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With this inexpensive kit, you can tinker in an hour a funny little children's piano that does not work with real keys, but with photoelectric couplers that are interrupted by the finger. |
Introduction to the 'photoelectric piano music player'
The end result of this kit
The video below, taken from a sales page on AliExpress, shows you how to play simple tunes with this little piano. The little instrument has only seven 'keys', but that is nevertheless enough to play some simple songs. Note that this little piano does not work polyphonically, but monophonically. So the instrument can only play one tone at a time. If you 'push' two 'keys' at the same time, nothing happens.
Where to buy and what does it cost?
The building kit can be bought via AliExpress, Amazon and IC-Station. You will pay for it, without shipping costs:
- AliExpress: € 9.04
- Amazon: € 6.31
- IC-Station: $ 12.59
At IC-Station, you can find this kit under the order number GY21489.
Delivery of the building kit
For once, the components are not delivered in the much too small plastic bag, the standard for Chinese building kits, but in a large cardboard box. The components are packed in several plastic bags. Thanks to this packaging, transport damage to components is out of the question. The one IC and the IC socket are pinned in a piece of foam to protect the pins. A construction description is not included, but can be downloaded.
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| The packaging of the parts. (© 2025 Jos Verstraten) |
The delivered components
In the picture below, all the parts included in the package are clearly displayed. The parts coloured brown are the six plexiglass panels that make up the housing of the piano. As you have seen in the video, these are black. The brown colour comes from the protective sheets stuck on either side of the plexiglass to protect it from scratches. On the bottom left you can see the power lead, which allows you to connect the device to a 5 Vdc power supply via a USB-A connector. On the far right is a piece of shrink tubing, which plays an important role in the construction of the light barriers that form the 'keys'.
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| Overview of all components supplied. (© 2025 Jos Verstraten) |
The manual
You can find a superb 17 page manual in English on the internet, where really every step of the construction is explained by a photo. We assume it was compiled by IC-Station, as most building kits supplied by this company are accompanied by such fine manuals. You can download this manual from the link below:
The PCB for the photoelectric piano
In the picture below, we have joined both sides of the 60 mm by 89 mm PCB together. The complexity is not too bad, even an absolute beginner in soldering can complete this PCB without any problems after some practice.
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| The two sides of the PCB. (© AliExpress) |
The electronics of this piano
Unfortunately, we could not find the schematic of this piano anywhere on the internet. However, the operation is clear. Heart of the circuit is an 89C52 microcontroller. The ‘keys’ consist of seven white LEDs shining equally on LDRs.
At rest, these have a very low resistance. If you press such a ‘key’, the light beam is interrupted and the resistance of the corresponding LDR rises into the MΩ range. This action is spotted by the microcontroller and the firmware causes the controller to generate a tone of a certain frequency. This signal is presented to a mini-speaker via a one-transistor amplifier.
Building the circuit
Soldering the components on the PCB
Although the full soldering of the PCB is described in detail in the English manual, here we give a short summary of the soldering sequence:
- Solder 7 x 1 kΩ resistor
- Solder 7 x 4.7 kΩ resistor
- Solder 2 x 10 kΩ resistor
- Solder 1 x 100 Ω, 1 W resistor
- Solder 1 x 12 MHz crystal
- Solder 7 x fully transparent white LED in locations D8 to D14, the shortest connecting wire being the cathode. However, the connecting wires must first be bent 90° in the right direction, see the figure below.
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Bending and mounting the white LEDs. (© IC-Station, edit 2025 Jos Verstraten) |
- Solder 2 x 22 pF ceramic capacitor
- Solder 1 x DIP-40 IC socket, note the correct position!
- Solder 1 x 10 μF electrolytic capacitor at location C2, you must mount it flat in the middle of the IC socket. Note the polarity!
- Then cut seven 1 cm pieces from the shrink tubing and mount them on the LDRs, see the figure below. Shrink these with a hot air gun and bend the connecting wires at a 90° angle. With these parts, it doesn't matter which direction you do it. Then solder these parts onto the PCB at locations 5516 and align the pieces of heatshrink tubing so that the white LEDs shine directly in.
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| Preparing the LDRs. (© IC-Station, edit 2025 Jos Verstraten) |
- Solder 7 x blue LED in locations D1 to D7. These parts have a slightly milky colour. Note the polarity, the shortest wire is the cathode.
- Solder 1 x S8550 transistor in location Q1. Note the position!
- Solder the second 10 μF electrolytic capacitor at location C1. Note the polarity!
- Solder the small speaker to location BEL1. Note the polarity here too!
- Solder the power connector to location DC5V.
- Solder the on/off switch to location K1. Mount the red button on the axis of the switch.
- Carefully push the microcontroller into the IC socket. You will first have to bend the pins slightly inwards. Note the position and make sure all pins are in the contacts of the socket.
The final result
The picture below shows the completely assembled PCB. Connect the PCB to a USB connector of your PC or laptop using the power cable provided. Of course, you can also use an inexpensive 5 V mains power supply. After pressing the on/off switch, the seven blue LEDs briefly run through two cycles, after which the device is ready to be played.
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| The completely soldered PCB. (© 2025 Jos Verstraten) |
Assembling the housing around the PCB
This is extensively documented with photos in the manual, so we won't waste words on that here. The end result, see the photo below, looks really cute!
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| The end result of this kit. (© AliExpress) |
Our experience with this electronic piano
The little device works as it should and is easy to play. Just one comment: the second ‘key’ sounds much louder than the others because the frequency of that ‘key’ is apparently close to the buzzer's resonant frequency.
The output signal
Of course, we could not resist studying the output signal sent to the speaker on the screen of our oscilloscope. You can see the result in the oscillogram below.
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The signal across the buzzer. (© 2025 Jos Verstraten) |
The generated frequencies
Our oscilloscope puts the frequency of the measured signal on the screen. So why not check the signals from the seven 'keys'? These are the measured frequencies:
- Key 1 (Do): 561.8 Hz
- Key 2 (Re): 627.9 Hz
- Key 3 (Mi): 794.2 Hz
- Key 4 (Fa): 738.6 Hz
- Key 5 (Sol): 837.5 Hz
- Key 6 (La): 937.1 Hz
- Key 7 (Si): 1,048 Hz
The piano is apparently tuned in the fifth octave, where the official frequencies of the notes Do and Si are:
- Do (C5): 523.25 Hz
- Si (B5): 987.77 Hz
So the little piano sounds quite out of tune, which, however, is hardly noticeable when playing simple children's tunes.
Photoelectric Piano Music Player Kit
