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

[Art Todesco <actodesco@xxxxxxxxx>]

On 7/1/2012 10:55 PM, Bob F wrote:
> Art Todesco wrote:
>> On 7/1/2012 3:04 PM, Bob F wrote:
>>> 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?
>>>
>>>
>> 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.
>
> Clarify for me - Is the power to the receiver pulsed also?
>
>
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.