Hi, Steve.
Detecting changes in current is quite easy. There are lots of other
ways to do this, but most of them would cost more (use digital signal
impressed on top of supply current,) so my guess still stands -- looks
at the current and decides what the state of the system is. Ditto the
up/down/light pushbuttons -- just put some resistors in series with
each button, and detect how much current flows. That is actually a
good clue to the technique.
(BTW, that's a lot of the way telephones work -- the mouthpiece
modulates the current and the telco detects that; they also measure
how much current flows when the phone rings; they can tell you are
off-hook because current is flowing through the circuit; they can tell
how many (approximately) telephone-type devices (actually
ringer-equivalents) you have connected in your home!)
Donald.
> Date: Sun, 23 Mar 2003 19:35:47 -0500
> From: Steven Trovato <trovato@computer.net>
>
> The SAME screw terminal that supplies the power to the sensor is also
> supplying the power to the IR beam transmitter. That means that there is
> always current flowing, because the transmitter must keep sending the
> beam. For you theory to be correct, the circuit would have to be
> differentiating between two different amounts of current. That's possible,
> I suppose, but I don't know if that's what it really does.
>
> The pushbutton control on my wall also does something similar. It has a
> button to open/close the door, and another to just turn on the
> light. Again just two wires. Somehow, it knows which button is pressed.
>
> -Steve
>
>
> At 07:26 PM 3/23/03 -0500, Donald H Locker wrote:
> >No guarantee, but if I were designing the circuitry, the lead that
> >carries power would also serve as the signal -- with nothing
> >interrupting the beam, current flows; when something interrupts the
> >beam, the current stops and my control logic watches for current
> >flow. This also either stops the motor or raises the door if the
> >wires to the sensors fail.
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