IR LED and Reciever

Palmer421

I am trying to make a optical switch (larger gap than I've been able to find off the shelf) and thought the IR Transmitter Kit with the IR·reciever would work well.· Does anyone know if you have to have it tuned to a frequency to change the On/Off signal or can it be set to "seeing light or not seeing light"?· Also is there a response speed limitation·to the reciever meaning·is there a limit to how many cycles between on/off could I get per second?

Thanks, Palmer421

Mike Green

The IR detector that Parallax sells is a modulated IR receiver. It responds to IR modulated at a specific frequency (38 KHz I believe - check the datasheet). The datasheet would have specific information on response time, but these are used to transmit remote control signals and asynchronous serial data and should be able to react to brief bursts of modulated IR, on the order of 1/4 to 1/2 ms.

If you want a simple on/off response, use a phototransistor.

Palmer421

Thanks Mike.· Now that I think about it I'll actually have to use the IR system since it does use·a specific frequency range.· The application I'm using this for will have very bright LEDs near it triggered by the optical switch so I think it needs some kind of filtering system like detecting a given frequency.

The documnetation I looked at last night said that you can detect different ranges to calculate distance from an object by bouncing the IR light off something which could be cool for other projects.·

I only saw·a photoresistor and a IR transistor available, were you talking about one of those?· I was worried that·something like that·wouldn't respond/reset fast enough.·

Thanks again, Palmer421

allanlane5

1. Photo-Resistor -- a variable resistance, whose resistance varies based on the amount of light hitting it. Slow. Easily confused by ambient light.

2. IR-Transistor -- basically a transistor, whose 'base' input is provided by IR light. Response is very fast. Easily confused by sunlight.

3. IR-Decoder -- A sophisticated 3-terminal device. It's 'looking' for an IR light, 'blinking' at a 38 Khz rate. This is called "a 38 Khz modulated IR signal". If it does not see the IR light 'blinking' at 36 to 42 Khz, it completely ignores it. This behavior is excellent for filtering out ambient light.

It's #3 that Parallax uses in their "IR" circuits. These devices were created to recieve TV (and other) IR Remote signals. As such they're pretty cheap, and very reliable. When they 'see' the 38 Khz modulated IR light, they output a TTL 'low' on their signal pin -- when they don't 'see' the signal, they output a TTL 'high' (though some need a 10 Kohm 'pull-up' resistor).

They're also pretty fast, able to faithfully recieve 1.2 mSec and 600 uSec 'bursts' of IR light. Note at 38 Khz, 600 uSec is about 23 'blinks' of the 'sending' signal. I'm not sure exactly how few 'blinks' it takes to activate the decoder.

Palmer421

So for the IR transistor I could turn the other LEDs in my system on or off with the presence of the IR LED? This action will be taking place inside of a tube but light could enter in. Now I'm debating between the two: IR transistor and the IR reciever.

Basically I'm trying to make my own Flash Tracer / Silencer for an MP5 Airsoft gun (ex: http://www.mfiap.com/airsoft/acce/tracer.htm). I can't seem to find them anywhere anymore. So the optoswitch will trigger some UV or really bright visible light LEDs to charge some glow in the dark BBs for a tracer round effect. Any thoughts?

Mike Green

In a well controlled environment where the light source would overwhelm any light leakage, the IR LED / IR phototransistor should work. Do put a "hood" around both the LED and the phototransistor to limit what they "see".

Parallax does sell a phototransistor (www.parallax.com/detail.asp?product_id=350-00018). I don't think I have a datasheet for this anymore and there's not a link on the webstore page, but e-mail Parallax support and I'm sure they'd send you one.

There are some bright UV LEDs available for charging phosphorescent stuff. The main problem is that they're potentially dangerous since you can't see the light from them and they can damage your eyes. Experiment with bright blue or bright white LEDs and see how well they do at it. The LED would be very close to the BB, so most of the beam would be directed to the BB.