Re: Need help understanding how to use opto-isolator

"John Fields" <jfields@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:560i61tfmof8mhjis826fj5q3dcghb53of@xxxxxxxxxx
> On 22 Apr 2005 05:57:14 -0700, "mjohnson" <crvmp3@xxxxxxxxxxx> wrote:
....
> Across the buzzer should be OK, but 395mV sounds awfully low, so I
> suspect it's being driven by AC. Measure it with your meter set to AC
> VOLTS and see what you get.

(To the OP:) I concur with that good advice.

....
> Before anything can happen we need to find out definitively what's
> happening at the buzzer when the alarm goes off, or we need to find a
> signal somewhere in the clock which changes state when the alarm goes
> off.

The power needed to drive a buzzer will be many
times larger than what needs to be picked off to
activate another circuit, (many mW versus uW).

....
>>I'm having a hard time getting a for the voltage accross the toggle
>>switch on the remote. I read 0 no matter if the switch is toggled or
>>not. I am probably not laying my probes across the switch correctly.
>>Do I need to touch both pairs of connectors at the same time?
>
> ---
> No, it looks like you only have to get across the terminals with trace
> connected to them. But, since we don't know how the switch is being
> used in the circuit, it would be best to use relay contacts (instead
> of the out of an optocoupler) across the switch terminals in order to
> activate the remote. Test it by shorting the terminals momentarily
> and see if it works the door. If it does, then relay contacts will be
> fine.
> ---
>
>>I'm assuming that I would use an opto-isolator to interface the pluse
>>circuit to the remote and that the output of the opto-isolator will be
>>enough to activiate the remote? I guess I'll just have to try it an
>>see.
>
> ---
> I wouldn't use an opto because of the current required for its LED and
> the uncertainty of being able to use its transistor output to trigger
> the remote.

Some such uncertainty is warranted, but I suggest that
there is reason to believe an opto-isolator will be fine.
The cheap (typically membrane) switches used in many
remotes are not asked to carry much current and, to
conserve battery power, large value pull-{up,down}
resistors are used. If a replacement for the contanct
had to carry more than 100 uA, I would be surprised.
The CTR (current transfer ratio) for opto-isolaters is
often guaranteed to be 100% or better, so a similar
current is all that the LED would need. Finally, the
signal sent thru the opto-isolator can be time limited
to just over what is needed for the remote in order to
conserve the battery.

> Here's what I see as a much simpler solution, with only the clock
> output needing to be defined in order to make it work:
>
> INTERFACE
> BATTERY
> CLOCK 4.5V REMOTE
> BATTERY | BATTERY
> 3V +---------+---------+ 3V
> | | | | |
> +--+--+ +--+--+ +--+--+ +--+--+ +--+---+
> |CLOCK|---|ALARM|---|1SEC |---|REED |---|REMOTE|
> +-----+ | DET | |MONO | |RELAY|---|SWITCH|
> +-----+ +-----+ +-----+ +------+

The opto-isolator would plug into that with little
change except reduction of the 4.5V battery drain
(unless my power surmises are completely wrong).

--
--Larry Brasfield
email: donotspam_larry_brasfield@xxxxxxxxxxx
Above views may belong only to me.


.

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