trip beam sensor [closed]

Question

I need a trip wire sensor, like a laser and a reflector, and or a infrared version. I am doing the cabling for a walk through attraction and will at some stage need also a PIR. Basically all these need to sense someones presence and switch a relay which can turn on an amplifier for audio or lighting. So where can I found a sensor and also a board that can turn on and off realys which can connect to amps etc. ?

Answer 1

This simplest sensor is probably one of those integrated units intended to detect signals from TV remote controls. Vishay TSOP is one product line that I remember, but other companies make these things too. These devices do a lot of stuff for you in a nice small and inexpensive package.

You could just use a light beam shining accross a hallway maybe 6 inches or so above the floor, but there are several problems with this. First, you probably don't want it to be obvious, so it should be invisible. That means infrared is a good candidate. IR LEDs are common, cheap, and more powerful relative to visible LEDs. Then there is the problem of ambient light. Even in infrared there will be some, and it might be stronger than the beam from the IR LED on the other side of the hallway. To get around this, you modulate the light and look for the specific modulation signature at the receiver end. Natural light isn't going to come in bursts of 40 kHz pulses, for example.

A TSOP does all this detection in one nice little module. It has a plastic lens that filters out light except for a relatively narrow range around typical IR LED wavelengths, like 940 nm. Then it has a detector that can take a wide range of ambient DC and find and amplify only the AC component. Then it goes further and looks for only a specific frequency in that AC component. You buy different variants that are preset to different frequencies, usually in the 35-50 kHz range. The bandwidth of these things is only a kHz or two, so again they can eliminate a lot of noise. Finally they produce a digital output when the specific carrier frequency has been detected. They also have a automatic gain circuit, so if the ambient light ever included this specific frequency, even that would get cancelled out over time.

Due to the automatic gain function in these detectors, you need to modulate the light with bursts of carrier cycles. 20 cycles or so on and 20 cycles off in a repeating pattern will work well. Let's say the detector is set to 40 kHz. That means each cycle is 25 µs long, and 20 cycles last 500 µs. That means the output of the detector would be a square wave with 1 ms period, or 1 kHz frequency.

You could probably create a analog circuit that reacts to anything near a 1 kHz square wave and turns on a relay. For better noise immunity, I'd use a microcontroller that looked for the presence of the 1 kHz square wave within some margin, and output a digital signal accordingly. That can be used to drive a relay very easily. I'd probably have the micro require that each edge is from 400 µs to 600 µs since the last one. I'd probably also add a little logic so that it would have to miss a few edges in a row to declare the beam interrupted. Even 10 edges in a row would still make the response time 5 ms, which is instantaneous in human terms and a lot shorter than the time a leg will interrupt a beam during normal walking.