A few weeks ago during one of my usual thrift store exploration I picked up a Kawasaki I-Soundz drum kit. Even before doing any bending I started having a great time with this toy. It has a large and varied vocabulary of samples and surprisingly high quality stereo audio driven by an internal TDA 2822 operational amplifier. I was also pleased to find that rather than the tactile buttons I’ve seen on many toy drum pads the Kawasaki kit is driven by piezo disks similar to higher end drum synthesizers. Unfortunately the device does not offer any polyphony but given the price point I did not expect it.
The drum pads also contain a number of interesting on board rhythm samples. Unfortunately these samples only play for about 30 seconds before stopping (regardless of whether you are playing the kit). Since I was looking for something I could use to set up repeating rhythms while I played other devices this left me with a need. I wanted to set up external triggers for the drum sounds. By using these external triggers in conjunction with my recently completed 4017 gate sequencer I could turn the Kawasaki drum pads into an 8 step drum machine and unlock a world of new rhythms.
The Build
The first step whenever you are setting up external triggers on any toy is to create a ground share point. This can be done by simply adding a banana jack or binding post and connecting it to any ground point on the circuit. If you are using grounded connections from your trigger source (such as 1/4 inch, 3.5 mm or RCA cable) then you can simply connect the ground point to the ground on your trigger input jacks rather than having a separate plug.
Because (in it’s normal operation) this toy is triggered by piezo disks most of our work is already done for us. When the pads are hit the piezo disk creates a trigger pulse. This pulse is sent to the base of an internal transistor (highlighted above), which switches the circuit on momentarily and causes it to play the sample associated with the drum pad hit. All we need to do to set up external triggers is send our external signal to the base of these transistors to switch them the same way the triggers from the piezo do.
One thing I am admittedly not incredibly comfortable with is soldering onto SMD (surface mount) circuits. That being said this is something that I feel I need to improve and develop my comfort with. Surface mount technology becomes more prolific and through hole circuitry becomes rarer and rarer each day. Further developing my comfort with SMD will open up near endless possibilities of new circuits I can work with. For this reason I have made it a goal not to shy away from these circuits. I will take the necessary care but am determined to become as familiar and comfortable with them as I am with more traditional components.
Where possible I soldered my leads to resistors adjacent to the internal transistors as there was less risk of damaging these components.
To solder I held my soldering iron to tinned wires to heat them up prior to touching the board. Once the solder on the tinned wire was liquified I lowered the wire and soldering iron to the soldering point together. I raised the soldering iron from the board almost immediately after touching the two down and held the wire in place until the solder solidified. The key here is to spend as little time as possible with the soldering iron on the board. The components are significantly smaller and the solder connections are significantly weaker than traditional through hole circuitry. This means any excess heat on the board can damage components or loosen their solder connection knocking them out of place.
Since my solder points were weaker and the circuit was so crowded I also added a small amount of hot glue to each connection. This gives each connection added strength and also insulates the wire from the other components to ensure it doesn’t touch any other solder points.
For reference the solder points I used for the triggers were on R10, R13, R8, Q3, R2 and R4.
Finally connect the wires from the trigger points to the trigger inputs of your choice. I was short on plugs so I have used bolts but you can easily use banana, 1/4 inch, 3.5 mm, RCA or any other type of input you have on hand.
Now that I have the external triggers set up on this drum pad I will be going back into the circuit and completing some more traditional circuit bending. I will be adding a pitch bend and hopefully will be able to find some other interesting bends and effects to give me an even wider range of sounds to use.
