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Turn Your Timid Toy Keyboard Up to 11

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Blasting through audio limitations is child's play. Photo by Aidan Collins/

Toy stores are a goldmine of cheap project fodder for DIY hobbyists. As children's musical instruments are manufactured for the part of the population that hasn't yet had its destructive impulses smoothed out by societal conditioning, modern-day kids' toys can provide the kind of basic electronic components that are resilient to thrashing little arms, and thus perfect for hacking. In this how-to, we'll teach you how to break open a generic toy keyboard and add a line-out so you can amp it up in ways inappropriate for delicate little ears.

This how-to was written by Aidan Collins, a musician and technologist who loves building things.


  • 1 What You'll Need
  • 2 Cracking the Case
  • 3 Measuring Levels
  • 4 Applying Resistance

What You'll Need

  • A screwdriver
  • Wire snips
  • Wire strippers
  • A soldering iron and solder
  • A panel mount audio jack - I'll be using this 1/4" mono jack, but you could also use an RCA connector, or 1/8-inch headphone size connector if you'd like.
  • Resistors (optional)
  • A Multimeter (not 100 percent necessary, but it's nice to have)

Cracking the Case


Here is a $20 keyboard I picked up featuring a cartoon sponge. This is what a phone recording of the built in speaker, a little thin, eh?

The first thing you need to do is to open the keyboard up. Use a screwdriver to remove all the screws holding on the back panel.


Here we can see the speaker is held in place by a plastic backing, so we’ll have to remove that as well.


This is an 8 ohm speaker, and it’s even labeled with which wire is the signal and which goes to ground, perfect!

On most audio cables, the signal will be the "tip" of cable plug, and the ground will be the sleeve. On the 1/4-inch mono jack I'm using, you can see the big arm part is connecting to the tip of the cable, so connect the wire to the contact point that is connected to the big arm. The sleeve, or ground, of the plug makes a connection with the ring of the jack itself, and you should be able to see which contact point is attached to that. The packaging of the audio jack might also have a diagram if you're confused.

Measuring Levels

Now, we’re going to use the signal being sent to the speaker to make our line out, so it’d be a good idea to see what voltage this keyboard is putting out. Take the leads of your multimeter and touch them to the solder points where the wires are attached to the speaker. Set the multimeter to read AC voltage, which might appear with this icon: “V~”. Audio signals are AC, which means they alternate between positive and negative values. If your multimeter only reads DC voltage (voltage only in one direction) it won’t be very helpful here. Play some sound while monitoring the voltage.

A line level signal should be around .2V, at most. You can see a lot of different levels on different keyboards and toys depending on how loud they are, or what kind of speaker they’re driving. In this case, I’m reading a maximum of .15 volts, which is pretty much right where I need to be. I won’t have to add anything to the circuit to get it to line level. I’m just going to remove the speaker, and put the audio jack in its place.

What would I do if the voltage read higher than .2V? This keyboard has a pretty puny sound output, so it’s not surprising the voltage was already low. A louder keyboard might read something like 2v, which would be too much for the input of your amp or stereo. In this case you would need to add a voltage divider to your circuit.

Applying Resistance


R1 and R2 are resistors, R1 connected from the signal lead to the signal connection on your audio jack, and R2 going between the signal and ground connections on the jack itself. The amount of voltage reduction depends on the value of these resistors and the ratio between them. The most common application is to use 100k ohms for R1 and 10k ohms for R2, which would give you a little less than a tenth of the voltage you started with. That would work pretty well if you had started with 2V. If you only needed to cut the voltage in half, use the same value for each resistor, like 50k and 50k.

Note: 100k and 10k are the most commonly used values because the output of a typical, decently loud, speaker toy is usually about 2V. The keyboard I had was much less than this, but I also picked the cheapest one I could find. The reduction in voltage is based on the ratio, R2 / R1 + R2, so this setup outputs one-eleventh of the input voltage, reducing it from 2V to under .2V, which is our target. Any two resistors that have the same ratio of values would reduce the voltage to the same level, but having relatively high resistance, in the 1k to 10k range, will also reduce the current by a fair amount. This will help your batteries last a bit longer as well.

Luckily, as noted above, my voltage is already in the target range, so I don’t need to add a voltage divider to the circuit. I just cut the wires to the speaker and strip them, then use my soldering iron to connect the matching leads to the signal and ground connectors on the audio jack.

I think this little plastic grill will make a nice place to hold my audio jack, and it’ll be easy to snip out an opening just about the right size.


Here it is all soldered and glued into place...


...and then put back together. Easy as pie!


Now, if you didn’t have a multimeter and followed the steps I did, be careful when you try the sound out. Start with the volume all the way down and slowly raise it as you start playing. If the sound is way louder than you’d expect, or if it sounds really distorted, it’s putting out too much voltage and you should go back and add that voltage divider to your circuit. If it sounds clean and nice, then you’re all set!

Here's a phone recording of the sounds playing through my stereo after I finished, where I’m free to tweak the treble and the bass. Nice!

All photos by Aidan Collins for Wired

This page was last modified 23:33, 18 July 2012 by amyzimmerman. Based on work by howto_admin.

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