UTMonkey

As my first project I want to have an LED shine onto a PhotoTransistor. when the beam is broken something happens.

I imagine using a standard LED and PHOTO TRANSISTOR would be problematic as there would be pollution from other light sources.

On the other hand there appears to be loads out there for infrared receivers and transmitters. this scares me slightly as the projects that use these require oscilliation for the LED transmitter (and receiver??).

So, do I build the LED\Photo transistor or try and tackle the infrared stuff?

Could I build the infrared circuits without an oscillator (continually on?)

Sorry to be thick.

Regards

Mark

ClydeCrashKop

Photo transistors like infrared. I usually get them as an emitter-detector pair. If you put the IR photo transistor in a tube that is painted flat black inside, very little ambient light gets at it. If you aim the IR LED at it, it will conduct to varying degrees and go almost open circuit when you block it. Start testing them close together and gradually move them apart. You will find out what the limits are.

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Torben

Hi UTM,

What Clyde said, plus: if you use a cheap ($2 here, maybe less where you are) laser pointer from a dollar store, you can get a laser diode on the cheap and use that for your light source. You'll still want to build a shield for your phototransistor, though. The laser isn't IR, but it dumps enough energy on the phototransistor to work with it. At least, mine do. Also, because it delivers so much energy, you get a greater gap between the on and off states (when light is reaching the PT and when it's not). And distance from the source becomes much less of a factor.

If you can remove the problem of ambient/exterior light interfering with the PT, you won't need to use modulation. This makes things a lot simpler.

You can remove the laser diode from the pointer (carefully) and power it yourself--it may have a current-limiting resistor in place which you can use, as mine do, or you may need to provide one. Alternately if you don't need long run times you can just tape down the laser pointer button.

There are a few threads on this board about beam-break detectors (also covered by the term "dark alarm". If you search for them you can find a lot of stuff which might help.


Hope this helps!

Torben

ericgibbs

hi Mark,
How far apart is the transmitter and receiver, whats the gap?

Whats the object thats breaking the beam?

Is it indoors or outdoors?

How's the 'crooked spire' these days.

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Nigel Goodwin

I'm sure it looks more twisted every time I drive past!

However, with the traffic chaos where they are building the new Hornsbridge roundabout you don't dare look very much, as you have to keep a sharp lookout for cars appearing from strange places!.

I was in Chesterfield last night, I'll be there again tonight! - daughter fetching!.

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UTMonkey

Wow, thanks for the response guys.

I suppose what I am looking for is something that detects someone breaking a 2 metre (or longer) beam.

Am I right in thinking then that the longer the distance between receiver and transmitter the more likely that "modulation" is needed (whatever that is!).

My goal is to have the transmitter and receiver on the same PCB pointed at a reflector.

TX> | Reflective
RX< | Surface

Torben

Pretty much. A regular LED (IR or otherwise) is fairly focused, but it does diffuse the beam quite a lot over distance. A laser will probably give you dramatically more distance.

Ever see those IR door dingers in stores, which use a transmit/receive unit and a reflector? They use lenses to focus the beam.


Torben

UTMonkey

Thanks to all.

I had better brush up on what "modulation" is.

Torben

In this case, it just means that you pulse the LED with a square wave of a certain frequency, and then use a circuit at the receiving end which ignores light which is not pulsed at that frequency.

Unfortunately (?) my installations so far have used lasers and good light shielding on the phototransistor, so I haven't had the need/opportunity to use modulation on one yet.


Torben