Using Power Source for Battery Operated Device

A friend brought me this light up balloon dog toy, and asked if I could make it powered by a power supply rather, because always having to change batteries was a pain and environmentally disastrous. It ran off 2 x AA batteries (3v in total).

I told him I was sure I could. I tried a few things, which I'll mention, but arrived at the simplest solution possible which is what the next few steps are about.

This will be a quick Instructable.

Step 1: Identify Your Power Needs and Cater to Them

How many batteries does your device take? What batteries are they?

In my case, it was 2 x AA batteries, each at 1.5v when fullok. This equated to 3v power needed. Finding a 3v power supply was not simple. I didn't go looking to buy one, just went through boxes of old power supplies.

Step 2: Take Apart Your Device, and Identify Where You'll Connect Your Power

I took apart the dog, and found it had much more electronics than expected. I thought it would just have batteries connect to a switch and the LEDs.

Anyway, inside it had a circuit board, and the battery terminals connected clearly to a B+ and B- place on the circuit board. This is where you'll want to connect your positive and negative wires from your power supply.

Step 3: Create Power Solution

I'd have to modify a 5v power supply, I thought, and went about doing so, trying voltage dividers first with a diode for the voltage drop.

The mission was unsuccessful. I'm assuming because the current decided it was less work to just go through the voltage divider than powering the dog.

I was concerned that I could have perhaps destroyed the dog, because it powered on briefly before dying again, so I pulled out a breadboard power supply module to test it. I set the power supply module to 3.3v, and applied the voltage to the circuit board and, hey presto! - it worked...

Then I had a naughty thought... could I just apply 5v? there are plenty of 5v power supplies at my disposal... I gave it a try and once again was met with success.

I'm not saying you should definitely do this! But, things that use batteries aren't so finicky about the exact voltage they receive, because (alkaline) batteries loose voltage as they are used.

Basically don't blame me if you fry something - but in my case this solution was perfect.

Step 4: Form Factor Decisions

I wanted my friend to be able to plug this in and unplug it at the dog, not only at the wall outlet, so I thought I'd give it a female jack of sorts and the power supply would terminate with a male jack. Doing this on the cheap I found scrap wires terminating in both RCA female and male jacks.

While looking for a 5v power supply I'd never likely use again, I realised I could just use a USB cable, as all USB ports will supply 5v. Now he could even use a battery bank to power it, or AA batteries (my cable and jack didn't interfere with the original battery case), or a mobile phone charger.... you get the point. Basically it could now be powered by any USB port.

I then found an old USB cable that Must have originally had 2 USB plugs on them but one had been cut off. Perfect, I thought. Can even use the USB cable for it's original purpose, if you wanted.

Inside the USB cable you will find at least a red and black wire (you can cut away and ignore any other wires - cut them away or whatever). These red and black wires are your 5v and earth respectively and will serve to power your device.

Inside RCA cables (should you choose to use them, as they are abundant and HDMI is killing off their species anyway) you will find a wire and a shielding wire. The wire may be red in colour, and thus I used it for positive and soldered it to the red wire from my USB cable. The shielding got soldered to the black. I used heatshrink to isolate the cables from one another and a larger heat shrink on top of the joint. You may use electrical tape, but heatshrink is wonderfully neat.

The female RCA wire was connected directly to the board in a way that didn't interfere with the original design, so he could use AA batteries again if he wanted to. The wire going to the B+ point was soldered to the PCB and the shield wire got soldered to the B- point.