Passive infrared (pir) door alert?

[Doktor Jones]

At work, we have a solid door that opens outward; unfortunately this means that if someone inside is heading out while someone else is outside fumbling for their keycard, they end up getting a door to the face. As a relatively inexpensive solution I was thinking of mounting a PIR sensor on the ceiling directly above the door on the outside and wiring that to a bright LED inside the door, so that the LED lights up whenever someone is within about 3 feet of the door.

With a 4V power source (regulated by a LM317L TO-92), could I just wire the LED directly to the output of this PIR sensor (with limiting resistor of course)? Or would I need additional electronics?

Does anyone know if this sensor outputs a "heartbeat" pulse? I've seen some other datasheets that indicate that the sensors periodically pulse high on the output (undesirable for my circumstances), so if that were the case I imagine I'd have to add a small timer circuit that only triggers the LED if the output is high for greater than the length of the heartbeat pulse.

Thanks!

 

[dr pepper]

I've used those pir's before they are allright for what they cost.
The output can be setup to come on and stay on for a preset amount of time on mine, not sure about the one you linked to, I got mine on ebay - it looks the same.
And I dont think it would power a high power led, the o/p is more for logic circuits or a uC, if you used something like a bc337 transistor and a 4k7 resistor you'd be able to switch an led.

I'd put led's both sides of the door.

Maybe even put a window in the door too.

 

[Doktor Jones]

This is a leased office though, so while we can get away with putting a hole in a ceiling tile for the sensor, hacking a hole in a solid wood door to install a window is probably not an option. There's also the matter of security/privacy -- we don't want just anyone peeking through the window into our office.

Thanks for the tip about the transistor. I assume a 2N3904 would be an acceptable substitute for a BC337 (i.e. general purpose NPN)?

What purpose would LEDs on both sides of the door accomplish? If you're near the door, you probably already know that. If you were suggesting have two sensors on either side with LEDs on the other side, that's not as useful because the door opens outward (and you can hear the beep of the card reader when someone's coming in) so there's not really any danger of smacking someone in the face with the door.

 

[dr pepper]

I was thinking you could see that the sensor has actually picked you up and someone the other side of the door is due warned.
Understandably you cant bash someone elses property.
The 3904 will be ok so long as you dont go over the max collector current, the bc337 can handle something like 800mA.

 

[Doktor Jones]

Ah, that makes sense; I'll probably skip that though because that'd involve an extra box to mount on the wall for the LED (since the PIR sensor will be attached to the topside of a ceiling tile).

I only intend to use a vanilla ~20mA super-bright LED, not like a piranha or Cree LED or anything... so the 2N3904 shouldn't have a problem managing that.

 

[jpanhalt]

I agree with your approach. Just a small detail...

If this is being installed with wiring above a suspended ceiling, you may want to consider using just a battery powered circuit. In the US, that space above a suspended ceiling is considered plenum, and there are fire codes regulating any wiring there. I don't know whether the code applies to low voltage battery devices, but it does apply to devices supplied with mains voltage. That is one reason you will sometimes see wiring hung with clips just below the ceiling rather than concealed above the ceiling tiles.

John

 

[Doktor Jones]

Looking at the MA building and electrical codes, this circuit should meet all requirements of a Class 2 circuit, so as long as the power supply is kept below the ceiling (which it will) and the wire used is plenum-rated (which it will be) it should be kosher.

Having scoped out where the nearest electrical outlets are, I am thinking of using a 9V supply to account for the voltage drop of the regulator (which will be somewhere between 1.0-1.5V) as well as the inherent voltage drop of the wire (up to 0.2V) since I'd need to run it ~30-40 feet; I'll also use a TO-220 regulator with heatsink instead of a TO-92, just to give myself plenty of thermal margin on that.