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

[Bob F]

I picked up a couple pairs of these Liftmaster 41A5034 garage door safety
sensors, which I thought I might be able to use for other sensing on my homebrew
home control system. Does anyone know what voltage/current the sender needs, and
what connections the 2 wire detector needs and how it signals. They both have
white 2 conductor wire with a black line on one of the conductors.

[Art Todesco]

I have them on my 2 garage doors. I installed an addition unit called
Garage Door Butler, which automatically closes the door if accidentally
left open. What I'd like to do, is to use the info from the safety
sensors to reset the Butler unit so that each time you break the beam,
the Butler would reset back to the 10 minutes I have it programmed for.
But, I too, don't know how they work. I would like to have a small
relay operate whenever the beam is broken. The contact on the relay
could easily reset the Butler. The one odd thing I've noticed is that
both the light source (IR) and receiver box have 2 wires each. They
connect in parallel and then go to the opener unit as just 2 wires.
I've want to put a voltmeter/oscilloscope across the wires and see what
it looks like, but just haven't had the time. I've also wanted to call
the guy that installed the doors (new house 3 years old) and see if he
has a schematic, but I haven't done that yet. I'll keep an eye here and
also will let you know if I find out something.

[Bob F]

My best guess so far is that the "receiver" conducts more current when the IR is
ON (or maybe off) and not the other way, so that the voltage at the end of the
wire provided through a resister will drop as the current increases.

[Bob F]

The above is pretty much the case. With 1k resister on either end going to the
"black/white" wire from the (+) terminal of a 9V battery, white wire to (-)
terminal, it senses properly. Feeding the receiver end signal (green LED) into
the opto-islator LED input of my home control should work fine.

[Art Todesco]

I went out in the 92 degree heat today to put out the flag and I decided
to checked my garage door photo sensors. When the beam is interrupted,
the voltage across the 2 wires read 6 volts DC. When NOT interrupted,
is read 5.75 volts DC. I thought there might be more to it than DC
volts, so I got out the scope. The 6 volts is actually interrupted
periodically. It shuts off for about .3ms every 6.5ms. When the beam
is interrupted, it is a steady 6 volts DC. Using an opto to the LED is
probably the easiest way to get info. I didn't look directly across the
green LED, but I think it is probably interrupted with the power
because, if you turn your head very quickly, it is definitely blinking.

[Bob F]

I wonder if they cycle it to detect other IR sources that could be jamming the
sensor?

[Art Todesco]

Actually, I think they just use it to know if the beam is interrupted or
not, plus you can send power to both the light source and the receiver.
As I didn't take anything apart, I don't know how the receiver can
stop the pulses across the power pair when the beam is broken. It would
be nice to have a schematic.

[Bob F]

Clarify for me - Is the power to the receiver pulsed also?

[Art Todesco]

I'm not sure where the pulses are coming from. In my case, the source
and receiver both have 2 wires and are actually connected together in
parallel and then go to the main opener unit on the ceiling. I'm sure
the 6 volts come from the opener unit, however, I really don't know
where the pulses come from. I just monitored, using an oscilloscope,
between the 2 wires. I'm guessing that maybe the pulses are put on the
6 volt power in the opener unit and when the photo receiver is blocked
from the LED light, it somehow shorts out the pulses. And when the
opener see that the pulses disappear, it reverses the door. But, the
pulses could be coming from the receiver box. Without a schematic, I'm
running blind. I think I will try calling the door installer to see if
he has some info.

[Bob F]

Wow! I never would have thought they would be in parallel. Gotta think about
that.

[Art Todesco]

Yeah, me neither. I called the garage door installer that put in the
openers and doors, and he was totally clueless. Didn't know anything of
a schematic. My guess is that the pulses come from the logic board in
the opener unit and the photo receive filters them out to power up the
receiver. Then when the beam is interrupted, the receiver puts clean 6
volts on the wire pair, thus 'shorting' out the pulses. The logic board
then detects this and reverses the door. But, as I said, just guesses.
Of course, if you are operating the units on a fixed 6 volt supply,
not from a LM opener, I don't think you'd see the pulses ... again, just
guessing. Do you see the green LED flickering on your units? If you
hold the 2 units together so that the receiver always sees the light
source, then move the 2 back and forth rapidly, you should be able to
see the green LED blinking (strobe effect). My guess is you won't
unless it is connected to a LM opener.

[Art Todesco]

I called Chamberlain/LiftMaster today and asked about a schematic. I'm
not sure the person even knew what a schematic was. It wasn't on her
script ... and she was in the US! She finally gave me another phone
number, which I called and the person said they didn't have them. I
sarcastically asked if they get built by magic. She then said the
schematic was proprietary. Most companies are usually good about giving
out schematics. I thanked her and said that if I reverse engineered a
schematic from the unit, I would be sure to post it on the internet! I
have a Garage Butler to auto close the door if left open, and I found a
schematic on the internet, but couldn't find anything on the LM photo
sensors. I really don't want to take apart my working units. I know
the LM opener will not work if the photo sensors are not connected and
working properly. So, if I find some cheap ones, I will work on it.

[Art Todesco]

> I'm not exactly sure, but I think they operate on current. A
> resistor in both sensors in parallel, one drops out and the
> current changes, but not the voltage.
>
It may be current change, but if you look way earlier in the thread, I
put a scope across the pair. There are pulses there, but when the beam
is broken, they disappear.

[Bob F]

> Did you try measuring current in both states?

Or, does the LED driving voltage from the opener change, and how, if you
disconnect the send from the circuit?

[Art Todesco]

On 7/4/2012 11:53 PM, G. Morgan wrote:

> When the beam breaks, only one LED goes out on the receiver - correct?
> Maybe the absence of the load changes the "safe state". I think the
> transmitter has the circuit set at a semi-precise current flow. If it
> always has 6 volts and internal resistance/load that also stays constant
> in a "safe state" - the logic board at the operator can always be
> looking for a current range. 6V/50 ohms = 120ma, I'm proposing the
> logic board is expecting to see 120ma +/- 10% (in this example) and a
> break in a wire or beam will do the trick.
>
I didn't actually measure current. I wanted to disconnect the 2 wires
from the opener to see what the opener is putting out. From what I've
read, if you don't have the photo sensors connected, the opener won't
work properly. Maybe that means it won't close, but I've not tested it.
But I think the detection of a broken beam really has something to do
with the pulses. As I said previously, the pulses are on the pair when
the beam is uninterrupted. Once the beam is broken, the pulses
disappear. The pulses repeat every 6.5ms and basically shut off the 6
volts for a very short 0.3ms. These can easily be filtered out in the
receiver with a simple RC circuit to power the receiver. My thought is
that the pulses come from the opener, but I haven't check that either as
of yet.

[servic...@gmail.com]

Liftmaster has a module called CPS that has a on or off type out put when photo beam is broken it connects two terminals like turning on a light switch

[servic...@gmail.com]

The CPS needs a 24 volt AC power source, standard for garage door transformers. Do not try to get power from push button wire this will cause trouble use a plug in transformer.

[eeew...@gmail.com]

The way that the sensors work might be this: The transmit module power leads and the sensor power leads are connected in parallel. The transmit module pulses on at a fixed rate. The detector module shorts out the power leads whenever it detects a light pulse from the LED module. The control unit watches for pulses on the power leads. No pulses means the light path is blocked.

[eeew...@gmail.com]

Each module can have some capacitance that is diode coupled from the power leads to power the module while the power leads are shorted.

[josephr...@gmail.com]

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 signal 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 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 pull 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 while resistors with gradually decreasing values were put across the terminals. Since the small boxes both have lit LED's when operating, the internal equivalent 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 voltage (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. Plus to the white/black wire, 0V (gnd) to the white wire. The signal was observed 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 resistor is expected or that the circuit is not linear (hence, Thevenin's theorem is not applicable). But it works, anyway.
photo of circuit To make a completed electric eye for use without the opener, 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 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 when 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 otherwise. 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 treaters?
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

[josephr...@gmail.com]

[josephr...@gmail.com]

[Art Todesco]

Fun to see some activity here, even if it is just a few people. For
Halloween, I used a Seco unit across the road to trigger the mechanical
stuff, like this one:
https://www.amazon.com/gp/product/B001LFPB0M/ref=pe_385040_128020140_pd_te_s_gr_ti/175-9736184-1310369
But it is a bit pricey. They also make a 45 foot unit, which is what I
use across the road. No need for power on the other side as you only
need a reflector which looks like a bike reflector.

[navy.v...@gmail.com]

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 that would be generated would be 5.5V.

[therealr...@gmail.com]

I know this is an old thread, but I actually measured how the ones on my Liftmaster Pro 1/2HP model work. The main unit outputs 6.2V open circuit which dops to 5.84 when both sensors are connected. The total draw is 15 MA, indicating the internal pullup resistor is about 25 Ohms. The transmitter and receiver each draw about 7.5MA. In normal operation the transmitter sends out about .4MS pulses at about 160HZ. When the receiver sees them it pulls the supply lines to about .25V. This develops a signal across the main unit internal resistor. The main unit needs to see these pulses in order to enable door closure. The yellow LED on the back of my unit flashes once about every 8 seconds then the sensors are disconnected and when you hook them up and the unit senses the signal, it flashes 4 times.

[therealr...@gmail.com]