Photodiode circuit help

[audioguru]

Hi Futterama,
Aren't you using the Fishay BPW82 IR photodiode in the black light-reducing case?
It should work pretty well but is very wide angle. It should be placed in a tube to shield it from daylight (the sun's heat reflected from clouds is IR).
As Awright says, a lens would reduce pickup from ambient heat and light and make it much more sensitive to on-axis IR.
You could also replace the 470k with 100k or 47k to reduce the sensitivity. :lol:

 

[awright]

RC car IR sensor

What you are seeing is simply that irradiance at your sensor from solar energy in the band passed by the epoxy "IR filter" of your sensor (roughly from 770 nm. to 1050 nm.) is much greater than the irradiance at about 880 nm. from your emitters. As I stated earlier, the world is very bright at near IR in daylight.

I was recently trying to measure displacement of an 880 nm. target (the Opto Diode OD-50L) at a distance of 30 to 50 feet on a sunny day using a 5" aperture reflecting telescope. Having difficulty getting a good signal, I viewed the target area with a Find-R-Scope IR viewer. While the OD-50L was clearly the brightest object in the view, the entire scene was very bright and highly visible. Passing a US$100, 880 nm., 10 nm. passband interference filter in front of the viewer objective made almost no difference in scene brightness! The "filter" on your BPW82 is EXTREMELY wide (280 nm.) and offers negligible effective filtering of ambient IR.

Don't forget, sunlight has a very broad spectrum, meaning that the amount of solar energy passing through a filter is (roughly) directly proportional to the bandwidth of the filter. Thus, if you use a filter with half the bandwidth of the 280 nm "filter" (really, just the natural transmittance of the epoxy encapsulant) of the BPW82, you will cut the solar energy immission in half, or get a 3 dB reduction of interfering solar energy. Not much benefit until you get down to a quite narrow filter bandwidth. Ideally, to get the best benefit from an IR filter, you need a filter passband that matches the emission bandwidth of your emitter. I had to resort to a HeNe laser, cube corner retroreflector target, and a 1 nm. passband filter to get a reasonable S/N ratio out of a silicon Position Sensing Diode in my telescope.

I think you will find that your emitters have a fairly broad beam angle. All the emitted IR that does not fall onto the sensor aperture is simply wasted by illuminating the landscape. You can get a reasonable idea of the emitted beam angle by observing the output from your sensor as you rotate the emitter around the vertical and horizontal axes at a fixed location in the field of view of the sensor.

The intensity of the IR from your emitter follows the inverse-square law. That is, at double the range, you get 1/4 the intensity due to spherical spreading. So, if you are having trouble with detection at a few mm. or cm. on your bench, you will be in deep stuff at a meter or two, even neglecting reflection losses. You can greatly improve this situation with optics on emitter and/or sensor, and with modulation/detection that allows detection of the desired emission buried deep in ambient optical noise. The latter is analogous to selective detection of a desired radio signal. (By the way, where did the "455 kHz" mentioned in your original post come from?)

Just shielding the light path and limiting the field of view of the sensor can improve your S/N ratio by reducing stray irradiance at the sensor from off-axis sources, but that does nothing to enhance the received signal level. Using a more directional source or external optics on the source can greatly improve the percentage of emitted IR that falls on the sensor.

Hope this helps.

awright

 

[awright]

IR RC Car Detector

I agree with L.Chung's suggestion regarding the benefit of using a laser pointer as the light source because it gives you a visible, very well defined beam and a strong signal at the sensor. However, if you use a visible laser pointer with wavelength in the deep visible red region around 650 nm., you will have to use a different photodiode. The "black" epoxy encapsulant of your BPW82 passes very little light at wavelengths shorter than about 770 nm., so you will want to use a photodiode that has a clear encapsulant that passes red with little attenuation.

With a low divergence laser pointer beam you will not need any optics at the emitter (that's already provided by the pointer mfgr.), and with all the laser energy contained in a spot size a few mm. in diameter at the sensor, you would not need and would not benefit from simple optics at the receiver. As suggested by L.Chung, a simple tube at the sensor with a small aperture for the laser beam will provide a lot of suppression of interference from off-axis light.

awright

 

[Futterama]

awright: Thanks for the long and detailed explanation.

I have thought of using a laserpointer, but it will be very hard to point it directly at something the size of a photodiode. If I use a laserpointer, I could perhaps use a LDR as the detector and just put it in a dark 10-20cm tube.

The cheapest laserpointer I can get my hands on is a 650nm 1mW and costs around $15.

I have a laserpointer already though, so I can test it with a LDR and tube in sunny weather (lucky it's summer in my country now).

Edit: The 455 kHz is from the table of stored components at my work where I got the photodiodes and emitter diodes.

 

[Futterama]

I've made a quick setup as shown below.

With the "sun in the detectors back" this would work.

With ambient light (sunshine from behind), the resistance in the LDR was around 5kohm, and when pointing the laserpointer directly at the LDR from a 2meter distance, the resistance in the LDR was around 200ohm.

I just have to make a stand for the laserpointer to keep it very steady and point it at the LDR in the tube. The LDR + tube should be adjustable in position, perhaps with some precision adjustment to adjust it in millimeters to make the laserpointer hit the LDR right.

What I hoped I could archieve with IR, was to eliminate the need for precision adjustment of the emitter and detector, but it seems to be very hard to get IR working right in the sun.

 

[Nigel Goodwin]

What I hoped I could archieve with IR, was to eliminate the need for precision adjustment of the emitter and detector, but it seems to be very hard to get IR working right in the sun.

The trick is to modulate the IR, usually at around 38KHz, and use an IR receiver IC to receive it. This minimises the interference from the sun, as it's AC coupled, so a constant high brightness isn't much of a problem, it's the changes which it amplifies and detects, not the ambient level. Bright sunlight does reduce sensitivity slightly, but it should work well enough, and it's VERY easy to do.

 

[Futterama]

Nigel, if it's so easy, then show me

Actually I have an IR reciever module (I think) from an old radio with remote. I think I have the remote too.

But the modules case says nothing but "LA" and thats pretty hard to find a datasheet from :lol:

 

[audioguru]

Purchase a shiny new IR receiver with a datasheet with it. It has Automatic Gain Control to deal with interference from those new compact florescent lights. If it receives more than about 70 cycles continuously of 38kHz modulation then the AGC cuts the gain way down. Therefore your 38kHz modulation must be sent in bursts of about 20 cycles, followed by about a 20 cycles pause. You can recover the on-off of your IR with a peak detector, but with about a 30 cycles delay. :lol:

 

[Nigel Goodwin]

Nigel, if it's so easy, then show me

Check the IR hardware pages in my PIC tutorials, it shows how to connect an IR receiver - it is really VERY simple. You can use a simple 555 set to 38KHz as the transmitter.

Actually I have an IR reciever module (I think) from an old radio with remote. I think I have the remote too.

But the modules case says nothing but "LA" and thats pretty hard to find a datasheet from :lol:

There's not a lot to find out really, there are only three connections, ground, power, and data out - by following the PCB tracks in the radio it's easy to find which is which.