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

[navy.vet.1959@xxxxxxxxx]

I wonder if they are using this like a switching power supply.  By turning =
it off for 0.5 ms out of every 6 ms with a 6V peak, the average voltage tha=
t would be generated would be 5.5V.

On Wednesday, February 22, 2017 at 10:08:38 PM UTC-6, josephr...@xxxxxxxxx =
wrote:
> With the devices hooked up, an oscilloscope was used to see what was goin=
g on. With the light blocked between the transmitter and receiver, the whit=
e/black wire is 6VDC above the white wire. When not blocked, a periodic sig=
nal is observed, where the white/black wire is 6V above the white wire for =
about 5.5 ms, then falls close to 0V for about 0.5 ms. This suggests that t=
he opener is supplying 6V through a resistor -- the receiver is "pulling" t=
he voltage to (near) 0V briefly and periodically. One cannot expect it to p=
ull too hard, so a resistor must be present.
> oscilloscope picture The devices were removed from the opener. With a DC =
voltmeter across the terminals on the opener, the voltage was measured whil=
e resistors with gradually decreasing values were put across the terminals.=
 Since the small boxes both have lit LED's when operating, the internal equ=
ivalent resistance can't much more than about 1k for a 6V source, so that w=
as the starting value for the external resistors. A significant drop in vol=
tage (though less than by 1/2) was noted for a couple hundred ohms across t=
he terminals. That means that the Thevenin equivalent resistance is less th=
an about a couple hundred ohms.
> A 6V power supply was wired through a 150 ohm resistor to the two boxes. =
Plus to the white/black wire, 0V (gnd) to the white wire. The signal was ob=
served to be virtually identical to what was observed from the opener, exce=
pt the maximum was closer to 5V rather than 6V. It might be that a smaller =
resistor is expected or that the circuit is not linear (hence, Thevenin's t=
heorem is not applicable). But it works, anyway.
> photo of circuit To make a completed electric eye for use without the ope=
ner, a small circuit board was used which included a 6V regulator (7806) an=
d a 555 (low-power) wired as a "missing pulse detector." The circuitry for =
the latter can be found on the 555 data sheet, and is easily found with an =
internet search. It has been copied and recopied by many. Referring to that=
 diagram, I used a 2N3906 PNP transistor, and a 0.1 uF capacitor and a 75k =
resistor 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 =
white/black wire to the remote boxes through a series resistor (I ended up =
using 150 ohms for that though a somewhat smaller value may work better). I=
 put in two LEDs. One just shows that the power is on, the other comes on w=
hen 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 othe=
rwise. So far it is just wired to light an LED. I am not sure what I might =
use the electric eye for as of yet. Perhaps something to do with trick or t=
reaters?
> Note that in the process of testing, I tried various power supply voltage=
s. The frequency of the pulses does depend on the voltage. A 5V source stil=
l seems to work, though the frequency is not as steady