Make music with lasers and a load of hot glue

The basic principle of a laser harp is simple. It’s a series of laser beams that, when broken, play a note. They vary from small hobby kits to huge performance instruments designed for stadium gigs. We purchased a small harp kit for £8.59 (including postage) from e_goto Processors Store, on AliExpress. This is a ‘framed’ style of laser harp, where the laser beams shine onto sensors. Some other laser harps are ‘frameless’, where the lasers shine out to the distance. Frameless ones are more complex, but can work better at larger scales, particularly for performances.

The instructions came in Chinese, but they’re not really needed. The PCB is marked with what component goes in which hole (everything’s through-hole). The only slight issue was the orientation of the LEDs, as it wasn’t clear which hole the plus sign in the silk screen referred to. Under close examination, we decided it was slightly closer to one hole than the other, and this proved correct.

The kit works by having a series of seven light-dependent resistors (LDRs) lined up on the bottom circuit board. These have a couple of centimetres of black plastic tubing slipped over them to block out most of the light. There are seven laser diodes, each lined up with an LDR, so that the beam fires straight down the tubing. When a laser beam is blocked, the light stops falling on the LDR and this triggers the harp to play a note.

Power trip

When working with lasers, it’s always sensible to consider the potential dangers. The laser harp came with 5 mW laser diodes – brighter than we were expecting – and the dots they create can be dazzling, even when hitting a non-reflective surface. The UK Government advice on this laser power (Class 3R) is: “The laser beams from these products exceed the MPE (maximum permissible exposure) for accidental viewing and can potentially cause eye injuries, but practically, the risk of injury in most cases is relatively low for short and unintentional exposure.”
This kit is right at the highest end of laser power that’s considered an acceptable risk for general use, and you should consider this before purchasing.

The laser diodes have to be aligned by hand, and secured in place with hot glue. This isn’t the best-looking system (and be careful not to burn your fingers as you do it), but it does work. The tubing around the LDRs means that they don’t have to be perfectly accurate – as long as they’re within the circle of tube, they should work. If you’re concerned about aesthetics, you might want to enclose the top part of the harp in a more pleasing-looking material. Alternatively, you could easily create your own upper section – there’s no need to use the PCB provided. The only wiring on this is to provide the laser diodes with power, so fitting this into wood or a 3D-printed structure should be fairly straightforward. However, if you do choose to modify the frame, you should consider whether this makes accidental exposure to the beams any more likely.


The board is based on an STC89C52 microcontroller. It’s programmable, so in theory you could update this to behave in any way you want. However, this isn’t a chip with much of a hobbyist community around it, so we wouldn’t recommend trying to reprogram it unless you’ve got experience with truculent microcontrollers. A far easier way of modifying this project (should you wish to) would be to take connections from the input pins on the STC to another board, such as an Arduino Uno (the STC is a 5 V part, so you’ll need appropriate protection if you use a 3 V microcontroller instead).

The biggest disappointment with this kit is that it’s only able to play one note at a time, so the classic harp sound of running your fingers across the strings is impossible to recreate unless you’re willing to modify the software. However, if you’re careful, it can be used to play a short tune. The harp creates a square wave which has a very computer-y bleep sound that’s perhaps most commonly associated with chiptune-style music. There is a certain charm to this sound, but it’s not very reminiscent of a harp. Close your eyes, wave your hand around in the harp, and the noises will take you back to the halcyon days of 1980s gaming.

This is a fun kit to put together, and makes an unusual curiosity. If you’re willing to go further, it offers some possibilities for hacking into more visually pleasing forms. However, unless you’re willing to tinker about with the microcontroller, you’re not going to get anything capable of making interesting music.

More features from HackSpace magazine