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

["Bob F" <bobnospam@xxxxxxxxx>]

Art Todesco wrote:
> 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.

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