555 Based Piezo Trigger

I’ve always been drawn to drum pads and kits. They are lots of fun and offer a slightly more tactile method of control then rows of pots and switches. So today while I was playing around on my breadboard I was drawn to pull out some piezoelectric disks and start experimenting.  What I’ve come up with is a very simple drum trigger circuit that you can build and experiment with.

This circuit uses a 555 timer set up in monostable mode. A monostable 555 timer will output a square wave pulse whenever it receives a trigger pulse from the piezo disc at pin 2. The pulse output from the 555 can then be adjusted through the 500K ohm pot placed between V+ and pin 7. The output pulse is then sent into the base of a 2N3904 transistor which works as a gate between the audio source and the speaker. This means when the pulse from the 555 is high the audio will pass through the transistor and when the pulse ends and the 555 output goes low the transistor will block the audio from passing.

If you are interested in adjusting the pulse length beyond what is available using the pot this can be achieved by adjusting the electrolytic capacitor between pin 6 and ground. By lowering the value of this cap you can shorten the range of pulse lengths available. Conversely by increasing it you can access a longer range of pulses.

By setting up 4 or 5 of these piezo trigger circuits you could create a fairly versatile set of drum pads. Since the audio source can be switched out or developed further there’s a lot that you can do to expand on the acoustic possibilities of your drum kit. You can try experimenting with different oscillators, Filters, LFOs, White Noise Generators or anything you want.

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Adding External Triggers – Kawasaki I-Soundz Drums

Trigger Inputs Drum Pad

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.

external triggers drum machine
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

external triggers drum pads
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.

External triggers solder points
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.

External triggers drum machine

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.

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Build A Simple Gate Sequencer

Gate Sequencer

For the past week or so I’ve been working on building a 4017 based matrix gate sequencer. I originally started thinking about this project after purchasing a set of Kawasaki electronic drum pads from a local thrift store. I wanted to create a tool I could use externally to trigger the drums in a continuous loop. As I began to design This build though i began to realize it’s full potential went well beyond that.

As this sequencer goes through each step it outputs a voltage (approximately 5V) at the top most pin on the matrix. By connecting this top pin to any of the 6 pins directly below it you can send this signal out through the associated output on the side of the gate sequencer. Because these signals are being sent at approximately 5V they are perfect for switching low voltage transistors such as the 2N3904 (essentially allowing it to turn on or off an electronic switch wherever you send it). By using these outputs to switch on or off transistors they could be used to trigger a sound from a toy, gate an oscillator, turn on or off a channel on a mixer, trigger an envelope or anything else you desire.

Click here for details on setting up triggers in a drum toy

Gate sequencer 4017

I’ve also included some basic controls common to more traditional sequencers like the Baby 8. These include a rate control to adjust the clock speed, a hold switch which pauses the sequence, a step selector switch which allows you to select how many steps the sequencer goes through before restarting and a clock out for syncing other sequencers or circuits to the gate sequencer’s clock rate.

Parts List:

  • 555 Timer IC
  • 4017 Decade Counter IC
  • 2 – 4.7 K ohm Resistor
  • 1 – 100 ohm Resistor
  • 200K ohm potentiometer
  • 1 – 10 uf Electrolytic Capacitor
  • 1 – 0.1 uf Ceramic Capacitor
  • LEDs (one for power and one for each step)
  • 1N914 Switching Diodes (One for each step)
  • Rotary Switch (number of positions equal to number of steps plus 1)
  • Toggle Switch – power
  • Toggle Switch – hold
  • Hook up wire (lots)
  • Ribbon Cable (Strands equal to number of steps)
  • Proto-Board
  • Clock out jack (I used 3.5 mm headphone jack)
  • Ground Connection Jack (I used banana)
  • 6 – Output jacks (I used bolts but banana jacks are ideal)
  • Matrix connections (I used pin headers but you can use whatever you have available, requires 1 out and 6 in per step)
  • 9V battery clip


This is the schematic I drew up while building the gate sequencer. For simplicity sake I did not draw out all of the steps but they will each mimic the first two shown on this schematic. Bear in mind though that the 4017 output pins do not go in order, make sure to check the pin out diagram to make sure you are setting up the steps in the correct order. For 8 steps you should be pulling from pins 3, 2, 4, 7, 10, 1, 5 and 6 in order.

I wanted to mention as well as it is not clear on this schematic. If you are using fewer than all 10 steps from the 4017 counter you will need to wire the output of the next pin higher than the ones you have used to the final position of your rotary switch so that the counter resets after going through the steps you have used rather than the full 10. For example my gate sequencer uses 8 steps (outputs 0 to 7 on the 4017) so I wired output 8 (pin 9) to the final position of my rotary switch.
AND gate for gate sequencer
If you are using this device to trigger circuit bent toys you may also run into an issue where you are not able to trigger the same noise for two consecutive steps. This is because if you send the signal to the same output for multiple steps the output will remain high rather than sending a pulse for each step. I was able to find the fix above from Peter Edwards of Casper Electronics who used it in a similar project he built a few years ago. In order to correct this you can place an AND Gate on each output and send the clock pulse into the second input on each AND gate as shown above. This will cause the output to pulse in time with the clock when the signal from the matrix stays high for multiple steps.

The Build:

The first step of the build was to populate the circuit. Following the schematic I had created while testing and designing my gate sequencer I placed and soldered all of the on board components. I also used a number of short leads to put the steps in order on the board so that I could work with them easier going forward. One thing I want to mention is the row of diodes shown in the above picture were actually removed and placed on a secondary board (more details to follow) to simplify the finished product.

At this point I also mapped out the surface of my project box and populated the off board components (switches, knobs and LEDs). Due to the number of components on the box I used a piece of graph paper cut to the size of the surface to plan the device locations then used a pin to mark each one through the paper. From here I drilled the holes for the larger components and secured them in place.

pin headers

Due to the sheer number of connectors required to build the matrix I was not able to use banana jacks (which would have been ideal). What I did have on hand though were a number of male to female jumper cables and a pile of pin headers. I cut 8 of the female heads for the top posts and used individual pin headers for the connections. To mount the individual pin headers i ran fairly high gauge solid core wire through the holes and soldered them to the short ends of each pin header. Next I pulled the wire back down the hole until the plastic guards on the pin headers sat securely against the top of the box. To secure them I poured a substantial amount of hot glue onto them from the underside of the box.

gate sequencer ribbon cable

In order to limit the rats nest I foresaw forming between the top of the box and the main board I used a small scrap piece of proto-board as a junction. From here I ran all the connections needed for each step. The ribbon cable shown here is attached back to the main board (orange is step 1 through to black for step 8). From the board there is an orange cable for each step to go to the rotary switch (attached to the reset pin), a green wire to connect to the LED for each step and a red wire to go to the top pin of the matrix for each step. Note the red wire is after a diode on each step while the orange and green are before it.

Here is a picture after each wire has been soldered to its place on the back of the lid. I also used hot glue to attach the small proto-board to the lid of my gate sequencer. Make sure you test all of the connections thoroughly prior to gluing it down. Check for any bleed between steps and that all the solder connections are strong. Once you glue it down it will be very difficult to modify.

I connected all of the pins on the matrix (excluding the top row) in rows and connected each row to the corresponding output on the side of the box. One more coating of glue and I was ready to make the final connections. First I connected the ribbon cable to each of the 8 steps on the main board. Then I worked my way around the components which needed to be connected to the main board. Once everything was connected I wired up the battery, power switch and power indicator LED. I secured the battery with some Velcro and after some brief troubleshooting (There was a faulty switch I had to change) it was ready to go.

I am currently in the process of setting up trigger bends on my drum pad. Once they are completed and running smoothly I will have another article up describing how you can use your new gate sequencer to trigger noises from circuit bent toys  and down the road you can expect to see me using gate and trigger voltages to control a variety of other devices. Within the next week or so I should also have a demo video of this device uploaded for you to check out.

Thanks for visiting and happy soldering!

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VTech Apple Part 5 – 555 Trigger Oscillator

circuit bending, loop trigger, 555

I have to admit for some time I have been stalled with my Vtech Alphabet Apple circuit bending project. I love the toy aesthetically and have always felt like there should be more bends available then what I was able to find. However even after hours of experimenting and dissecting this toy I was left feeling somewhat underwhelmed with the results. Over the weekend though I brought it back out determined to turn it into a more functional instrument, and to do that I needed to create a trigger oscillator.

If you want to get caught up before going forward don’t hesitate to visit my previous posts on this toy:

VTech Apple Part 1 – Kill Switch and Line Out

Vtech Apple Part 2 – Exploration and Pitch Adjustment

Vtech Apple Part 3 – Voltage Starve

Vtech Apple Part 4 – Body Contacts

Since I hadn’t had any luck finding a loop on the board I moved on to less straight forward methods. I decided what I needed was a way to send a signal at repeating intervals to one of the contacts on the button matrix to trick the Vtech Apple into thinking a button was being repeatedly pressed and trigger a repeating sound. By generating this signal independently I could manipulate its frequency and control to suit my needs.

Once I had a clear definition of what I needed to get the job done the solution seemed all to clear, what I needed was a 555 timer. By setting up an astable 555 timer as a trigger oscillator I could route the square wave signal into the button matrix to repeatedly trigger the button (or buttons) of my choosing. Further by using a potentiometer I would be able to adjust the frequency of the square wave and therefore control the time between button presses as needed.

555 astable
This is a simple mock up of the circuit I used. Note that for this to work the positive voltage must be supplied by the toy itself and the ground must be share with the toy as well. This is easily accomplished by running the power from either the positive power connection on the Apple’s circuit board or directly from the kill switch installed in Part 1 . Just ensure it is connected at or past the kill switch so that the kill switch will remove power from the oscillator as well. Similarly the ground can be connected to any ground point on the circuit.

Astable 555 circuit
After testing my plan using a breadboard I put together this small 555 timer circuit on a scrap piece of perf board I had laying around from a previous project. I did my best to keep everything as small and compact as possible as my space inside the toy is somewhat limited. I’ve seen some circuit benders using what is called the “Dead Bug Method” in these situations to further minimize the size of the circuit. When building a dead bug circuit the components and connections are soldered directly to the pins on the IC rather than onto a piece of perf board. This can be an excellent way to shrink the circuit for those really tight fits but also leaves you with a more fragile product so since I could get away with using a board in this toy I did in order to get more stability and durability from the circuit. I will revisit dead bug circuits in a future post but in the interim there are many demonstrations of the method on YouTube if you deem it necessary.

Wiring 555 Oscillator
Once I had my 555  trigger oscillator circuit built the next step was to install it in my Vtech. I started out by planning positions and drilling holes for the potentiometer, switch and LED. Once these were in place I began the process of wiring the leads I had left on the 555 trigger oscillator circuit to their respective locations on the toy. On the diagram below I have marked the approximate paths of each wire upon installation. In planning this mod I did my best to limit the number of wires crossing between the back and front sections of the toy as these wires tend to get put under a lot of stress when the toy is being opened and closed. To achieve this I pulled power from my kill switch and sent ground directly to the negative terminal on the battery box. Since the switch and pot are mounted on the back portion this means only the pulse out wires have to cross over to the front half of the apple.

Wiring Vtech Apple

You may also notice that there are two pulse out wires leading from the switch to the button matrix. I used a 3 position on-off-on switch for this bend which allowed me to send the pulse to two locations based on the switch position (as well as nowhere in the off position) By connecting the opposing sides of the switch to different positions on the key matrix I am able to choose between two different buttons when running the oscillator. If you were so inclined and had the space to work with you could take this even further using a rotary switch or patch bay to allow you to select where the pulse was being sent. If there is a specific key you are after which you are not finding by touching the pulse to the solder points on the matrix try using your probe to connect sets of two points on the matrix together. If you find that this is necessary to get the input you desire this can be done by bridging the two points with a transistor and feeding the pulse into the base (more on that in a future article).

Finished Vtech Apple Loop
Once everything was wired I secured the 555 circuit and LED with hot glue, taped down all the loose wires and closed up the toy. I have to say after playing with it a bit I am really enjoying this modification. I’ve been able to produce a host of strange noises and effects and without having to use a hand to continually press the buttons I feel like I’m finally able to take full advantage of the other bends on this device. I’ve been having particular fun using the power starve to produce glitches in conjunction with the continuous oscillating noise produced by raising the rate of the 555 to high frequencies.

That’s all for today but I hope you guys have fun. Happy soldering!

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