Building an optical pickup using a photodiode

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lingyueqing
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Joined: Thu Feb 08, 2018 8:04 am
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Building an optical pickup using a photodiode

Post by lingyueqing » Thu Feb 08, 2018 9:04 am

I am building an optical pickup using a photodiode connected to a LM741 op-amp(datasheet:http://www.kynix.com/uploadfiles/pdf9675/LM741CH.pdf). My circuit is similar to this one:
Image
photodiode circuit

Except that I've added a passive high-pass filter after the output of the op-amp, to eliminate DC (since I am using 0V and +12V as my V- and V+, respectively). I use Rf=500K Ohm (is this way too much?). In addition, I have an LED adjacent to the photodiode that serves as a light source. The LED is powered by 5V and the op-amp is powered by 12V, both from a PC power supply. The photodiode and LED are connected to the circuit using a 2m long guitar cable ("PL").

The circuit works and produces audio signal when I modulate the intensity of the light shining on the photodiode, but my problem is that the signal is very noisy. I can hear/see two types of noise:

Electrical noise similar to a noisy electric guitar pickup. I suspect that it originates in the long cable (or the tip of it, where the photodiode and the LED are conected) collecting ambient electromagnetic noise. This noise is present all the time, even when no light is shining on the photodiode.
Another noise is present only when a signal is generated, i.e only when I modulate the light intensity. I suspect it is a result of amplifying thermal noise, since my gain is very high.
I would like to know what is the best approach, or in other words where to start in my effort to eliminate the noise:

Improving the signal to noise ratio at the source, i.e by optimizing the physical conditions (ambient light, precision of the position of the photodiode, etc.).
Using a different circuit - I've seen many suggestions on the web and started with the simplest.
Using a different op-amp, one that is more suitable as an audio pre-amplifier.
Improving the shielding of the pickup itself, to eliminate electromagnetic ambient noise.
Using batteries as power source instead of the PC power supply (I am thinking maybe part of the noise is coming form the mains).
If none of the above, What would be your suggestion?

steve
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Re: Building an optical pickup using a photodiode

Post by steve » Thu Feb 08, 2018 12:04 pm

lingyueqing wrote:Electrical noise similar to a noisy electric guitar pickup.
You mean "hum" / "buzz"?
If so, the first measures to take are:
  • Use batteries to power the circuit rather than a mains transformer. If you have to use a mains transformer, you will need to use adequate smoothing and voltage regulation (which
  • may be a bigger problem than the rest of the project put together).
  • Use shielded cable to connect the circuit to your sound card (cheap microphone cable is usually good enough).
  • Enclose the entire circuit (including batteries) in a metal enclosure, and connect the shielding of the mic cable to the enclosure. (Cut or drill a hole in the enclosure to let light hit the sensor).
  • Keep cable runs to a minimum.
lingyueqing wrote:Another noise is present only when a signal is generated, i.e only when I modulate the light intensity. I suspect it is a result of amplifying thermal noise, since my gain is very high.
This is "hiss"?

A small value capacitor in parallel with the "Rf" feedback resistor will limit the bandwidth gain of the circuit. It allows high frequencies to feed back from the OpAmp output to the inverting input, so you get no more than unity gain at very high frequencies.
The value of the capacitor should be selected large enough to cut out unwanted high frequencies, but not so large as to interfere with the signal that you want to pass.

You could also use "post processing" in Audacity to clean up the signal (such as Noise Reduction), though not in real-time.
9/10 questions are answered in the FREQUENTLY ASKED QUESTIONS (FAQ)

Trebor
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Re: Building an optical pickup using a photodiode

Post by Trebor » Thu Feb 08, 2018 1:56 pm

lingyueqing wrote:I am building an optical pickup using a photodiode connected to a LM741 op-amp ...
If the light you shine on it is mains-powered, and imperceptibly flickering at audio-frequency, that that could induce main-hum noise in the optical-receiver, analogous to LiFi, (such noise would only occur when the device was illuminated).

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