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

[Art Todesco <actodesco@xxxxxxxxx>]

On 6/29/2012 7:17 PM, Bob F wrote:
> Bob F wrote:
>> Art Todesco wrote:
>>> On 6/25/2012 5:37 PM, Bob F wrote:
>>>> 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.
>>> 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.
>>
>> 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.
>
> 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.
>
>
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.