Re: Need help understanding how to use opto-isolator

On Fri, 22 Apr 2005 09:42:24 -0700, "Larry Brasfield"
<donotspam_larry_brasfield@xxxxxxxxxxx> wrote:

>"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).

---
Depends. The OP's advocating using the signal driving the buzzer to
also drive the LED in an opto, which will be milliwatts VS milliwatts.
---

>...
>>>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.

---
Not necessarily, CTR falls off quickly as LED forward current
diminishes and there are temperature effects which need to be taken
into consideration which can largely be ignored with a comparator-reed
switch solution. Also, with the reed switch solution there is no
saturation voltage VS LED If problem since it's either on or off.
---

>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.

---
Either solution will require the generation of a timed pulse to the
remote, so that's probably a wash.
---

>> 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).

---
Could be. I'll defer judgement and wait until the OP comes back with
something definitive on the buzzer signal to post my design. If he
doesn't, I can always fall back on the acoustic thing I've already
posted. You may want to ask him about the current being conducted by
the remote's switch switch to see whether you can use an opto in
there. An easy way to determine the current would be to jump the
switch contacts with a milli/microammeter...

--
John Fields
Professional Circuit Designer
.

Relevant Pages

  • Re: Need help understanding how to use opto-isolator
    ... The OP's advocating using the signal driving the buzzer to ... >would still be a small fraction of the buzzer power. ... Additionally, if the remote used pull-down resistors, the opto's ... with the reed switch solution there is no ...
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  • Re: Need help understanding how to use opto-isolator
    ... > Across the buzzer should be OK, but 395mV sounds awfully low, so I ... >>switch on the remote. ... >>circuit to the remote and that the output of the opto-isolator will be ...
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  • Re: Need help understanding how to use opto-isolator
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  • Re: Need help understanding how to use opto-isolator
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