Simple Photodiode On/Off circuit

[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?

[Jumper]

If you have 8 A/D channels avaliable on your pic you can easily detect a 0.2V change. It should give a difference for aprox 30-40 steps in the reading if you power your pic from 5V.

Disadvantage? YES several - slow and needs you to scan the A/D frequently to detect if the laser is on or not.

External comparators - Cheap, easy to use and fast. Use a potentiometer to set the reference level on one input. Use the other input for the LDR (4.6 to 4.4V signal) and have one comparator for each LDR.
Connect each comparator to a digital pin and then you can read the port to get all 8 values at the same time. You could also try to connect the 8 outputs from the comparators to pins that have interupt function and then you can wait for an interupt to fire and then you know you have a hit on a target.

It all depends what you are trying to make, ambient light can be a problem and there could be better ways to solve your design.

8 LDR targets ... and a laser.. my guess is Laser Tag system or maybe some kind of Laser fence

Any one else up for some guessing?

[bodgetts]

Thanks for the help. I will try your idea with the comparators, as it sounds like the easiest to implement. Can you recommend the best chip for this particular job?

I am making a shooting target game, so when they pull the trigger I will turn on the laser and see if it has hit any of the targets. I will pulse the laser at a slightly different rate each time to stop people cheating.