Simple Photodiode On/Off circuit

[readitaloud]

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

[bodgetts]

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.

[bodgetts]

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?

[cncmachineguy]

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

[aratti]

You can use the pic comparator as an amplifier and solve the high impedence problem. See Microchips comparator tips & tricks @ page 31.

Cheers

Al.

http://www.google.it/url?sa=t&source...TP7z0_z8fDfPxA

[bodgetts]

Thanks for the suggestions guys. The problem is that I am going to have 8 targets. If these were all just LDRs, then it wouldn't be an issue and I could hook them up with 1k pull ups and it would be fine.

Because I need so many, I can't really use the comparators, etc of the PIC. Is there a simpler way where I can use these photodiodes on the general I/O pins?

I am also hunting around for some diodes that maybe have a narrow frequency range, so as to only respond to the laser. Then when there is no laser it would be a lot closer to 0 instead of 2 MOhms, etc.

[bodgetts]

Maybe there is another way?

If I measure the voltage coming through the photodiode, it is 4.6v normally, and drops to 4.4v when the laser is directly on it. How could I detect such a small voltage change?