> Hi all,
>=20
> I want to make a simple, cheap, quick, reliable (no one dare say you can
> only pick only x) garage door open detector that will light up an LED in =
the
> house/beep a speaker if the garage door is open. I know all it takes is a
> mechanical switch, but I'd rather not use any moving parts. I'm thinking =
a
> very bright infrared LED and IR transistor with a piece of foil mounted o=
n
> the inside of the garage door. If the door is closed, a series of LED pul=
ses
> will hit the aluminum foil reflector and bounce back to the IR transistor
> and signal the device that the door is closed. Will this work? I've never
> done any IR work, so I don't really know what power LED is required, what
> the distances are, etc.
>=20
> As for the micro controller unit, I can make it wireless and signal the
> indoor unit via something like this
> http://www.sparkfun.com/products/10534but I'd have to impliment
> encryption on the micro controller itself. It
> would also interfere with other units in the same band, which is very ver=
y
> undesirable. How hard is it to implement simple encryption?
>=20
> Xbee is cool, but it will add $40 at least to the cost of the project.
>=20
> Any other ideas?

On the simple side of things, but... I set one up for my parents garage doo=
r (tilt=20
door) many years ago. Used a long lever microswitch that the door operates =
when=20
fully closed, normallly open and normally closed contacts arranged so that =
a green=20
LED lights for fully closed, red LED for door open any amount. Stole 12V DC=
 power=20
from the door opener somehow, 3 wires in underground cable to remote LED's =
in=20
house. The microswitch does need pushing around and readjusting every now a=
nd=20
then for I don't know what reason, but basically still working fine after m=
aybe 25=20
years.

Get calculator... that means the green LED has probably done 200,000hrs~!=20
(Assuming door closed 90% of the time). Assuming 15mA current that's 3000Ah=
rs,=20
LED has resistor of 12VDC supply, say 90% PSU efficiency that's approx 140A=
hrs=20
@ 230VAC, assume power factor unity then that's 32kWhrs,  @ say $0.20 avera=
ge=20
per kWhr unit =3D $6.40 electricity for 25 years of operation.

--=20
Brent Brown, Electronic Design Solutions
16 English Street, St Andrews,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: +64 27 433 4069
eMail:  brent.brown@clear.net.nz



--=20
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