard to figure out the pinout and wire for 120V service instead, jimmying a MagSafe cable of course being also included. Other than that, I haven't seen too many magnetic breakaway cables like those being produced.

Given that my MacBook Air charger puts out 3A, the 2.5A you require is within limits, although you might want to put a good smattering of epoxy on the terminals to prevent arc-overs.

As to your inductive charging idea, it isn't too hard to build an inductive charging set, although the "isn't too hard" part relies on your immersion heater being of the "dumb" type; that is, a simple resistive heater that is capably powered by DC as easily as it is AC. Here, you'd wire up a primary coil of wire on a "former" for an inductive charging base with some resistance in play to make sure you don't smoke the wire with too high a current. Then, you'd wire the secondary former with a few more turns to account for air-core losses as well as voltage losses due to the primary resistor. Once you had the inductive couple "transformer" wound and drawn up with magnets or what-have-you, you'd bridge-rectify the secondary and install a simple diode clamp or crowbar circuit to limit the voltage across your heater to something safe, in case your inductive couple one day decides to be less lossy than usual.

If you'd like I can hand-draw a schematic for that and post it up; not in a eCAD/EDA mood at the moment.

  • $\begingroup$ Your Macbook idea sounds very nice, but would it be able to handle the 300W the heater needs? I assume the wires are pretty thin, thus pretty high resistance. I'd also prefer to keep this as cheap as possible since it's just a little project I'm playing with on the side, and the $21 for the port plus cables and a charger is getting a bit more expensive than I'd like. $\endgroup$
    – IzBelle
    Aug 30 '15 at 17:16
  • $\begingroup$ I would very much like to see a schematic for your inductive charging idea. I saw some diy induction chargers from Adafruit when researching, but all of them were for iPhones and the like, so were made for only 5V/500mA. I'm also pretty new to electrical engineering, so clarity in the schematic would be awesome. Thanks so much for your help! $\endgroup$
    – IzBelle
    Aug 30 '15 at 17:20
  • $\begingroup$ @IzBelle Okay, I'll try to get one together and upload it sometime today. $\endgroup$
    – ecfedele
    Aug 30 '15 at 18:07

I would suggest that your robot activate a relay which controls power to the heater. This is simpler than solving the original problem of developing the mechanical force to plug it in.

You would use a "power relay", rated for at least 120Vac and 2.5A, something like this. A mobile robot will communicate via internet or local network to a controller which activates a power relay. You will need another small logic board like an ESP8266 to receive communication from your robot and control the relay.

You may need a cascade of relays, i.e. controller sends 5Vdc control signal to a 12Vdc relay, which sends a 12Vdc signal to a 120Vac relay, to then power the heater. There may be some prepackaged "power relay controller" out there that already helps accomplish this as well.


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