The LM324 Low-Pass Filter

I know, I know… It has been far too long since my last post.

The weather is cold and rainy so it’s CIRCUIT BENDING TIME!!!

I have been working on the Kawasaki Dual-CoolKeys lately. It’s a great keyboard with many independent features. By independent I mean, the keys are tied to a different timing resistor then the drums. Also, there is the ability to have one side play one set of instruments and the other side play another set. HOW COOL IS THAT?!?!?

I was digging through the usual circuit bending suspect’s blogs and was, as always, blown away by The man is a super genius. And best of all, all his circuit bent gear looks so good.

He always has a filter (or two) in his projects. So, I wanted to have one too. How hard could it be? The answer is NOT HARD AT ALL!!!

I dug up some schematics for an LM324 Quad Op-Amp low-pass filter circuit and tried it out. I ran into one major problem with the integration of the low-pass filter and the Dual-CoolKeys. The Coolkeys has the drums output as the negative terminal going into it’s internal speaker. The keys are on the positive terminal. WHAT?!?!?

I spent a day just trying to figure out why the drum beats weren’t playing into the filter.

Once I figured out that each line into the speaker was actually individual sound outputs it made things easier. However, another issue is that if you connect the two outputs together with resistors to mix the sound, it shorts out the keyboard. I don’t know if it needs higher resistor values or what but that was annoying too.

Ultimately, I think I will just run each into an amplifier and mix them after that.

Back to the low-pass filter…

The filter is a simple build. Nothing fancy. The circuit only uses one op-amp from the LM324 so you still have 3 op-amps for other things. A great thing to add would be another filter for the drums. Or a high-pass filter… We will see.

lm324 opamp low-pass filter design

lm324 opamp low-pass filter design

I bread boarded everything up and it worked great.

LM324 Low-Pass Filter

I played around a little with the values of the potentiometer and the other resistors. It took a while to find the right balance. If I remember correctly, the original schematic worked pretty well though.

LM324 Low-Pass Filter

It’s a great little circuit and super easy to build. Try it out and let me know how it goes.

Happy bending!!!


Kawasaki Dual-CoolKeys – Bending Heaven.

I was recently asked about the Kawasaki Dual-CoolKeys. The reader was asking if I had any experience with it or if I knew of any cool bend points.

Coincidentally, I had been messing around with a Dual-CoolKeys, not long before. I took some pics to document the device and wanted to share them.

kawasaki dual-coolkeys

I picked two similar Kawasaki keyboards up at Value Village for about 10 bucks. The guy looked at me super funny when I bought them. But hey, that’s part of the fun of circuit bending.


Upon opening up the Kawasaki Dual Keys keyboard, I noticed an amazing thing. The Dual Keys seems like it was built by someone who loves to circuit bend. Everything is modular and the whole keyboard comes apart beautifully. The screws are easy to reach and there ins’t much that is glued down. Really, the only glue is supporting the wires.

kawasaki dual-coolkeyskawasaki dual-coolkeyskawasaki dual-coolkeyskawasaki dual-coolkeys


I did some basic bend searching and found a resistor of particular interest. The thing that makes this perticular keyboard so perfect for circuit bending is that there is not one but at least two different timing resistors. This means that the timing for the drum track and the timing for the keyboard sounds are independently controlled .

kawasaki dual-coolkeys

I will be honest, it’s been a while since I was messing around with this Dual-CoolKeys, so I can’t remember if this resistor controlled the drum track timing or the keyboard sound timing (I’m pretty sure it was the keyboard) but either way, this is AMAZING!


This is just the beginning. It’s easy to see the potential for circuit bending the Kawasaki Dual-CoolKeys into a much more amazing piece. I just wanted to get the ball rolling on the Dual-CoolKeys documentation. With some time and research, the Dual Keys, and the rest of the Kawasakies could be as popular as a Hing Hon or some of the more known synths out there.

If you have any information on the Dual-CoolKeys, feel free to shoot me an email or make a comment. If you write about, or have written about it, let me know and I’ll link to you.

Should you ever come across any of the Kawasaki line, (I have a few and they are all awesome) I highly recommend you pick it up.

DIY Drum Machine – Circuit Bending

I have been working on a diy drum machine for a while now. I wanted to post this video to show it’s functionality.

The core of this drum machine is from an electronic drum stick. You swing the stick and it makes drum sounds. The stick made 3 sounds; Snare, bass drum, and high hat.

DIY Drum Machine

When you swing the drum stick, it plays a snare sound. There are two buttons on the stick, that when pressed AND the stick is swung, play two more sounds. So this was easy to hook up to a 3-way switch with up as one button, down as another button, and middle was just like swinging the drum stick without pressing a button.

DIY Drum Machine

I rigged it up to a 555 timer and a 4017 decade counter and some pots and switches, and away it went. There is still some glitching that happens I think because of some switches which aren’t wired to ground to de-bounce them.

DIY drum machine - time signature controlerDIY drum machine - switches to control sound typeDIY drum machine - 555 timerDIY drum machine - 4017 decade counter

The 555 timer triggers the drum sound and advances the 4017. The 4017 sends a pulse through one of 10 3-way switches to hold the particular sound. The sound selection is set with the 3-way switch. I hope that makes sense.

Like I said, it’s a work in progress.

DIY drum machine - control board layout design

The case is from a kids toy (big surprise, I know). It was a light box for tracing. It use to have two lightbulbs inside and a semi-transparent screen. The cool thing about is that it has a battery compartment built in. All I had to do was modify it for the voltage I was using. I just shrunk it down. Easy (yeah right).

I painted it up white first and then spray painted a stencil of a fist on top of it. The body was also painted red. The final step for the paint job was to do some clear coat. All in all, a pretty sweet paint job.

Circuit Bending a DIY Drum Machine

In this video, you can see holes for the LEDs that will indicate which point in the sequence it’s at. I am having a heck of a time with that. It seems like, when connected, the sound plays but the LEDs don’t light up. I don’t know if it’s a lack of current or what. I have tried the LEDs in series and in parallel. Neither work now. The joke is, it all worked at one point when it was on a bread board. Upon soldering everything up, everything started changing. I suppose that is the nature of the beast.

I would love to hear your thoughts.

Happy Bending,

Arduino Sequencer


To quote noystoise…

wow, its been a long time since i have posted anything here.

It has been ages since my last post. So much has happened (Including planning a wedding and getting married). With that said, I have still been circuit bending and building things.

My Arduino Uno Micro-Controler

Recently, I purchased an Arduino Uno micro-controller. This thing is awesome. It doesn’t have much to do with true circuit bending (I know someone will have a problem with that) but it is insanely powerful.

Here is what I have been playing around with in the last few weeks.

This little Arduino Sequencer project that I have been working on is super simple to build. Really all you need is an Arduino Uno (and a computer to program it with), breadboard, speaker, potentiometer (any value) and some wires.

All of the functionality comes from the programming. The code is simple too. Basiclly, you use the built in tone generator (it’s just a square wave generator) to read the value of a potentiometer and place the value of that the potentiometer into a variable. Then you play a tone based on that value, pause, play another tone, pause, play another, etc.

The subsiquent tones are based off the original tone, they are just multiples of that first value.

The program loops and starts all over again. That’s it!

Arduino uno diy sequencer

Here is the link to the tutorial that I followed originally. It has some schematics and in depth instructions about how to set it up and how the code works.
Arduino Tone Follower Tutorial

Here is the code:

SecondHandSynth Digital Arduino Sequencer

Plays a pitch that changes based on a changing analog input

* 8-ohm speaker on digital pin 8
* Breadboard
* Some resistors and wire

This example code is in the public domain.

Original code before modification came from


void setup() {
// initialize serial communications (for debugging only):

//boot up sound
int var = 100;
while (var < 500)
tone(8, var, 50);


void loop() {
// read the sensor:
int sensorReading = analogRead(A0);
int delayTime = 175;
int durationTime = 5000;

// print the sensor reading so you know its range

// map the pitch to the range of the analog input.
// change the minimum and maximum input numbers below
// depending on the range your sensor's giving:
int thisPitch = map(sensorReading, 0, 1023, 50, 2000);

// print the mapped range

// play the pitch:
tone(8, thisPitch * 2.5, durationTime);
tone(8, (thisPitch * 2), durationTime);
tone(8, (thisPitch * 1.5), durationTime);
tone(8, (thisPitch), durationTime);

//second string
thisPitch = thisPitch * .5;

tone(8, thisPitch, durationTime);
tone(8, (thisPitch * 1.5), durationTime);
tone(8, (thisPitch * 2), durationTime);
tone(8, (thisPitch * 2.5), durationTime);

//second string
thisPitch = thisPitch * 1.5;

tone(8, thisPitch, durationTime);
tone(8, (thisPitch * 1.5), durationTime);
tone(8, (thisPitch * 2), durationTime);
tone(8, (thisPitch * 2.5), durationTime);

//second string
thisPitch = thisPitch * .5;

tone(8, thisPitch, durationTime);
tone(8, (thisPitch * 1.5), durationTime);
tone(8, (thisPitch * 2), durationTime);
tone(8, (thisPitch * 2.5), durationTime);


Try it out for yourself and let me know how it works. If you have any questions, don’t hesitate to ask.

Thanks for reading and HAPPY BENDING!

Delay circuit with the melody making sequencer!

It was suggested to me by Iowanchef to add a pt2399 circuit to this crazy little bend.

Ask and thou shalt receive!

I had a soldered up pt2399 delay circuit just laying around doing nothing so I threw it in parallel with the sequencer’s speaker. I ran an output off to my guitar amp. It worked perfectly. CHECK IT OUT!!!

The video is a little long but there is some really cool sounds toward the end.

Now that I think of it… why didn’t I turn on the drum machine? Oh well, next time.

Happy Bending!

Little Composer – The Melody Making Sequencer

I can’t really finish my drum machine right now because I am waiting for some 3-way switches to arrive from ebay. I need to learn to stock up. I am way to impatient for this whole “12-21 days shipping from Thailand” thing.

What to do instead?
I picked up this second hand synth the other day. It’s called the Little Composer by Leap Frog.

05_03_2010 001

It’s really more of a musical learning toy for children. It has an 8 note measure that you can adjust each not from low to high C (only in the key of C, of course). The Little Composer has 7 voices. There are three instruments; piano, guitar and violin. There is also a woman voice that tells you what the note is. A, B, C, D, E, F, G… That is really annoying. And finally there is a dog, a cat, and a frog. These only sound good when they are clocked really high or really low.

05_03_2010 004

There are also a bunch of songs too but they are all lame. I suppose that they could be kind of cool clocked real low because the would go for a really long time but I don’t think I will include them in the final build.

05_03_2010 005

The ocatave has no sharps or flats. which is unfortunate. It would be awesome to have a full octave. Of course, if one were to add a pitch bend, one could hit any western key that they wanted.

05_03_2010 007

Naturally, that’s just what I did. I removed the timing resistor and added a pitch bend in series with about 5k ohms (i think),

Here comes the cool part. When you press the play button, this melody-maker only goes through the measure once. This is a real bummer. How is a child suppose to learn music if they can only play one measure at a time. So, I added an LFO from a 555 timer.

05_03_2010 003

It took me a little while to figure out how to make it work. I ended up just running a 10k out of the output from the 555 timer and going strait to the pin on the control circuit that handled the play button. That worked great. I thought I was going to have to do some weird thing with a transistor that I wouldn’t fully understand for the next couple of years. Luckily, that wasn’t the case.

So, if you tune the LFO frequency, as I am doing in the video below, you can get the melody maker to act as a sort of 8 note sequencer. SWEET RIGHT?!

Next, I think I am going to have to add the usual LFO to modulate the pitch and maybe even a tremolo. I’m also waiting on some frequency divider chips from ebay. Maybe one of those will make it in too.

05_03_2010 002

Happy Bending!

Drum Machine – Work in Progress

This is the project I am working on right now.

This is a drum machine that I am making. The core is from an old electronic drumstick that I found at a thrift store. It is opperated by swinging it just like a real drum stick and stopping it in mid air. When the electronic drumstick stops, it makes a sound.

The default sound is a snare but there are also two more sounds. If you hold one button and swing the electronic drum stick it makes a bass drum sound. The other button has a crash symbol.

The drum machine is being timed by a 555 timer and a 4017 decade counter. It will have a 3-way switch for every step in from the 4017. One for each sound. In addition, it will have an on/off switch and (if all goes well) a pitch adjustment for every step.

Happy bending!