Hello everyone. I made a bass guitar with optical pickups. This was intended as an exercise to familiarize myself with optical pickups and to see if a slide bass would work. I’m happy with where I got with this and am ready to put it to bed. There are others working on optical pickups professionally (Light4Sound) and otherwise (Infrared String Bass). My work was not meant to compete with these, but to simply find out how it all works. I share it here in hopes that some other tinkerer could get inspiration from it, and for myself to put down this project and move on to the next.
I've attempted to attach pictures of the completed bass. I knew I would need plenty of space underneath the strings to be able to experiment with different types and arrangements of pickups. Because of this I made no attempt to give it frets and instead used the limitation as a way to explore if a slide bass would work (spoiler, it didn’t). The bass is simply a 2x4 with some metal bits welded together and some heavy bass strings. The tuning pegs are just bolts with a hole drilled through them for the string to pass before it is wound. The lack of gearing in the tuning pegs made it a challenge to tune, but it was workable.
The optical pickup circuit is completely analog and is, at heart, just a voltage divider. The photoresistor and the fixed resistor in series will produce a signal where they meet. I isolate the signal with a capacitor and then sum up the four signals at the end. It’s powered by a 9 volt battery which gives plenty of signal, on par with a traditional pickup. No extra amplifier is required.
The tone is exceptionally clean with no noticeable interference. I’d like to elaborate on how free from interference it is, but it’s difficult to describe the nothingness that is added to the tone. The signal comes only from the vibration of the string and nothing else. One interesting side-effect is that because the pickup only detects the lateral movement of the string, the sound of dragging a pick or a fingernail along the string is muted as compared with a traditional pickup. It is possible that the shadow of the string can either be larger than the photoresistor or misaligned. Both situations cause a portion of the vibration not to be picked up. This can lead to some interesting “optical distortion” tones and I think is worth exploring.
A big challenge with optical pickups is finding a clean source of light. Whatever signal is in the light source will make its way into the pickup. This is extremely obvious with any source drawing power from a 60hz mains outlet. Even LED lights that rectify the mains power do not do so well enough to eliminate the 60hz hum from the signal. Battery power is a requirement. I used a small bright lithium powered rechargeable spotlight that I hung from the ceiling. With the other lights off I was able to get a clean shadow on the photoresistors.
There has been effort made by others to contain the light source and pickup into a single package which has the added benefit of not allowing “dirty” light to be sensed. I made no such effort. I wanted to see the effect of the environment on performance and tone. I think there could be interesting ways to play with the tone by playing with the light source. For example, a fan in between the pickup and the light source makes a neat on-off effect. One thing I wanted to research is the wavelength dependence of the photoresistors. If they are dependent on wavelength then a color changing light source could give an interesting “color” to the tone. While isolating the light from the environment may be a requirement for a professional musician, it should be realized that not isolating the light can be a rich source of novel effects.
As for experimenting with a slide bass I was not able to get the results I wanted. I assumed that I would be able to make some good, if simple, music by using a slide. I wasn’t. I could not get the sustain I wanted and had to use too much pressure on the strings to get any sustain at all. That combined with my lack of slide guitar skill made it extremely difficult to compose anything interesting.
Overall I am pleased with the results I got and the lessons I learned. This project or something like it is well within most people's capabilities. The circuit is simple and the parts cheap. I hope people keep experimenting with this and I am interested to see where the optical pickup goes in the future.
I've attempted to attach pictures of the completed bass. I knew I would need plenty of space underneath the strings to be able to experiment with different types and arrangements of pickups. Because of this I made no attempt to give it frets and instead used the limitation as a way to explore if a slide bass would work (spoiler, it didn’t). The bass is simply a 2x4 with some metal bits welded together and some heavy bass strings. The tuning pegs are just bolts with a hole drilled through them for the string to pass before it is wound. The lack of gearing in the tuning pegs made it a challenge to tune, but it was workable.
The optical pickup circuit is completely analog and is, at heart, just a voltage divider. The photoresistor and the fixed resistor in series will produce a signal where they meet. I isolate the signal with a capacitor and then sum up the four signals at the end. It’s powered by a 9 volt battery which gives plenty of signal, on par with a traditional pickup. No extra amplifier is required.
The tone is exceptionally clean with no noticeable interference. I’d like to elaborate on how free from interference it is, but it’s difficult to describe the nothingness that is added to the tone. The signal comes only from the vibration of the string and nothing else. One interesting side-effect is that because the pickup only detects the lateral movement of the string, the sound of dragging a pick or a fingernail along the string is muted as compared with a traditional pickup. It is possible that the shadow of the string can either be larger than the photoresistor or misaligned. Both situations cause a portion of the vibration not to be picked up. This can lead to some interesting “optical distortion” tones and I think is worth exploring.
A big challenge with optical pickups is finding a clean source of light. Whatever signal is in the light source will make its way into the pickup. This is extremely obvious with any source drawing power from a 60hz mains outlet. Even LED lights that rectify the mains power do not do so well enough to eliminate the 60hz hum from the signal. Battery power is a requirement. I used a small bright lithium powered rechargeable spotlight that I hung from the ceiling. With the other lights off I was able to get a clean shadow on the photoresistors.
There has been effort made by others to contain the light source and pickup into a single package which has the added benefit of not allowing “dirty” light to be sensed. I made no such effort. I wanted to see the effect of the environment on performance and tone. I think there could be interesting ways to play with the tone by playing with the light source. For example, a fan in between the pickup and the light source makes a neat on-off effect. One thing I wanted to research is the wavelength dependence of the photoresistors. If they are dependent on wavelength then a color changing light source could give an interesting “color” to the tone. While isolating the light from the environment may be a requirement for a professional musician, it should be realized that not isolating the light can be a rich source of novel effects.
As for experimenting with a slide bass I was not able to get the results I wanted. I assumed that I would be able to make some good, if simple, music by using a slide. I wasn’t. I could not get the sustain I wanted and had to use too much pressure on the strings to get any sustain at all. That combined with my lack of slide guitar skill made it extremely difficult to compose anything interesting.
Overall I am pleased with the results I got and the lessons I learned. This project or something like it is well within most people's capabilities. The circuit is simple and the parts cheap. I hope people keep experimenting with this and I am interested to see where the optical pickup goes in the future.
Attachments
Many years ago I tried using some opto-interrupters similar to these on a conventional 6 string electric guitar since I needed a separate output for each string. The results were less than satisfying, so I gave up and created some single pole magnetic pickups. They got traded in for some piezoelectric bridge saddles from Graphtech which were OK.
The opto interrupters gave different results depending on how the string was picked, and the output varies as the string goes through varying vibrational modes during decay.
The opto interrupters gave different results depending on how the string was picked, and the output varies as the string goes through varying vibrational modes during decay.
@cricha5 - this looks great! As the general principle is fairly simple, I've built a 1-string prototype for myself and it indeed works like a charm (lot of bass, no noise). I've used a white LED as the light source and it looks like the key factor in success is to find one that provides a sharp shadow (mu intuition is that an led without a lens or a small, smd led, should provide better results than a regular led).
I have two questions, though:
1. you're using capacitors and resistors - whats their function? Do they serve as some kind of hi-pass filter to filter out possible DC offset?
2. Do you combine the outputs in parallell or in series?
Best,
Krzysztof
I have two questions, though:
1. you're using capacitors and resistors - whats their function? Do they serve as some kind of hi-pass filter to filter out possible DC offset?
2. Do you combine the outputs in parallell or in series?
Best,
Krzysztof