I want to be able to detect when a bronze key (Balinese Gamelan) gets struck by a mallet (hard wood) without effecting the keys vibration, so a piezo is likely out.

Example of the instrument:

enter image description here

Is there a cheap optical vibration sensor that could go under the key and detect when the key has been struck?

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    \$\begingroup\$ is a directional microphone out? \$\endgroup\$ – Grady Player Mar 21 '13 at 4:32
  • \$\begingroup\$ Do you require just the information that a specific key (or keys) has been struck, do you also need the intensity of a strike, or is the actual vibration profile needed too? \$\endgroup\$ – Anindo Ghosh Mar 21 '13 at 4:57
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    \$\begingroup\$ have you tried an electric guitar pickup? (the magnetic field will dampen the vibration, probably imperceptibly) \$\endgroup\$ – markrages Mar 21 '13 at 5:03
  • \$\begingroup\$ why do you think piezo will affect vibration? The one in my guitar bridge doesn't seem to affect the strings much. \$\endgroup\$ – markrages Mar 21 '13 at 5:03
  • \$\begingroup\$ @markrages I think the OP implies that piezos bonded to the keys as contact pick-ups will change the vibration characteristic of each key - which they will. The piezo pickups on guitars are mounted on the bridge not on the strings, and do not need to distinguish between individual strings, just pick up the entire sound pattern. \$\endgroup\$ – Anindo Ghosh Mar 21 '13 at 5:20

A simple optical solution is to use an IR LED / Sensor paired module, such as used in various hobbyist projects for heartbeat sensing.

For example, the Vishay Semiconductor TCRT5000:

TCRT5000 (Source: Digikey)

These emitter-sensor pairs sell on eBay.com for as little as $1.89 for 10 pieces, including shipping.

As you will see from the Heartbeat Sensor project linked above, the implementation of a system that senses really small movements is fairly easy. Whether the amplitude of oscillation from the Gamelan keys is sufficient to be detected thus, can only be determined by experiment.

In order to maximize sensitivity, the reflective optical sensors would need to be positioned perhaps 1 mm away from the bottom surface of each key, exactly midway along the length of the key as measured between the suspension points - that is the point with maximum amplitude of vibration.

An alternative approach, given that the keys are brass, is to have a pair of tiny wire coils fixed under each key, again perhaps 1 mm below the bottom surface: One coil at one end of the key (the emitter), the other at the dead center (the sensor). Alternatively a single emitter coil wound in a stretched narrow rectangle could span the ends of all the keys at once.

Energize each of the emitter coils with a small AC sine wave well beyond human hearing, say 50 KHz. The center coils will pick up a varying amount of signal depending on the varying distance of the center of the key from the coil. The brass key will work as an induction shunt, bridging the emitter and sensor coils.

This approach is experimental at best, and will require some trial and error around number of turns of coil, ideal frequency to drive the emitter coil at, and interference with other electronic equipment such as microphones or pick-ups.

The advantage will be size, using really thin and tiny coils of fine enamel wire, since the currents involved will be minuscule.

Another advantage is that the actual sound, or velocity / intensity of striking each key can be picked up - these can be used as audio pick-ups and not just impact sensors.

For the latter purpose, an emitter frequency higher than 50 KHz would be preferable - Nyquist limit comes onto play.

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Unless there are considerations like amazingly low latency, I'd consider a microphone w/ some frequency processing to differentiate notes.

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I agree with Anindo Ghosh, an infrared transceiver is one of the best ways to detect these small movements without touching the surface. But I'd like to add some advice on how to use these properly.

Firstly, the one pictured is not the best type. You really need one like this:

Opto transceiver

It's flat on top, meaning that light doesn't leak in and out as much as the more open design.

Secondly, you will need to choose a suitable reflecting surface. Something like shiny metal works very badly because it simply reflects the light right back into the transmitter. The optimal reflective surface is actually white paper, so that some amount of light is scattered sideways into the receiver. So if you can stick a small round white paper sticker onto the back of each key without affecting the sound, then that's a good idea. However, the keys don't look like shiny metal, the actually look a little more diffuse, so you may not need the paper.

Thirdly, the optimum distance to place the reflector: You actually need to select the distance of the device, the current flowing in the LED, and the pull up resistor needed by the phototransistor. Here's how I would go about it. Firstly, choose the LED's resistor by consulting the datasheet for the device. How much current do they recommend for the LED? Next decide on the distance you need between the sensor and the brass. How much do these keys actually move when they're hit? Do they flex a lot? (sometimes things can flex a lot more than you expect).

Cymbal Crash

Once you've determined the distance required so that the brass can't touch the detector, you can choose the pull up resistor for the phototransistor. Choose this value so that the Output signal is 50% of the supply voltage when the brass is stationary.

IR detector circuit

Lastly, you'll probably want to ac-couple the signal from these sensors to the rest of your electronics, so that things like temperature fluctuations don't cause a false detection.

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