DJ Baja Blast’s Desktop Laser Harp Measures Hand Speed While Adding Tilt and Distance Controls

Photo credit: DJ Baja Blast
Derrick, who goes by DJ Baja Blast, spent years turning a lingering idea into a finished desktop instrument after seeing a laser music installation at Meow Wolf in Santa Fe. Version 2 of his laser harp sits in a frame of cherry and walnut, finished with hardwax oil after careful sanding and some epoxy work to close imperfect miter joints. Fourteen low-power lasers aim downward at light-dependent resistors mounted in the base. Each resistor sits in a voltage divider so the Teensy 4.1 microcontroller can watch the analog signal in real time.

When a hand crosses a beam, it blocks some of the light that would otherwise reach the resistor, causing the voltage to rise dramatically. When the hand moves away and the beam snaps back into place, the voltage lowers back down. The time between these two events is then converted into the velocity of the MIDI output note and the volume of the synthesis, both of which are handled onboard. A quick flick produces a strong attack while a slower motion yields a softer one. Two 74HC4051 multiplexers keep the wiring manageable by routing all fourteen sensors through just a few pins on the Teensy. The same board runs the Teensy Audio Library, which generates Karplus-Strong string sounds along with pad and synth patches that can be switched among presets.

We also have an MPU6050 accelerometer within the instrument that acts as a tilt sensor, allowing him to control effects such as reverb, delay, and even bit crushing. Alternatively, he can map it to change the pitch, as it is all in the code. The ultrasonic distance sensor at the bottom allows for theremin-style control of all of the effects we’ve discussed. An encoder wheel on the side provides more continuous control, and a normal pedal connection allows us to plug in a hold pedal so he can just start recording and overdubbing loops as needed. If someone wants to connect to an external synth, he can simply use the MIDI output, while the onboard speaker or auxiliary port will handle the local audio.

A small fog generator is installed at the rear of the instrument, and it emits brief bursts whenever he plays a note, which is useful for keeping track of the laser lines during practice or performance. He also modified the infrared remote for the fogger so that the Teensy can control it remotely using optocouplers. An RGB light strip beneath the beams illuminates whenever a string is interrupted, which is useful for keeping an eye on things. Then there’s the 128-by-64 OLED display, which he had to be creative with because it was already attached when he chose to install a mirror to pull the display out from behind the panel. When the side panels need to be removed, they simply unbolt and connect via JST connectors, which is a bit different from how he had it initially. He also had to take steps to protect the sensitive analog lines from interference from neighboring wiring and displays, and copper tape was the preferred option.

Cheap laser diodes have a horrible habit of burning out eventually, so each one is inserted into a PETG housing with a beautiful ball-end tip that is locked with a thumb screw once the beam is properly oriented. The builder discovered that even when you print parts with great care, they don’t always line up properly, so being able to make a few tweaks made all the difference. There’s no point making perfect alignment a major priority when you can just experiment with it till it works. Cable routing, color coding, and the fact that the connectors index together all help to keep people from making stupid mistakes later on, like having a display go up in smoke, which, believe us, is no joke. The first version of this device was fully 3D printed a few years ago and was on the modest side, but he moved to Fort Collins, got some time in a local maker space, and resulted in a much better version that played just that little nicer and looked heaps better.

Future plans include making the most of the added PSRAM by allowing it to sample audio from a mic or auxiliary input, then plugging it into those fancy string models, sorting out the underperforming vocoder and polishing it up, swapping out the ultrasonic sensor for a laser distance unit because why not, and adding a fan to help the fog spread out more evenly. Files and a comprehensive write-up are available on GitHub and the builder’s website, so check out the pinouts, voltage dividers, and code. The best part is that it still fits on your desk, but with all of the MIDI, pedals, and sensors, you can start creating truly distinctive sounds without having to reach for a bunch of other gear.
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