LaserHarpKit

#700 Laser Harp Kit

Build and test an 8051-based laser harp kit.

Here's a quick demo..

Notes

There's a common C51-based "laser harp" kit that is available in various packagings from sellers on aliexpress, icstation and other market-places.

I picked up one of the kits from an aliexpress seller to check it out. There are various build and demo videos out there, for example:

Parts

The kit comes with all the parts needed to build the harp, including USB power cable and speaker. Note: in the final build, I replaced the speaker with a piezo buzzer - the sound quality is not much different!

Item	Qty	Ref
1kΩ	11	R1-7, R16-19
10kΩ	8	R8-15
30pF	2	C3-4
10µF	2	C5-6
12MHz crystal	1	Y1
3mm blue LED	10	D1-10
STC89C52	1	U1
50 pin DIP socket	1	U1
wire	2	U1
40mm copper pillar	2	U1
speaker	1	SPK
5V laser	7	JG1-7
power jack	1	DC002
USB power cable	1
8550 PNP	1	Q1
5516 LDR	7	GM1-7
pushbutton	2	K1-2
small copper pillar	4
DPDT switch	1	KG
screw	4

Circuit Design

The circuit is quite straight-forward:

• 7 lasers are pointed at 7 LDRs monitored by 7 GPIO ports
• 2 switches are wired to GPIO ports
• the microcontroller polls/reads the LDR inputs and switches
• the microcontroller generates the appropriate sound output (square wave)

Here is a transcription of the circuit in Fritzing:

Testing the Behaviour

The two buttons control the mode of the harp:

• K1: play a tune. Rotates between "Happy Birthday" and a Chinese nursery rhyme "世上只有妈妈好" (Mother is the Best)
• K2: selects between 3 note ranges octaves (low, medium, high)

The output frequencies:

Note	Octave 1	Octave 2	Octave 2
1	141	281	561
2	153	317	627
3	178	355	705
4	187	375	740
5	211	421	837
6	237	473	937
7	266	530	1047

Here are some sample scope traces of the lowest and highest notes in the first octave:

Conclusion

After playing around for a few minutes, a couple of limitation are quite apparent:

• why 7 notes?? There are plenty of spare GPIO pins, so why not 8 for a full octave?
• tuning: I don't think the note frequencies match standard tuning
• button responsiveness: the code doesn't handle concurrently pressing the range button and playing a note, making it very hard to play a tune across multiple octaves
• "string" sensitivity: it does not appear very responsive to making/breaking contact with a laser

Next Steps?

I haven't found the 8051 source code or tried to download it fro the chip. Hoever given the problems with the laser harp - most of which can be solved in code - I don't think that's a big loss.

The obvious next step is to redo this project but with:

• 8 lasers/notes
• new code that is more responsive and handles concurrent button presses while playing a note. Maybe attempt this with 8051, or switch to an AVR/Arduino.

Credits and References

• DIY Kit C51 MCU Laser Harp Kit String DIY Keyboard Kit Electronic Parts 7 Strings Electronic DIY Kit Technology Piano Music Box - aliexpress
• DIY Kit C51 MCU Laser Harp Kit String Electronic Keyboard Kit DIY Module Electric Production Sensor Music Controller - same product from icstation
• STC89Cxx series MCU Data Sheet