John Fields said:
John Fields said:
On Sat, 23 Apr 2005 00:31:26 -0700, "Larry Brasfield"
On Fri, 22 Apr 2005 09:42:24 -0700, "Larry Brasfield"
...
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 was not addressing use of the optoisolator in that
position. However, if the current taken through the
opto LED is limited to a few 100 uA, such usage
would still be a small fraction of the buzzer power.
As you point out, CTR would be reduced, but no
more than a few uA of output would be needed.
---
That's not the point. As you've already stated, large value pullups
or pull-down resistors may be used in the remote in order to conserve
battery power during switching, and it's precisely that which makes
using an opto in other than a saturated mode problematical. Consider:
+V
|
[100k]
|
+----->Eout
|
O |
|<-
O |
|
GND
In this case, if the load on Eout is insignificant, (CMOS, say) Eout
will be either +V or, assumong GND is at 0V, 0V.
However, in this case:
+V
|
[100k]
|
+----->Eout
|
C A
B <--[LED]
E K
|
GND
Eout will depend on the collector-to-emitter resistance of the
transistor, and if it isn't driven low enough (if the current through
the LED isn't high enough) Eout may not cross the switching threshold
of the driven device.
I've seen commercial uses of photo-transistors
used as switches with 1M pullups. The transistors
themselves do fine at such levels, suffering only
slight beta reduction. Saturation resistance is
very nearly inversely proportional to excess
base current (or the equivalent photocurrent
for a phototransistor), so I see no reason to
expect the problem you allude to here.
---
Without adequate drive the phototransistor will never go into
saturation, so whether the output of the opto can pull the driven load
down (or up) far enough to cross the switching threshold becomes the
problem.
Agreed.
As for using the phototransistor in a mode
other than saturated (or nearly off), I have
not suggested that. It might be useful, when
the receiver can deal with a non-switching
input, but that is not the case for the position
I suggested the opto for.
---
I know you haven't suggested that, but it may well be the position the
opto finds itself in if there's not enough photocurrent to drive its
output into saturation. That is, operating linearly, its
collector-to-emitter resistance may be too high to cause the driven
device's switching threshold to be exceeded.
I think we can noisily agree that using sufficient
current is necessary with the optoisolator and
that it should be more or less saturated when
used to replace a switch.
I think we have merely a terminolgy issue here.
You are willing to call a two terminal circuit a
follower. I use the term for common {emitter,
source, cathode} amplifiers.
However, maybe you think the underlying facts
impact the operation, and that being a follower
limits the voltage in the pullup configuration more
than in the pulldown configuration. In that case,
I suggest you connect a 1T resistor from the
base of the 4N25A in your simulation to ground
and run it, then observe the base voltage. You
may be surprised to see it more positive than the
other two terminals by about a diode drop.
You might be right though, if the LED drive current is high enough,
but with the microamp drive levels you're proposing, I don't think so.
I've only made one explicit statement about the
LED current, suggesting it might be "limited to
a few 100 uA". More on this in a moment.
Run this:
Version 4
SHEET 1 880 680
WIRE -192 208 -192 176
WIRE -192 400 -192 288
WIRE -112 176 -192 176
WIRE -16 400 -16 272
WIRE 32 176 -32 176
WIRE 32 272 -16 272
WIRE 288 240 224 240
WIRE 288 272 288 240
WIRE 288 400 288 352
WIRE 384 176 224 176
WIRE 384 192 384 176
WIRE 384 400 384 272
FLAG 384 400 0
FLAG 288 400 0
FLAG -192 400 0
FLAG -16 400 0
SYMBOL Optos\\4N25A 128 240 R0
SYMATTR InstName U1
SYMBOL voltage 384 176 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 3
SYMBOL res 272 256 R0
SYMATTR InstName R1
SYMATTR Value 100k
SYMBOL voltage -192 192 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 4.5
SYMBOL res -16 160 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R2
SYMATTR Value 100k
TEXT -226 506 Left 0 !.tran 0 .1 0
Interesting. It certainly demonstrates the reduction
of CTR at lower currents. When I reduce the LED
current setting resistor to 36K to get about 100 uA
thru the LED, the output saturates nicely. And with
that 1T resistor, the base goes to 3.53 V.
---
I disagree. Doing it your way requires knowing the values of the
pullups and/or pull-down resistors, the contact resistance of the
switch(es), the switching thresholds of the driven device, the drive
level and voltage available from the buzzer to drive the OPTO LED (if
that's how you were planning to do it) and...
My way only requires me to know what the drive voltage to the buzzer
is, or if there's an ALARM ON signal available, what its voltage is.
I've already agreed that the relay solution is easier
to apply. My reason for stating so is exactly the
sort of required knowing you mention. For that
reason, (and the capacitance issue I've posted), I
think your solution is entirely appropriate for the
OP's purpose.
---
Take a look at the contact life specs of any reed relay hot switching
microamps and you'll probably get a grin on your chops when you
consider how many centuries worth of garage openings and closings that
comes out to!^)
Ok, that's funny too. I could quibble about centuries
being the proper unit of measurement. (What falls
between millenia and eons?) I never claimed my
dislike of moving parts was rational!