Dummy batteries let you use an AC adapter

dummy-batteries-work-as-AC-adapter

We find it frustrating when battery operated consumer electronics don’t include a way to connect an external power supply. We try not to purchase disposable alkaline cells if we can avoid it, and this dummy battery AC adapter hack will aid in our mission.

The battery compartment shown above is for a motorized baby swing. It accepts C sized batteries (who has those just lying around?) and lacks a barrel jack to connect a wall wart adapter. [Jason Smith] mentions you can get around this by connecting your positive and ground wires directly to the conductor springs. But using a dummy battery makes it a bit easier to remove the adapter if you do want to use battery power.

Each of the orange dummy is a wooden dowel with a screw at each end. The screws are connected with a piece of jumper wire, shorting the two terminals. This completes the circuit in the battery compartment and allows him to power everything from the adapter cell at the bottom. The adapter uses an LM317 adjustable voltage linear regulator. He used fixed resistor values to dial in his target voltage. The equipment should be rather forgiving as battery voltage starts higher than the printed value and drops as the cells are used up.

This technique has been around for a long time. One of our favorites was a hack that converted an Apple Magic Trackpad to USB power.

Comments

  1. Phil says:

    I directly wired the one for my daughters, the batterys only lasted an hour or so but the swing was so much help when they were younger.

  2. Dan says:

    I had this same problem – I added a DPDT switch to toggle b/tw the wall wart (for which I added a barrel jack), and the batteries, in case I should ever want them again.

    Spoiler: I never did, I kept using the wall wart.

  3. Famine says:
  4. PC1911 says:

    Dan, In addition to your DPDT switch idea, a panel mount power jack like the RadioShack 274-1582 which contains a built-in switch (uses a standard 5.5mm OD 2.5mm ID power connector) to switch between internal and external power could have also been used.

    Also when did baby swings become motorized? I seem to remember them being windup, no batteries or power needed for those.

  5. Phil says:

    Another way of doing this would be to cut the dowel to length, drill a hole down the center and push a long nail through.

    • Cement Truck says:

      Actually, if you just run a wire from the first battery’s positive spring to the 3rd battery’s negative spring and and used 4th battery’s pos and neg springs to attach the female jack there would be no need for dowels and nails

  6. ejonesss says:

    you may be able to eliminate the voltage regulator if the power supply puts out the correct amount.

    you do have to be careful because some devices may split the batteries to create a + and – voltages for example some cheap rc cars instead of using 4 transistors to make the motor reversal drive they may center tap the middle of the battery pack and use 2 transistors to connect the + end and – end to the motor and you can get reversal that way too.

    by taking the device apart you can determine if all the cells are being used to make 1 big voltage or multiple smaller voltages.

    if it is 1 voltage then you will see only 2 wires come off the battery holder.

    multiple voltages will have 3 or more wires

    another example of multiple voltages maybe some cheap rc cars that instead of a 9 volt and 6 aa you may have 12 aa and center tapped somewhere is a wire to power the radio electronics and the remainder to power the motors or other accessories like lights or siren if it a police car for example.

    • Adi th tinkererrr says:

      dont think many people would have a RC car connected to a wall wart but suppose it makes sense to check your device to make sure it dont have diffrent taps from the battery pack

  7. Peter says:

    We had a baby swing that ran off a spring & wind-up crank.
    Electric power is not always the best solution to a design problem.

  8. chango says:

    We had the exact same swing, and I wrapped wire around the battery terminals that matter like a lazy schlub. Also, unregulated 6V linear wall warts FTW.

  9. Rich says:

    I picked up this ancient video cam set up, camera+VHS player/recorder, and some sort of power supply/controller/battery pack and a multitude of proprietary cables for $5, which had a surprisingly good picture and performance but it was just too unwieldy to use for much of anything. The whole deal weighs about 25lb, “portable.” The power set up has a huge 12V rechargeable battery, about half the size of a brick (and weighed more) which was dead, so I made a chunk of wood with some copper contacts to fit in the battery holder, I could either hook it to a car battery, a lighter plug, or a battery charger, to run the thing. I had this idea to use it as a vid recorder in my truck, but the thing is so huge it made no sense.

    But many times a hack makes no sense other than to hack something for fun.

    • chango says:

      Fun fact: that “brick” is a sealed lead acid cell. You used to be able to get a clip that fits over the terminals to adapt it to a lighter socket. Young whippersnappers don’t realize that there was a time when NiCd was considered futuristic.

      Also, the viewfinder module on those cameras tend to be self-contained black and white NTSC CRTs. There are loads of howtos for finding the pinout. Perfect for a retro-80s Glass-alike with a RasPi.

      • Bobby B says:

        not want to be picky, but please understand what he wrote, “12V rechargeable battery, about half the size of a brick” key words: “about half the size of brick” – means hat the battery’s size is comparable to the 1/2 of brick, more, following that text further, it the battery weights more than 1/2 of the brick, that has been used earlier for comparison.

        • Rich says:

          I’m not sure what that means, but the battery is about 1.25″ thick and maybe 3″x6″ or 7″ and quite heavy, so I am guessing it might contain a stack of nicads or maybe lead acid, for whatever ephemeral purpose this vague knowledge will provide.

          I think the cam has a color viewfinder in it if I recall, maybe not. It sits in my garage for a someday further hackathon, though because it does actually work pretty well I am loathe to tear it apart. Not that I need a 25lb video camcorder setup for anything particularly useful, but who knows…

  10. You don’t need dummy batteries to do this. I did the same thing – utilized an old Blackberry P/S. The batteries are in series so you just go to the beginning and end of the +/- chain. The rest of the in-between springs and jumpers are just for completing the battery linkages!

    • peter says:

      Agreed that you don’t need all the dummy batteries but if you don’t want to permanently modify the device, it is easier to use two dummies– one at the positive and negative terminals. You don’t need the ones in the middle.

  11. eatith mee says:

    Done this more times than I can count over the years with battery operated stuff. However; I find a nice spot with clearance for a switched barrel jack, drill a hole, wire it up and then I can use batteries or a wall wart of the proper output.

    A couple of things that bother me about this hack is there is no heat sink on the voltage regulator, which a regulator is probably not needed as 6V DC wall warts are everywhere and the little motor in this thing could care less about a well regulated supply to begin with. TO-220 regulators are typically only good to 1.5A and thats with a good heatsink (a block of wood is not a good heatsink). Batteries can supply more inrush current than a voltage regulator as well, so the motor in this thing must have very low draw for this to work. I’ve never had much luck powering motors with LM317s or LM78XXs, ever… So color me surprised that this even works.

    • polossatik says:

      a seriously over sized cap helps most of the time, once the motor runs current draw falls trough the floor.. (use a fast multimeter with peak hold)

      if this does not need a heatsink then I assume it draws almost nothing from current , hence why it works without *any* caps (which is a bit surprising indeed)

  12. This is quick and easy for the novice hacker, but my solution is to put a type M or type N connector in that disconnects the batteries when you plug in an AC adapter. They’re tiny and almost always very easy to mount. And if you can solder three wires, you can fit these power connectors right in, no problem.

  13. Steve says:

    Be careful that your linear regulator doesn’t start a fire. It doesn’t seem safe to have pressed up against a block of wood. They can get VERY hot.

  14. Blondie says:

    Here’s another approach:
    http://quinndunki.com/blondihacks/?p=576

  15. Lunaras says:

    In the late 90s I had an MP3 player that would only run a couple hours on a pair of AA batteries. I got tired of buying replacements and had no rechargeables, so I took two 3/8″ wooden dowels, put a screw in one end of each, taped them together, wired the screws to a 3V wall wart. It worked great and was easily removable when I wanted portability.

  16. Tyler says:

    Cartridge fuses often make very good dummy batteries for AA and AAA.

  17. Bob says:

    Scraping the barrel here HaD…
    This is yet another one of the almost daily hacks that you shoudl put on a new subdomain:

    basic.hackaday.com

    Novice hacker? lmao.

  18. Hirudinea says:

    It seems so many people do this they should make an adapter kit, come China, we need some more crap for the dollar store, get on it!

  19. My dad made a ‘dummy battery’ wallwart adapter for my microscope when I was a kid, because it needed batteries for its light. It took 2 AAs, so he just cut a block of balsa to fit the battery holder, and then took the stripped wires from the wallwart and rolled them into little ‘pads’ that he tacked to the end of the block of wood. Worked like a charm. He even notched the base of the microscope so the wire could get out without disturbing the fit of the rubber cap that went on the bottom. :D

  20. Augur says:

    Really stupid question…. Wouldn’t the wood slowly heat up with the motor is under load since the screws are attached to the dowl rod as a base?

  21. andres says:

    hmm. I have never been one to be a safety nazi, but i question the idea of putting an lm317 on a block of wood inside an enclosed box on a swing with a baby in it. i think a better idea would be to put the lm317 external closer to the wall wart. also, why bother bridging all the battery spaces? why not just wire 2 wires to where the end of the battery series would go?

  22. NewCommentor1283 says:

    i agree with all the safety hazards but…
    you HAVE to realize that this “huge current” you speak of
    is only for less then a half second, every two seconds.

    thats ONE QUARTER of the AVERAGE heat
    that you’d expect from such a linear regulator!

    if it draws 10A from a standstill (each end of the swinging action)
    it will NOT run down ALKALINE “C” batteries in 30 minutes

    it will run for 2 hours aka half a day’s usage
    because once it is up to speed(each direction) it takes less then 1A
    until it starts the whole thing over(it goes back and fourth)

    PS: im assuming ALL alkaline “C” batteries are around 5AH
    (“AA” are around 2.25AH)

  23. Sven says:

    Anything with more than 2 batteries in series you can just leave the middle batteries out and use two used batteries to clamp the wires to the active terminals. Put the outermost batteries in and stick the wires between the cell and the terminal.

  24. anon says:

    I performed this exact same hack four years ago on the same kind of baby swing, but didn’t think it worthy enough to send into HaD. Only instead of going through all the trouble to create dummy batteries, I soldered alligator clips onto the wires and dremeled out a small hole in the battery door for the wire to go through.

    • asheets says:

      I went with the dowels and wall-wart, and it lasted through 5 babies (my three, and the two whose parents got the swing after my kids were done with it. I got the idea from an ancient issue (early 1980′s) of Popular Photography (or something like that) where some other guy built something similar for a portable flash unit. It used dowels to replace the AA batteries. If I remember right, the flash hack may have even been a “for sale” product.

  25. kissmyass says:

    I did this years ago with a VHS-C camcorder that had almost zero battery life using its standard battery pack – cut the leads in the pack and wired it to an external source (a small (7A) lead-acid battery).

  26. Zellot says:

    Hahaha. Knew I recognized this. I did it 15 years ago for my sons baby swing. Except I soldered in a female barel adapter and installed NiCad batteries. That way it could be plugged in at home or used on the road without having to change out batteries

  27. Scott says:

    Did something like this a (long) while back, only with a block of wood instead of individual dowels, copper foil tape and a female jack. Just be certain that your wall-wart is supplying a voltage near your target WHILE at the current load of the device. If your device draws a current much lower than the rating of the supply, you may find that the voltage will be MUCH higher than you expect if it is unregulated. Learned this the hard way on something else.

  28. interstellarsurfer says:

    I did something similar but much less refined for my second boy a few months ago. I couldn’t find a 6v supply in my parts box, so I just installed half of a USB cable with some hot melt glue and soldered jumpers, whittled a pass-thru into the battery cover for the cable, and in less than 15 minutes had a USB powered baby swing.

    Of course, my wife asked if it was going to catch fire, or throw the baby into the wall. ^_^

  29. David Henderson says:

    I need to built a few of those for 9 volt batteries.. I am thinking of taking an old dead 9 volt battery and gutting it. Soldering the wires to the inside of the old contact plate and them filling the old casing with hot melt glue.

    • Dan says:

      Dave, why not just buy those 9v battery clips? They work in reverse, and you don’t need the whole case, just the contacts.

      • David Henderson says:

        What I am wanting to use them with does not have the snap type 9 volt connections. It just has two metal plates the the battery contacts. I know what you are talking about. I have that set up for my clock.

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