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

[G. Morgan <G_Morgan@xxxxxxxx>]

Bob F wrote:

>G. Morgan wrote:
>> Art Todesco wrote:
>>
>>> On 7/4/2012 9:48 AM, G. Morgan wrote:
>>>> Art Todesco wrote:
>>>>
>>>>> On 7/3/2012 7:42 AM, Art Todesco wrote:
>>>>>> On 7/2/2012 2:12 PM, Bob F wrote:
>>>>>>> Art Todesco wrote:
>>>>>>>> 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.
>>>>>>>
>>>>>>> Wow! I never would have thought they would be in parallel. Gotta
>>>>>>> think about
>>>>>>> that.
>>>>>>>
>>>>>>>
>>>>>> 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.
>>>>>>
>>>>> 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.
>>>>>
>>>>
>>>> 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.
>>
>> 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?


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.