Hi everyone. I am nearly there on my laser harp project. I only have the sensor left. (albeit this may be the most difficult part) I am thinking of using a light to voltage sensor like the TSL257 or a light to frequency sensor like the TSL235R. I'm not sure which would be best or if either would work. I was thinking about adding something like a bandpass filter to filter out any light that isn't on the wavelength of my green laser(562nm) but I am unsure how to do this in combination with one of the light sensors. Any ideas out there from someone who has experience with either of these sensors or with bandpass filters?
It is always a good idea to remove extraneous influences from sensors, in your case light of the wrong wavelength.
A narrow bandpass filter would be best, but they tend to be expensive. Edmund is one source.
yeah they do look kind of expensive. Is there a correlation between wavelength and voltage or frequency output from the light sensors that I can then feed through a cap/resistor bandpass filter? I read the datasheets but the one seems to produce higher voltage the brighter the light and the other says something about outputting a square wave which I'm not really sure how to use. Just trying to think of another way to get this done.
Is there a correlation between wavelength and voltage or frequency output
Not enough to be useful, even if light of only one wavelength is involved.
If you have many wavelengths simultaneously impinging on the sensor, it is utterly hopeless to sort them out after they interact with the photodiode. The filter MUST go between the sensor and the light source.
Narrow filter bandwidth = better discrimination = more expensive. Go with what you can afford and see if it works. Perhaps green cellophane will work well enough.
You can filter more than just the color of the light. Look at how TV remote controls work. The transmitter sends data with a specific carrier frequency. The receiver responds only to that frequency, like a radio tuned to only one station.
I guess that you want to use green light for the look. That's probably not going to excite a regular IR receiver properly. If you could use red, then that's probably close enough to infra-red to make it work. But I'm sure that you could find a "remote control" receiver that does respond to green light.
Light-to-frequency converters sound like very old technology. Probably expensive too. Modern IR receivers are available for cents or micro cents.
If it gets really difficult/expensive to detect the green light, then perhaps you can put an IR laser alongside the green one and use that to detect the fingers.
Ha. I was on the same track last night. I am looking at a differentiator circuit. Output is based on the rate of change of the input. So I can use a light to voltage converter and then run the output voltage from that into a differentiator circuit. I am thinking this might take care of ambient light(hopefully) and give a larger voltage when a flash of light hits the sensor. Any thoughts?
If you can modulate the laser source in the kHz range, use a bandpass filter to make your receiver sensitive to the frequency of modulation. That will vastly improve the sensitivity and noise reduction. Even in that case, an optical filter is still a great help.
You won't be able to use a light to voltage or frequency chip in that case, you will need a photodiode and proper amplifier/filter. See this overview on photodiode amplifiers.
Brewskio:
Hi everyone. I am nearly there on my laser harp project. I only have the sensor left. (albeit this may be the most difficult part) I am thinking of using a light to voltage sensor like the TSL257 or a light to frequency sensor like the TSL235R. I'm not sure which would be best or if either would work. I was thinking about adding something like a bandpass filter to filter out any light that isn't on the wavelength of my green laser(562nm) but I am unsure how to do this in combination with one of the light sensors. Any ideas out there from someone who has experience with either of these sensors or with bandpass filters?
I don't see any description of what is REFLECTING the laser light. IF it is a common back-silvered glass mirror, you will get two reflections. One from the front glass surface and one from the back surface.
Paul
Brewskio:
Hi everyone. I am nearly there on my laser harp project.
I'm guessing musician's fingers are reflecting the light.
Another post reminded me that LEDs used as photodiodes are very wavelength sensitive. So, you could try an orange or red LED as a photodetector for your green laser.
LEDs are most sensitive to light about 50 nm blue-shifted from their respective emission maximum, so look for an LED with emission maximum of around 610 nm, to match your 562 nm laser line. Try several different LEDs as their efficiency varies greatly.