Garage door receiver shorts its own power supply

I have an old Sears garage door opener, and the receiver appears to trigger the door to open or close by shorting the two wires that run to the mechanicals box. But those wires also provide the power for the receiver - in this case it measures about 30VAC, which the schematic says is a nominal

24VAC off the transformer secondary.

It appears that newer boxes work the same way, and new replacement rolling code receivers also do that.

It would seem that this system would be problematical, particularly for the newer receivers that need to remember stuff.

Have I misinterpreted what appears to be going on here? If not, how/why does this work?

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You'll find that the sensors are fed from the main unit via current-limiting devices such that a heavy current draw causes a 1 ->

0 transition ;-) ...Jim Thompson
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

Something to keep a short from burning stuff up (see JT's comment), a big- ass cap to hold power during the "power off" interval, and flash memory.

Or, don't short it out completely -- just draw enough current to suck it down to 5V or so.

Tim Wescott 
Wescott Design Services 
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Tim Wescott

You don't need power available to "remember stuff".

Try this:

- install your receiver

- give it whatever "secret" it needs to remember

- SHORT the terminals to the receiver FOR A LONG TIME (seconds) [chances are, the GDO will either start opening when you first APPLY the short *or* when you remove it. But, probably won't REstart when the GD completely closes/opens -- i.e., it looks for the *transition* to start the mechanism]

- now try your receiver to see if it still "remembers".

Or, easier still: simulate a power failure by unplugging the GDO (which removes power to the receiver!) for MANY HOURS.

All you have to do is be able to create a "temporary short" that is long enough for the GDO to sense that fact. I.e., you need to have some energy stored "outside" the shorted system that will allow you to maintain the short (via a FET or transistor) for a long enough time to cause the GDO to notice it.

Electronic doorbells also do this (in a few different ways).

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Don Y

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