Simple Photodiode On/Off circuit


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  1. #1
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    Default Simple Photodiode On/Off circuit

    I have a photodiode that I will be hitting with a laser diode, and I need to be able to determine when I have hit the 'target'.

    What is the simplest way to do this with a PIC? ie. Can I hook it up with just a pull-up resistor in a similar fashion to a simple button input? I only need to know when it is on and off.

    Thanks

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    Default Re: Simple Photodiode On/Off circuit

    Quote Originally Posted by bodgetts View Post
    I have a photodiode that I will be hitting with a laser diode, and I need to be able to determine when I have hit the 'target'.

    What is the simplest way to do this with a PIC? ie. Can I hook it up with just a pull-up resistor in a similar fashion to a simple button input? I only need to know when it is on and off.

    Thanks
    Yes, as you said, it would be one of the simplest ways to do it.
    Instead of a pulll up or pulll down resistor, use a trimmer pot to adjust the voltage level on the PIC pin when there is light on LDR.

    And for the code, use a delay routine to avoid false readings from ambient light(s) or sudden flashes.
    "If the Earth were a single state, Istanbul would be its capital." Napoleon Bonaparte

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    Default Re: Simple Photodiode On/Off circuit

    Bodgetts

    There are a lot of ambient noise sources these days. TV remotes use a modulation frequency of around 53 KHz to modulate their controllers light source. An "on" signal is actually modulated light. Very long code sequences of the mfg's choosing are used for differentiating button functions. The code sequence is unnecessary for your application. You may want to modulate the laser on/off at some carrier frequency. Use some form of limiting after the detector. Feed the output of the limiter A/C coupled into one of the PIC timers to correlate correct modulation frequency. If you come up with the correct count after integrating over a period of time then you can say you have hit the target.

    - Martin

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    Default Re: Simple Photodiode On/Off circuit

    Ok, the photodiode that I have is tuned for the wavelength of the laser (or close enough).

    When I hook the diode up to a multi-meter, the resistance is sitting about 2-2.4 M Ohms. If flash a regular light on it, it doesn't really change. As soon as I flash it with my laser, it jumps up to 4.5-5.2 M Ohms.

    Should I be able to make this trigger the PIC input if I use say a 3M Ohm resistor? I have tried this with 3 x 1 M Ohm resistors in series, and I can't seem to get the PIC to recognise the change.

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    Default Re: Simple Photodiode On/Off circuit

    If I use a standard LDR, I can use a 1k resistor and detect the difference between light ( < 1k Ohm) and dark, which is in the 10 M Ohm range. I understand that this works because when 5v comes in through a 1k resistor, it will go to the PIC because the 10 M Ohm of the LDR is much higher.

    However, the photodiode that I am using has a range of 2 M Ohm for dark/no laser, and about 4-5 M Ohm when the laser strikes it. If I use 3 M Ohm, the voltage that does get through to the PIC is way too low to trigger an input. Any ideas here?
    Last edited by bodgetts; - 6th August 2011 at 00:51.

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    Default Re: Simple Photodiode On/Off circuit

    maybe set it up as a voltage divider, but use the comparater or A/D to decide if it's on or off?
    -Bert

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