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February 16, 2018 09:06

Rapid response light sensors for use with an Arduino

I am trying to build a laser harp MIDI controller using an Arduino, small laser modules and photodetectors. I have constructed one using CDs photoresistors but the response to the interruption of the light beam is too slow to allow fast tunes to be played. I am trying to identify a suitable sensor (probably a photodiode or phototransistor) which would be suitable for direct connection to the inputs of the Arduino, would react quickly to the interruption of the light beam and would be reasonably affordable. Can anyone recommend anything suitable from the rs-online catalogue?

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February 22, 2018 08:36

You are welcome, let us know how you get on with the project, sounds interesting!

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February 20, 2018 10:33

Thanks for all your work, Boss. I think you've pointed me in the right direction.

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February 20, 2018 10:33

Hi Geoff,

The wiki article was jus to show a photodiode and general discussion, probably wasn't a good article in this case. I have searched high and low to find a good simple circuit to interface to the Arduino. The best example I can find at the moment is in this article, https://arxiv.org/pdf/1606.02774.pdf see fig 6. p16 just the photodetector part which will need three wires +5V Gnd and the analog input. I hope this helps.

Totally different.... I also found this method which is a light to frequency converter.
http://www.instructables.com/id/Highly-sensitive-Arduino-light-sensor/ which I share just as an alternative method that could be of interest.

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February 19, 2018 08:53

Hi Boss, thanks for answering, I think I understand what you're saying but I have a couple of questions, I would be grateful if would answer them.
1) The wiki article talks about the gain of the amplifier but I don't understand where the power to produce the gain come from unless there is power input at Vout which seems counter-intuitive since that is the output from the circuit. Could you explain please?
2) I believe that, If I built one of these circuits to act as an input sensor to an Arduino, I would have to connect Vout to the Arduino input terminal and the earth connections to the earth (negative) terminal of the Arduino. I also believe that the output would be high (5v) when light was striking the photodiode and low (near 0v) when the light was interrupted. Am I right?
3) There are three variables in the equation relating Vout, Rf and Ip and I believe that Vout needs to be 5v, however this leaves me with two variables which are interdependent. How can I determine the value of one of them tor my application?

Thanks in anticipation
Geoff

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February 19, 2018 09:03

CdS or LDR are slow, often around 10mS for small light changes and over one second for large changes. I would have thought any photo diode (with amplifier) would be considerably faster with a MHz bandwidth which you may need to limit.
Have a look at the first circuit here:
https://en.wikipedia.org/wiki/Transimpedance_amplifier
Choose Rf for the gain that matches your light source and add a C across Rf to slow it down.
The BPW34 may be suitable
https://uk.rs-online.com/web/p/photodiodes/0247240/
but almost any visible spectrum diode could be used, if you know the laser frequency you could possibly optimise to match that but most photo diodes are broad spectrum.

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