If you have questions, watch the video, it has extra detail as well.

Safety first

When it comes to safety, it pays to double check your work and error on the side of caution. I can give some guidance on how to choose equipment safe for your application, but it doesn’t mean it is safe for your person if you get in the middle of your electric circuit. if you have concerns about doing this on your own particular project, ask for help.

Some things to keep in mind for safety:

Voltage does not kill, current kills, but voltage is required to push the current along. So, while a 5 Volt 1 amp power supply may be reasonably safe to work with, a 100 volt .1amp is potentially lethal if it ‘bites’ you in the wrong spot. Ask for advice if you do not know. At all times, practice safety.

Batteries/power supplies have specific direction/polarity (positive or negative) that the moves the current along. If you wire your circuit backwards, the current will attempt to flow backwards through your device and could damage it. Check the polarity of your batteries and power supply to ensure they match. There will be more on this in a later step.

Before replacing batteries with a power supply, consider where the device or toy is used. Will it be sitting on a desk or near the bathtub? Would your kids put it in the bathtub? Will it be a shocking experience for them if they do?

The problem:

In general, batteries make my life a little more mobile and less tangled. Sometimes however, there are certain devices that I wish came with a plug. Most recently my kids got an awesome night-time toy that projects stars, the moon, galaxies, etc on the ceiling. The problem is the toy is battery operated only and there’s no automatic shutoff so when kids use it like a night-light it’s dead in a few days. I don’t like buying or replacing batteries. (because I’m a cheapskate).

This toy just sits on the desk, so it’s a good candidate to modify to accept a DC power supply instead of batteries. This idea is not well suited to something like an R.C. Car, but in a pinch, you can use it on the remote control for your TV.

Step 1: Some Theory

Wall outlet power is generally alternating current, or ‘AC’. Batteries are direct current ‘DC’ and only push the current in one direction. An AC to DC power supply can change AC wall power to DC power. Many common devices that have batteries (laptops, smart phones, etc) only accept DC power. They use a AC to DC power supply to allow us to charge the device by plugging it into the wall.

Ohm’s law is a formula in electronics that relates the voltage (V, volt), current (I, amp) and resistance (R, ohm) of a circuit. Ohm’s law tells us that Voltage in an electric circuit is equal to the product of the current flowing through the circuit and the resistance of the circuit.

V=I*R

For a given circuit, in my case, the toy, the resistance R is a constant. If I replace my batteries with a power supply of equal voltage, then the current in the system also stays the same. This project uses this relationship to replace Voltage, V supplied by a battery with voltage supplied by a DC power supply – nothing else is changed.

Another way to think about this is that voltage is how much the power supply pushes and current is how much the device pulls the energy. If your power supply says 5 Volts and 1.5 amps, it can power a 5 Volt circuit and it won’t burn up until the device pulls more than 1.5 amps from the power supply. In a later step, I will show you how to verify the current draw of your device.

Pro tip: AAA, AA, C and D cell batteries all have the same voltage rating and are interchangeable. That is, they are interchangeable if you can make them fit. My brother and I used to wrap AA’s in paper strips to make them fit the place of a C cell ?. The difference between the batteries is just the physically larger batteries last longer/have more ‘juice’, they don’t push more current/amperage through your system.

Step 2: Understand What Voltage Your System Uses and How the Electricity Flows

My device has 3 AA batteries which are 1.5 volts each. All added together (assuming they are connected A to D to E to B to C to F) I should see 4.5 volts. The first thing to check is how the batteries are wired and find the beginning and end of where the energy flows.

Consider the above battery diagram. If you put your volt meter such that one lead is on A and the other D you will see either +1.5 or -1.5 volts (or something close unless your batteries are dead), Now check point A and point B do you see +/-1.5 volts or +/- 3 volts? If it is +/- 1.5 volts, then point A and B are essentially the same point and must be electrically connected. If A to B is +/- 3 volts, then you are adding the voltage from batteries #1 and #2 together. Use this same logic to trace the batteries to the end and determine if points A to F or points D to C incorporate all 3 of the batteries (+/- 4.5 volts with full charge batteries). This example are for batteries in series (connected A to D to E to B to C to F for example) Most of the kids toys batteries seem to come this way, but I’m sure there are exceptions. Let me know if you need some help troubleshooting the batteries for a different circuit.

Keep note of which points show you the 4.5 Volts or -4.5 volts and which voltmeter lead you are using at each point. You will need to maintain the +/- polarity when you hook up your new power supply.

To measure the current draw of your system, connect your multimeter inline with the batteries. You’ll have to disconnect one end of a battery and use your multimeter to bridge the gap between the disconnected end and where it should be. Note that current draw may change for your device if it has multiple modes of operation. For example, a computer at startup draws more current than one that is hibernating.

Step 3: Find Your Power Supply

Now find a power supply that supplies the needed voltage you measured and has a high enough current rating that you won’t burn it up with your device’s amp draw.

I’ve had good luck finding power supplies at thrift stores. Some power supplies even have multiple voltage settings. In my example, I am using a 5V 1.2 Amp power supply – it is a little over-sized, but for my application, I think it will be just fine. Often times batteries at full charge put out a little more than their rated voltage as well, so that’ something else I’m considering when choosing a slightly higher voltage power supply.

Unplug the power supply, snip the plug end off (unless you are going to use it!), divide the wires and strip them at the tip. Plug in the power supply and check the voltage using your volt meter making sure to note which way gives you +4.5 or -4.5 volts (like in the previous steps example). So, if you put your red voltmeter lead on A and your black on F and saw -4.5 volts, then you want to want to see -4.5 coming out of your power supply and then whichever wire the red lead is on will go to point A.

Step 4: Assemble

You could solder, or otherwise attach the power supply to the points determined in Step 2, or like I do in my example, I fit a plug to the power supply and a jack to the toy. You may consider if you want this process to be easily reversible if you would like to add batteries later. ***NOTE*** Don’t do this with batteries in the device, take them out.****

Solder and assemble and you are done, no more need for batteries!

Thanks for reading and happy making!
-instructodad