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alternatives to PIR motion detector? photo sensor pairs?

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danjel

Member
I have a project with a hallway that is 30-40m long and 6ft wide. I was planning to use PIR motion sensors to detect motion in the hallway but I am worried they may not be focused enough (they are easy to trigger over a wide area).

I was thinking instead of using distance sensors but they are limited to about 150cm and they are analog values.

Another obvious option is to use cheap lasers and photodiode pair to basically make "tripwires".

What would be the cheapest and easiest option to implement?

Maybe there is another solution I should consider?
 

mneary

New Member
A friend of mine monitors his hallway with a light beam sent lengthwise down his hallway like you suggest. The obvious design is to put the transmitter in the center of one end wall and the receiver in the center of the other end. It's obvious, but impossible, because the ends are likely to be doors, desks, or decorations.

If nobody is expecting it, it's almost as effective to put them in opposite corners.

Cheapest and easiest is to send 38kHz bursts from an IR LED to be received by an IR module made for a remote. This is tricky if you don't know what you're doing.

1) Put a lens in front of the LED to focus most of the energy towards your receiver. You don't want to waste energy screwing up your TV in the adjacent room anyway.

2) Put the IR LED in a socket. Replace it with a red LED while you focus your optics.

3) Send 1ms bursts of 38kHz, separated by silent periods of 4ms. A pair of 555s or a 556 will do this easily. The 38kHz sensors don't like continuous signals.

4) The receiver should be producing 1ms pulses with 4ms pauses. Look up 'missing pulse detector' here or on the net to see what to do with the sensor output.

5) Note that the transmitter and receiver can be independent, and on different power sources.

Good luck.
-Mike
 

danjel

Member
my sensors would be positioned width wise so they are all only spanning 6ft.

Also this hallway is relatively empty... this is for an art installation not a burglar alarm (the triggers are going to a computer controlling a projector)
 

mneary

New Member
I would put the transmitters on one side, as low as possible (hide in the baseboards where you can hide the wires too?). Opposite, the string of receivers. Transmitters can be inside a 1" black tube so they don't spray adjacent sensors, or you could centrally trigger one transmitter at a time and only look at the corresponding sensor.

I'm assuming that you only want a trigger and not an accurate count or timing (how long did they study this pic?). I'm also assuming people won't be walking briskly and will tend to shuffle their feet.

The reason for transmitter and receiver on opposite sides is my lack of knowledge how much (electrical) interference a receiver module can tolerate from a nearby LED driver. Maybe someone has some experience.
 
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Mr RB

Well-Known Member
You could buy any of the commercial IR reflector systems they sell for use in shop doorways (to ring the bell when a customer enters etc). They cost$50 but are fully assembled and look reasonably pro mounted on a wall so it would be easy to use for your art installation.
 

mneary

New Member
The Maxbotix unit looks cool. Looks like you would maybe daisy chain PW to RX, and connect TX all together open collector. Of course, that's at least two signal wires, plus power and ground!

It says one of the interfaces is RS232, which is totally untrue. It's open collector TTL, 9600 bps, serial, in the opposite polarity of RS232.
 
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