Sunday, November 19, 2023

Analog to Digital Adapter for Alesis Turbo Mesh Kick Drum

Earlier this year we bought an Alesis Turbo Mesh electric drum kit. The kit works well, but my drum teacher advised me to switch from the spring/trigger kick pedal to a proper kick drum pedal. I ordered a kick pedal and sensor from eBay, but was bummed to discover that the Turbo Mesh kit only works with a "switch" style kick pedal

Fig 1: The Goal (Images via

The Alesis Turbo Mesh kick pedal connects using a standard 1/4" mono jack. When the pedal is depressed, the two conductors in the mono jack are connected and the control module recognizes it as a "kick." You can easily demonstrate this by touching both parts of the kick drum 1/4" connector with a wire. 

The higher quality kick sensors are analog, meaning instead of a binary on/off (like a switch) they change the voltage on a signal wire, which is interpreted by a microcontroller. This is how electric drum kits get different volume levels for different intensity strikes. In this case, I needed to convert the analog signal from the kick pedal to a binary on/off before the signal reached the Alesis drum controller. 

Fig 2: Connections (Images via and

I bought an Arduino Redboard from Sparkfun for about $20. The Redboard has analog and digital I/O pins, making it an easy choice for this project. I also had some relays left over from the Fort Lock project. I bought a couple of 1/4" jacks. 

Part List

Digital Resources


Redboard Setup

The Arduino IDE didn’t work out of the box with the Redboard, which requires an additional support package. 

I had to add this line to the Arduino IDE preferences to add additional boards to support.

After updating the preferences, I was able to select the Redboard and connect!

Reading Analog Values

I connected the kick drum sensor to an analog pin on the Redboard and used the example serial reader code to read the voltage levels from the kick sensor.

Fig 3: Serial reader sample code from Sparkfun

Using the built-in LED I was able to reliably detect "kicks" using the analog sensor and looking for sudden increases in voltage. The Arduino software does not provide time guarantees, but the main loop ran fast enough that any latencies were not noticeable. 

Writing Digital Values

To trigger the "kick" I needed to "connect" the two conductors from the Alesis control unit's kick cable. There were a couple of options available to accomplish this. Using a multimeter, I found that the Voltage difference between the two kick sensor conductors was 3 Volts. 

To make the signal consistent, I had to create a common ground connection between the drum control unit and my Redboard. I connected a solid wire from the Redboard to part of the breakout connector on the bottom of the drum control unit. 

Fig 4: Connection point to establish common ground

I tried sending 3V directly using the Redboard's digital output pins, but the latency was too high (it sounded like about 300-500 milliseconds). My suspicion is that the Redboard could not supply the voltage quickly enough the internal resistance in the drum control unit. 

To overcome this issue, I used a relay that acted like a switch to close the connection between the two signal lines in the kick drum cable. I can't find the exact product I used (it was from eBay) but it was similar to this (Amazon). I understand from talking with friends that an OpAmp could have achieved the same thing, but I have not learned how to use those yet. 

I designed a simple case in FreeCAD and printed it. 

Fig 5: 3D Model of Enclosure

Fig 6: Wiring Diagram

Saturday, July 8, 2023

Build Log: Keycode Access Control for the Fort

 My sons requested a keycode-based access control system for their backyard fort. I'm pretty happy with what we created. In this blog post I'll talk through the details of what we built and why. 

The keypad and custom mounting bracket

Parts List


  • Adafruit Metro ESP32-S2
  • The Adafruit Metro ESP32-S2 is a fairly inexpensive ($20) controller that includes wifi and Adafruit's Qwiic I2C connector system. 


Electronic Lock


  • Fuses (PTC Fuse Resettable Fuses 72V 0.5A 500mA RXEF030 Series)
  • Diodes
  • Qwiic Button



I worked with my friend Peter to develop requirements for the lock. The requirements are written up on github here. In summary, the lock: 
  1. shall not allow a child to get stuck
  2. shall be resistant to critters/teenagers
  3. shall not require physical keys
The current solution meets all three of these requirements. 

Cyberphysical System Models

This project gave me a good excuse to spend time learning the Assume Guarantee REasoning Environment (AGREE). AGREE is a model-based tool that helps you build and verify an argument for correctness in your architecture. 

This specification, written in the Architecture Analysis and Design Language (AADL) describes a magnetic lock actuator that physically actuates (locks) only when power is applied to the system and the lock gets an actuation signal. The first section, "lock_actuator" describes the desired behavior in terms of assumptions and guarantees. The section section, "lock_actuator.magnetic_actuator" describes the state transitions of a particular lock design. 

This type of reasoning is helpful when thinking through design choices. For example, I considered using a solenoid lock like this version from SparkFun: 

But this kind of lock does not meet the conditions described in my assumptions and guarantees. Namely, (as I verified through bench testing) the SparkFun solenoid lock physically actuates (locks) when power is not applied. Such behavior would violate the requirement, "shall not allow a child to get stuck," because a power loss could cause a child to become stuck in the fort.   

Electrical Architecture

Architecture Figure 1: Electrical System

Electrical Summary

A MOSFET relay with a dedicated 12VDC source controls power to the electronic lock. At Peter's recommendation, both sides of the 12VDC supply are protected with resettable fuses to avoid shorting the system (if, for example, something is wired backward). 

The ESP32 board uses one of its General Purpose Input/Output (GPIO) pins to trigger the relay. Input to the ESP32 comes from two sensors (a button and a keypad) both of which are connected via the I2C protocol using Sparkfun's Qwiic cable system (which I absolutely love). 

Flyback Diode

When the magnetic field of the electromagnetic lock collapses, it can induce a current in the reverse of the normal direction. To avoid damaging the microcontroller, I used a Flyback Diode as described on this page. Normally current flows in the B to A direction (illustrated on the figure), but when the lock's power is disengaged, the collapsing magnetic field induces a current in the A to B direction, making a circuit back through the electronic lock to dissipate the remaining power. 

Physical Assembly

The lock I purchased was intended to by mounted on the interior of a door frame. The fort door is an exterior door, so this mounting approach would not work (the lock would be exposed to weather). To mount the lock entirely inside the fort, I welded a "Z" bracket from two pieces of "L" steel. 

Architecture Figure 2: Installation. After taking this photo I had to shim the steel plate (attached to the black painted steel) so that it made good contact with the magnet.

I installed the microcontroller, relay, and exit button in a standard 2 gang electrical box. 

Architecture Figure 3: Exit button and installation box

My son and I designed the keypad mounting bracket in TinkerCAD (this was great practice for him using a micrometer). You can find the .stl file here

Architecture Figure 4: Mounting Bracket for SparkFun QwiicKeypad

Software Architecture

Architecture Figure 5: Software

The software design is pretty simple. As shown in Architecture Figure 5, the microcontroller simply waits for input from either the keypad or the exit button, storing up keypresses to compare to a saved code, resetting if input times out. 

Development Environment

This was my first time using CircuitPython and the Mu Python editor. Overall I'm still on the fence about CircuitPython. It is certainly convenient once it is working, but the set up process is not as straightforward as I'm accustomed to with Arduino. 

1. Install CircuitPython to Board Flash Memory

  • Download esptool (I used version 4.4)
  • Find ESP32 in my device manager (on Windows this was COM8)
  • Write circuitpython bootloader to flash
    • .\esptool-v4.4-win64\esptool-v4.4-win64\esptool.exe --port COM8 --after=no_reset write_flash 0x0 .\tinyuf2-adafruit_metro_esp32s2-0.12.0\combined.bin
Once CircuitPython is loaded on your board's flash memory, the board will appear as a storage device.

2. Install Mu Editor

You can download Mu here. Overall I'm content with Mu, but the CircuitPython model seems to require I only use a single file, "" which makes version control cumbersome.

3. Install Required Libraries

CircuitPython libraries are not the same as regular Python libraries. You can find many CircuitPython libraries here. Copy the files for the devices you are using from the lib directory of the CIrcuitPython library to the lib directory on your board. For example, I copied sparkfun_qwiickeypad.mpy to the lib directory of my board so that I could do:

import board
import sparkfun_qwiickeypad
i2c = board.I2C()
keypad = sparkfun_qwiickeypad.Sparkfun_QwiicKeypad(i2c)


Access Testing

My kids helped me test the lock - both the entry PIN and the exit button work as desired. We also talked through the emergency protocol if they need to leave the fort and the exit button is not working (just unplug it). 

Power Consumption

I used a KASA wifi power outlet adapter to measure power consumption. It holds steady at about 9 Watts for the lock alone (not including the controller). 

Final Thoughts

I learned a ton doing this project. The practice doing circuit design was particularly valuable, as I made (and corrected) lots of mistakes. 

Saturday, June 3, 2023

How I built a stand for the UCS Millennium Falcon

Freestanding Millennium Falcon display with lighting

The Ultimate Collector Series (UCS) Millennium Falcon is an incredible LEGO set. It is absolutely huge, 33" by 23" and 28 pounds. My office has one in our common room, and this year I decided that I wanted to build a stand for it; a standard office table was not doing it justice. 

Acceptable, but boring, way to display the coolest set ever

I started by making paper sketches of stand options. I wanted it to be free standing so that visitors to the office could be greeted in proper geeky fashion. I wanted the 'Falcon to appear to be taking off and easy for viewers to see. 

Initial design (digitized)

I purchased some scrap steel from Coremark Metals and set about making a small scale prototype from 1/4" and 1/8" hot rolled steel. I used Metal Inert Gas (MIG) process welding with the Hobart Handler given to me by my brother Graham, who also makes some pretty cool stuff. 

Small-scale prototype

I was pleased with the small-scale prototype, and my coworkers were too. Bolstered by their feedback and the stability of the small design, I started work on the full size version. I scanned the 'Falcon with Polycam to get measurements and locations of its landing struts, then went back to Coremark to purchase enough steel for the full size version. 

Polycam lets you pull measurements from a 3d model

I purchased about 50 pounds of steel for this project:
  • 1/8" Steel sheet for the landing pad
  • Squares of 1/4" steel bar for the base (to make it heavy)
  • 4' Length of square steel tubing for the upright
I assembled the base first. I used seven square 1/4" steel bars together to make a U shaped base that allowed space for wiring to come down through the upright, with a larger square 1/4" steel bar on top. I drilled a small hole in the center to run wires through. I tacked the small squares together before doing most of the welding, which helped to minimize warping of the steel from welding. 

Base design

I used several additional pieces of 1/4" and 3/16" steel to build a small box to house electronics at the base and cover the gap left at the bottom of the base. I was happy with the upright angle I used on the prototype, so I copied it to the angle to the upright and cut it with an angle grinder. I beveled the edges a little bit to try to minimize the profile of the weld. 

Wiring box

Attaching the landing pad was tricky, as my welding magnets and clamps did not work well to situate the upright. Eventually I made it work.

Full size stand welded and cleaned for painting

I spray painted the stand with a metal primer and glossy black paint (I was excited and probably would have gotten a more durable paint coat if I'd waited longer between primer and paint). I brought it into the office and was pleased with the result.

Fit test

I taped off the locations of the landing struts, then brought it back home again to trim the landing pad and assemble the lighting. I purchased a bunch of pre-wired LEDs and some speaker wire. I set the lights up in three strings, so that I could have multiple lighting settings. I decided to run a positive and negative line for each string to maximize flexibility. 

Trimmed, taped, and wired

I installed a couple of (unnecessarily) large switches at the base, as well as a motion sensor. For the motion sensor I 3D printed an additional mounting bracket, since it had to be located above the base to detect movement. 

Bracket for motion sensor, with rectangular hole at the base for a bar magnet and round hole for wiring. 

I used 1 1/2" bar magnets for several parts of the assembly. First, I used the bar magnets to help anchor the 'Falcon's landing struts so that the force wasn't entirely on the rear struts. Second, I used a bar magnet to attach the motion sensor bracket to the base. Third, I used a bar magnet to hold the battery in place inside the base. Finally, I used bar magnets to attach a steel cover to the wiring chamber of the base. 

I ran blue LEDs to the front and rear of the ship, and red LEDs to the upper gunner's station. I'm pretty happy with the result. 

The gunner's station lights up when people walk by

Everything is powered by a single 9v battery. The only power consumption is from the LEDs and motion sensor, so I am hoping it will last several weeks on one charge.