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IR beam break detector

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MikeMl

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I need it quick. Don't have time to mail order a finished product or a kit.

I have some Lite-On LTE-5208A IR LEDs and matched Lite-On LTR-3208E IR Phototransistors. Driving the Emitter at 20mA DC, and using a 220K load on the phototransistor gives me clean detection a 1m range in room light with no optics. (less outdoors in daylight?) I need to get a range of about 3m.

I'm thinking of modulating the LED at about 1kHz, and using an AC-coupled gain of 100 op-amp amplifier, followed by a diode detector, followed by a dc amplifier to increase the range.


Anybody have any other suggestions?
 

ericgibbs

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I need it quick. Don't have time to mail order a finished product or a kit.

I have some Lite-On LTE-5208A IR LEDs and matched Lite-On LTR-3208E IR Phototransistors. Driving the Emitter at 20mA DC, and using a 220K load on the phototransistor gives me clean detection a 1m range in room light with no optics. (less outdoors in daylight?) I need to get a range of about 3m.

I'm thinking of modulating the LED at about 1kHz, and using an AC-coupled gain of 100 op-amp amplifier, followed by a diode detector, followed by a dc amplifier to increase the range.


Anybody have any other suggestions?

hi Mike,
In a like IR system [ but using optics] over 6 mtrs outdoors, I found that a 'slicer' gave me good results in all levels of ambient light.

In addition to the 'ac' coupled amp I have a low intensity IR emitter pointing into the IR detector. The output power of the 'dc' emitter is controlled by the 'dc' component of the received signal and ambient light.

Brief description, hope it makes sense.

EDIT:

This is another 'slicer' option.
 

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crutschow

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Do you have a red filter for the detector to minimize ambient light?
 

MikeMl

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The detector is in a dark plastic housing which allegedly blocks visible light, i.e. the detector looks similar to ones used in the 40kHz remote controls.
 

crutschow

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In addition to the 'ac' coupled amp I have a low intensity IR emitter pointing into the IR detector. The output power of the 'dc' emitter is controlled by the 'dc' component of the received signal and ambient light.
What is the purpose of adding more dc offset to the IR detector signal?
 

MikeMl

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What is the purpose of adding more dc offset to the IR detector signal?

I can take a stab at this:

Power saving and dynamic range. If you put an agc loop around the average IR level at the receiver, you can run the emitter at a low level during dim ambient illumination, and crank it up as needed during bright sunlight which tends to desensitize the receiver.
 

crutschow

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Power saving and dynamic range. If you put an agc loop around the average IR level at the receiver, you can run the emitter at a low level during dim ambient illumination, and crank it up as needed during bright sunlight which tends to desensitize the receiver.
Guess I'm dense, but I still don't see how adding a DC light signal to the input helps the situation your are describing. So you add more DC component when the ambient level is low?
 

MikeMl

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Think of it like the ALC (automatic level control) in my kilowatt RF amp. The drive level is automatically set to the sweet spot on the tranconductance curve. During periods of high ambient light, it takes more drive signal to maintain the SNR; less drive at night.
 

ericgibbs

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What is the purpose of adding more dc offset to the IR detector signal?

hi Carl.
The 'dc' offset is not added to the IR detector signal, its subtracted, so that operating point of the main IR receiver amp is biassed to give the maximum output of the received pulse.

The systems I have working [2] outdoors, have been trouble free for over 25 years.
 

ericgibbs

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The detector is in a dark plastic housing which allegedly blocks visible light, i.e. the detector looks similar to ones used in the 40kHz remote controls.

Mike,

The ambient daylight filter will take out most of the visible light, but there is a strong IR component in daylight that can cause problems.
 

ericgibbs

Well-Known Member
Most Helpful Member
I can take a stab at this:

Power saving and dynamic range. If you put an agc loop around the average IR level at the receiver, you can run the emitter at a low level during dim ambient illumination, and crank it up as needed during bright sunlight which tends to desensitize the receiver.

Mike,
I dont use the 'dc' component to control the IR emitter, its used to move the bias operating point of the IR receiver amp, which is a 'dc' coupled amp.

Later stages are 'ac' coupled.
 

Mr RB

Well-Known Member
You don't need any DC amp or gain control. Just modulate the LED, then the photodiode through a high pass filter into a high gain AC amp, then diode detector into a small cap into a NPN transistor base.

Any sunlight or ambient IR component will be very low freq, so will be completely negated by the high pass filter. Likewise there is enough AC gain so even a hint of the high freq IR coming through will get amplified and operate the output.

I would use a much higher freq than 1kHz, so you can make the filter more extreme which will remove more of the low freq AC noise picked up by the amp, like mains noise, vehicle ignition noise etc.
 
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MikeMl

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I got it working. I wanted a beam-break circuit which could detect a runner running down a narrow path, as in the attached picture. This is for a 100 mile foot race called the Wasatch 100, which is happening this weekend. I want to know that a runner is about 1/2mi from our checkpoint, so we can get ready to welcome the runner.

I'm with the ham radio group that provides communications for the event. We team with other volunteers at one of the aid stations on the course. Our checkpoint is at mile 83, so the runners are quite spread out, and they arrive all night into the next day. I'm providing a travel trailer, antennas, radios, computers, packet modems, etc. for the event.

I hooked the beam break detector to a low-power VHF transmitter. The transmitter sends a coded message once each time the beam is broken.
The coded message opens the squelch on a scanner receiver at the checkpoint, giving us about 5 min warning that a runner is about to arrive.

To get reliable detection across the trail, I used this IR detector for the receiver. I made an IR transmitter by building an oscillator that runs at 40kHz, and driving a Lite-On LTE-5208A IR LED at 40mA at a 50% duty cycle.

The IR transmitter is left on continuously, aimed at the IR receiver. The IR receiver output is high with a steady signal, and goes low and then back high with a momentary interruption of the beam, such as passing a hand or body through the beam. The time constants in the receiver work just fine for a short interruption. I stretched the output pulse to key the radio transmitter.
 

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