How to use Liftmaster garage door IR safety sensors for other uses?

[josephromero14@xxxxxxxxx]

With the devices hooked up, an oscilloscope was used to see what was going =
on. With the light blocked between the transmitter and receiver, the white/=
black wire is 6VDC above the white wire. When not blocked, a periodic signa=
l is observed, where the white/black wire is 6V above the white wire for ab=
out 5.5 ms, then falls close to 0V for about 0.5 ms. This suggests that the=
 opener is supplying 6V through a resistor -- the receiver is "pulling" the=
 voltage to (near) 0V briefly and periodically. One cannot expect it to pul=
l too hard, so a resistor must be present.
oscilloscope picture The devices were removed from the opener. With a DC vo=
ltmeter across the terminals on the opener, the voltage was measured while =
resistors with gradually decreasing values were put across the terminals. S=
ince the small boxes both have lit LED's when operating, the internal equiv=
alent resistance can't much more than about 1k for a 6V source, so that was=
 the starting value for the external resistors. A significant drop in volta=
ge (though less than by 1/2) was noted for a couple hundred ohms across the=
 terminals. That means that the Thevenin equivalent resistance is less than=
 about a couple hundred ohms.
A 6V power supply was wired through a 150 ohm resistor to the two boxes. Pl=
us to the white/black wire, 0V (gnd) to the white wire. The signal was obse=
rved to be virtually identical to what was observed from the opener, except=
 the maximum was closer to 5V rather than 6V. It might be that a smaller re=
sistor is expected or that the circuit is not linear (hence, Thevenin's the=
orem is not applicable). But it works, anyway.
photo of circuit To make a completed electric eye for use without the opene=
r, a small circuit board was used which included a 6V regulator (7806) and =
a 555 (low-power) wired as a "missing pulse detector." The circuitry for th=
e latter can be found on the 555 data sheet, and is easily found with an in=
ternet search. It has been copied and recopied by many. Referring to that d=
iagram, I used a 2N3906 PNP transistor, and a 0.1 uF capacitor and a 75k re=
sistor for the timing. The power was supplied through a surplus wall cube (=
labeled 9VDC, 200 mA). The regulated 6V powers the 555 directly, and the wh=
ite/black wire to the remote boxes through a series resistor (I ended up us=
ing 150 ohms for that though a somewhat smaller value may work better). I p=
ut in two LEDs. One just shows that the power is on, the other comes on whe=
n the beam is broken. More LEDs is better, right?
Pin 3 of the 555 goes low when the light beam is broken, and is high otherw=
ise. So far it is just wired to light an LED. I am not sure what I might us=
e the electric eye for as of yet. Perhaps something to do with trick or tre=
aters?
Note that in the process of testing, I tried various power supply voltages.=
 The frequency of the pulses does depend on the voltage. A 5V source still =
seems to work, though the frequency is not as steady