Puzzled by opto-coupler.

[[email protected]]

I was toying with an opto-coupler from damaged machinery and
am puzzled by its behaviour. It has the SHARP logo, and was
used as a tachometer with a slotted wheel revolving in a slot
between IR LED and photo-transistor. It has three terminals
which I've deduced to be LED anode with a series resistor,
phototransistor collector, and a common terminal for LED
cathode and emitter.

When I apply an adjustable current source at the LED input,
and a 3V supply with a series resistor at the phototransistor
collector, the transistor current rises with LED current.

However, until the LED current reaches about 6 mA, it makes no
discernible difference to the transistor current whether it's
shielded from the LED or not. That is, the transistor current
stays high even when it's completely blocked.

When the LED current exceeds this critical level, the transistor
current is switched on and off by blocking and unblocking the
IR light from the LED.

At first I thought the device was defective, or that I'd
incorrectly identified the terminals. But this doesn't seem to
be the case. Can anyone please explain this behaviour ?

 

[Baron]

I was toying with an opto-coupler from damaged machinery and
am puzzled by its behaviour. It has the SHARP logo, and was
used as a tachometer with a slotted wheel revolving in a slot
between IR LED and photo-transistor. It has three terminals
which I've deduced to be LED anode with a series resistor,
phototransistor collector, and a common terminal for LED
cathode and emitter.

When I apply an adjustable current source at the LED input,
and a 3V supply with a series resistor at the phototransistor
collector, the transistor current rises with LED current.

However, until the LED current reaches about 6 mA, it makes no
discernible difference to the transistor current whether it's
shielded from the LED or not. That is, the transistor current
stays high even when it's completely blocked.

When the LED current exceeds this critical level, the transistor
current is switched on and off by blocking and unblocking the
IR light from the LED.

At first I thought the device was defective, or that I'd
incorrectly identified the terminals. But this doesn't seem to
be the case. Can anyone please explain this behaviour ?

Since you don't give a part No:, I'll hazard a guess that the transistor
has a built in Schmidt trigger !

 

[[email protected]]

Since you don't give a part No:, I'll hazard a guess that the transistor
has a built in Schmidt trigger !

Unfortunately, the part No. is no longer legible. I thought
of some hysteresis effect too. But why should that cause it
to refuse to turn off even when the IR beam is completely
blocked with a metal plate ? And it does turn off when the
LED current supply is cut.

Also, when I gradually increase the LED current, the photo-
transistor current increases in approximately direct proportion.
This suggests that there is no Schmitt trigger effect.

 

[Baron]

Unfortunately, the part No. is no longer legible. I thought
of some hysteresis effect too. But why should that cause it
to refuse to turn off even when the IR beam is completely
blocked with a metal plate ? And it does turn off when the
LED current supply is cut.

That suggests a common circuit !

Also, when I gradually increase the LED current, the photo-
transistor current increases in approximately direct proportion.
This suggests that there is no Schmitt trigger effect.

Still guessing ! Can you positively identify the connections on both
sides ? You said that the LED has a series resistor ? Is it built in
or an external part ?

 

[Jonathan Kirwan]

I was toying with an opto-coupler from damaged machinery and
am puzzled by its behaviour. It has the SHARP logo, and was
used as a tachometer with a slotted wheel revolving in a slot
between IR LED and photo-transistor. It has three terminals
which I've deduced to be LED anode with a series resistor,
phototransistor collector, and a common terminal for LED
cathode and emitter.

When I apply an adjustable current source at the LED input,
and a 3V supply with a series resistor at the phototransistor
collector, the transistor current rises with LED current.

So something like this:

: adj. +3V
: current |
: source |
: | \
: | / R1
: \ \
: / R2 /
: \ internal |
: / series +---> to voltmeter
: | |
: | |
: D1 --- ~~ |/c Q1
: LED \ / ~~ -| NPN
: --- ~~ |>e
: | |
: '-------------+
: |
: gnd

There is no pin for Q1's base, I take it, and you are providing an
external R1 but not R2 (which is internal to the device.) You are
monitoring the collector voltage, right?? Or are you directly
observing the current through R1?

However, until the LED current reaches about 6 mA, it makes no
discernible difference to the transistor current whether it's
shielded from the LED or not. That is, the transistor current
stays high even when it's completely blocked.

Okay. Clarity issue. You wrote earlier, "the transistor current
rises with LED current." Here just now, you say "no discernable
difference to the transistor current" AND you say, "the transistor
current stays high."

I think you are confusing terms, for one thing. I am going to go out
on a limb and guess that you are measuring the voltage at that
junction I show above as "to voltmeter." And that when you write "the
transistor current stays high" that you really mean that the
"transistor VOLTAGE stays high." In other words, that there is no
appreciable current flowing through R1 and you are actually measuring
voltages and not currents.

When the LED current exceeds this critical level, the transistor
current is switched on and off by blocking and unblocking the
IR light from the LED.

By this, I take it to mean that you see the voltage at the collector
of Q1 readily changing.

At first I thought the device was defective, or that I'd
incorrectly identified the terminals. But this doesn't seem to
be the case. Can anyone please explain this behaviour ?

Are my guesses correct as you understand it?

Jon

 

[Baron]

So something like this:

: adj. +3V
: current |
: source |
: | \
: | / R1
: \ \
: / R2 /
: \ internal |
: / series +---> to voltmeter
: | |
: | |
: D1 --- ~~ |/c Q1
: LED \ / ~~ -| NPN
: --- ~~ |>e
: | |
: '-------------+
: |
: gnd

There is no pin for Q1's base, I take it, and you are providing an
external R1 but not R2 (which is internal to the device.) You are
monitoring the collector voltage, right?? Or are you directly
observing the current through R1?


Okay. Clarity issue. You wrote earlier, "the transistor current
rises with LED current." Here just now, you say "no discernable
difference to the transistor current" AND you say, "the transistor
current stays high."

I think you are confusing terms, for one thing. I am going to go out
on a limb and guess that you are measuring the voltage at that
junction I show above as "to voltmeter." And that when you write "the
transistor current stays high" that you really mean that the
"transistor VOLTAGE stays high." In other words, that there is no
appreciable current flowing through R1 and you are actually measuring
voltages and not currents.


By this, I take it to mean that you see the voltage at the collector
of Q1 readily changing.


Are my guesses correct as you understand it?

Jon

Hi Jon. That makes sense to me. Not realised that he was measuring
voltages ! His mentioning 6ma LED current didn't help.

 

[[email protected]]

Hi Jon. That makes sense to me. Not realised that he was measuring
voltages ! His mentioning 6ma LED current didn't help.

Hi, Baron and Jonathan. Thanks for your interest, but no, I
did not mix up current and voltage in my description.

First, when I talked about 6mA and current source, I used a
simple regulated current source I designed and constructed
a long time ago to test zener diodes (it has served me well
for a variety of purposes I never envisioned). It can be
adjusted to supply 0.5 to 15mA up to a max of 50V.

I deduced that the LED has a series resistor by measuring
the voltage drop at different current levels. By subtracting
an estimated LED voltage, I deduced that the series resistor
is about 220 ohms.

Jonathan, your ASCII diagram is correct except that I
measured collector current, not voltage. Well, I did also
monitor the collector voltage with a 'scope, but when I
mentioned current, I do mean current as displayed by a meter
in series with the collector supply.

Regarding collector voltage, the CRO showed that it was
driven down to saturation, and stayed down even when the
phototransistor window was completely blocked, until the LED
supply was removed.

One possibility is that the common pin does not go directly
to cathode and emitter, but through a small resistor for
some hysteresis effect. I have not yet worked out the rest
of the circuit in my head that will fully explain its
behaviour. Maybe the 3V supply was too low for proper
operation. I'll try again with a higher voltage, and note
down numerical values.

 

[Jonathan Kirwan]

Hi, Baron and Jonathan. Thanks for your interest, but no, I
did not mix up current and voltage in my description.

Accepted. I guessed wrong.

First, when I talked about 6mA and current source, I used a
simple regulated current source I designed and constructed
a long time ago to test zener diodes (it has served me well
for a variety of purposes I never envisioned). It can be
adjusted to supply 0.5 to 15mA up to a max of 50V.

In that case, you are probably in a better position to examine your
problem (with us watching) than any of us. You know enough to design
an adjustable current source for diodes (which can set overly simple
feedback designs into oscillation.) And you have the part, too.

I deduced that the LED has a series resistor by measuring
the voltage drop at different current levels. By subtracting
an estimated LED voltage, I deduced that the series resistor
is about 220 ohms.

I'm glad you are adding in fuller details, as you understand them
currently.

Jonathan, your ASCII diagram is correct except that I
measured collector current, not voltage. Well, I did also
monitor the collector voltage with a 'scope, but when I
mentioned current, I do mean current as displayed by a meter
in series with the collector supply.
Okay.

Regarding collector voltage, the CRO showed that it was
driven down to saturation, and stayed down even when the
phototransistor window was completely blocked, until the LED
supply was removed.

So no ambient light getting in, scope properly ground referenced, no
hot lead of a reversed, non-ground-prong plug placing hot side AC
through your weak device and ruining it, etc. Assume valid
procedures. Got it.

Have you considered the possibility that the IR LED emits light that
can pass through your block? (For example, I think steel passes
around 20cm wavelengths and longer about like glass and can be used to
lens those wavelengths.) I know it is a longshot... but you haven't
said what you know about your barrier and how certain you are that it
blocks the emitted wavelengths. (I assume this is a near-IR LED, so I
suspect that your block works okay... but I have to ask.)

Do you have a way of observing the IR LED, more directly? A separate
photodetector, for example, that is wired to a simple transimpedance
amp?

One possibility is that the common pin does not go directly
to cathode and emitter, but through a small resistor for
some hysteresis effect. I have not yet worked out the rest
of the circuit in my head that will fully explain its
behaviour. Maybe the 3V supply was too low for proper
operation. I'll try again with a higher voltage, and note
down numerical values.

Well, there are various possibilities. Including Baron's hint towards
something more complex in the circuit. On this assumption, I took the
liberty of looking at Sharp's web site. I previously assumed you'd
done that and didn't find anything helpful, so I didn't want to waste
my time, too. But in this case, I took a chance.

Try this:
http://sharp-world.com/products/device/lineup/data/pdf/datasheet/gp1a073lcs_e.pdf

Look on page 2 at the schematic. This is a three-pin device with an
OPIC output. What do you think of this possibility?

Here is a general page for photointerruptors from Sharp:
http://www.sharpsma.com/Page.aspx/a...00e-4400-b23a-ee90c054389a/Photointerrupters/

Jon

 

[Eeyore]

Regarding collector voltage, the CRO showed that it was
driven down to saturation, and stayed down even when the
phototransistor window was completely blocked, until the LED
supply was removed.

It's not a simple phototransistor output device in that case. Their output
conducts when the light path is unobstructed.

Graham

 

[[email protected]]

[snip]

Regarding collector voltage, the CRO showed that it was
driven down to saturation, and stayed down even when the
phototransistor window was completely blocked, until the LED
supply was removed.

[snip]

Have you considered the possibility that the IR LED emits light that
can pass through your block? (For example, I think steel passes
around 20cm wavelengths and longer about like glass and can be used to
lens those wavelengths.) I know it is a longshot... but you haven't
said what you know about your barrier and how certain you are that it
blocks the emitted wavelengths. (I assume this is a near-IR LED, so I
suspect that your block works okay... but I have to ask.)

I used mild steel plate about 0.5mm thick. It must be
effective as a barrier because it turns the output on
and off when the LED current is above 6mA.

Do you have a way of observing the IR LED, more directly? A separate
photodetector, for example, that is wired to a simple transimpedance
amp?

I didn't have anything handy by way of a photodetector.
But I shone a TV remote control on the phototransistor.
It showed the expected pulsed waveform on a CRO.

[snip]

Well, there are various possibilities. Including Baron's hint towards
something more complex in the circuit. On this assumption, I took the
liberty of looking at Sharp's web site. I previously assumed you'd
done that and didn't find anything helpful, so I didn't want to waste
my time, too. But in this case, I took a chance.

Try this:
http://sharp-world.com/products/device/lineup/data/pdf/datasheet/gp1a073lcs_e.pdf

Look on page 2 at the schematic. This is a three-pin device with an
OPIC output. What do you think of this possibility?

Thanks for the link. I did look for a datasheet, but I must
have looked in the wrong places. It *was* about 3:00 am. The
device shown looks very much like the one I have. The
simplified internal schematic accounts for its behaviour,
especially the way they connect the LED cathode in series
with the receiver instead of to the return terminal.

I thought of using it in a project and would have made a
mistake if not for that datasheet. The 7V max rating implies
that it's meant to operate at TTL level. The project is an
analog-digital hybrid using a 12V supply throughout.

Now I have the options of changing the digital P.S. to 5V
and adapting the signal levels at the analog-digital
interface points, OR to provide a 5V supply just for the
optocoupler and let its output drive a transistor operating
from a 12V supply rail.

 

[Rich Grise]

I was toying with an opto-coupler from damaged machinery and
am puzzled by its behaviour. It has the SHARP logo, and was
used as a tachometer with a slotted wheel revolving in a slot
between IR LED and photo-transistor. It has three terminals
which I've deduced to be LED anode with a series resistor,
phototransistor collector, and a common terminal for LED
cathode and emitter.

When I apply an adjustable current source at the LED input,
and a 3V supply with a series resistor at the phototransistor
collector, the transistor current rises with LED current.

However, until the LED current reaches about 6 mA, it makes no
discernible difference to the transistor current whether it's
shielded from the LED or not. That is, the transistor current
stays high even when it's completely blocked.

When the LED current exceeds this critical level, the transistor
current is switched on and off by blocking and unblocking the
IR light from the LED.

At first I thought the device was defective, or that I'd
incorrectly identified the terminals. But this doesn't seem to
be the case. Can anyone please explain this behaviour ?

It sounds like there's more to the circuit than you can see by
looking at 3 leads, like the PHT is getting some kind of bias
from the current supply. Is it acting like a threshold thing?

But anyway, if you intend to use it, I'd go ahead and supply
it with whatever current makes it work, and just go for it -
you intend to use it as an interruptor, not a linear coupler,
right?

Good Luck!
Rich

 

[Baron]

Baron wrote:
Jonathan Kirwan wrote:

On 5 Nov 2006 13:29:50 -0800, [email protected] wrote:

I was toying with an opto-coupler from damaged machinery and
am puzzled by its behaviour.

[snip]

Regarding collector voltage, the CRO showed that it was
driven down to saturation, and stayed down even when the
phototransistor window was completely blocked, until the LED
supply was removed.

[snip]

Have you considered the possibility that the IR LED emits light that
can pass through your block? (For example, I think steel passes
around 20cm wavelengths and longer about like glass and can be used
to
lens those wavelengths.) I know it is a longshot... but you haven't
said what you know about your barrier and how certain you are that it
blocks the emitted wavelengths. (I assume this is a near-IR LED, so
I suspect that your block works okay... but I have to ask.)

I used mild steel plate about 0.5mm thick. It must be
effective as a barrier because it turns the output on
and off when the LED current is above 6mA.

Do you have a way of observing the IR LED, more directly? A separate
photodetector, for example, that is wired to a simple transimpedance
amp?

I didn't have anything handy by way of a photodetector.
But I shone a TV remote control on the phototransistor.
It showed the expected pulsed waveform on a CRO.

[snip]

Well, there are various possibilities. Including Baron's hint
towards
something more complex in the circuit. On this assumption, I took
the
liberty of looking at Sharp's web site. I previously assumed you'd
done that and didn't find anything helpful, so I didn't want to waste
my time, too. But in this case, I took a chance.

Try this:
http://sharp-world.com/products/device/lineup/data/pdf/datasheet/gp1a073lcs_e.pdf

Look on page 2 at the schematic. This is a three-pin device with an
OPIC output. What do you think of this possibility?

Thanks for the link. I did look for a datasheet, but I must
have looked in the wrong places. It *was* about 3:00 am. The
device shown looks very much like the one I have. The
simplified internal schematic accounts for its behaviour,
especially the way they connect the LED cathode in series
with the receiver instead of to the return terminal.

I thought of using it in a project and would have made a
mistake if not for that datasheet. The 7V max rating implies
that it's meant to operate at TTL level. The project is an
analog-digital hybrid using a 12V supply throughout.

Now I have the options of changing the digital P.S. to 5V
and adapting the signal levels at the analog-digital
interface points, OR to provide a 5V supply just for the
optocoupler and let its output drive a transistor operating
from a 12V supply rail.

The data sheet explains the hysteresis and the apparent strange
behaviour.

 

[[email protected]]

On Sun, 05 Nov 2006 13:29:50 -0800, pjdd wrote:


But anyway, if you intend to use it, I'd go ahead and supply
it with whatever current makes it work, and just go for it -
you intend to use it as an interruptor, not a linear coupler,
right?

That's right.

Good Luck!
Rich

Thanks.

 

[[email protected]]

The data sheet explains the hysteresis and the apparent strange
behaviour.

Yup.
Regarding pulse level conversion, I think I'll go for the
option of providing +5V supply for the device and use the
circuit to convert the signal level. It's simpler than
using a dedicated level converter IC. There will be phase
reversal of the pulse, but that's not important for the
intended application. Can you see anything wrong with it ?

+5V +12V

| |
| |
\ \
47k / / 10k
\ \
/ / ----- to f-v converter
\ \ |
| |-----|
| | |
From | |/ ----- to counter
photo-tr --------| NPN
collector |\
|
|
_|_ Ground

At first, I thought of inserting a diode in series with the
transistor base to raise the threshold level. But the
low-level output of the phototransistor is specced as 0.35V
max, so inserting the diode seems to be rather superfluous.

Resistor values are uncritical. Dark current is not specified
for the photo-tr, but I think 47k would be a reasonable
starting point. Just sharing my thoughts. Comments welcome.

 

[Baron]

Yup.
Regarding pulse level conversion, I think I'll go for the
option of providing +5V supply for the device and use the
circuit to convert the signal level. It's simpler than
using a dedicated level converter IC. There will be phase
reversal of the pulse, but that's not important for the
intended application. Can you see anything wrong with it ?

+5V +12V

| |
| |
\ \
47k / / 10k
\ \
/ / ----- to f-v converter
\ \ |
| |-----|
| | |
From | |/ ----- to counter
photo-tr --------| NPN
collector |\
|
|
_|_ Ground

At first, I thought of inserting a diode in series with the
transistor base to raise the threshold level. But the
low-level output of the phototransistor is specced as 0.35V
max, so inserting the diode seems to be rather superfluous.

Resistor values are uncritical. Dark current is not specified
for the photo-tr, but I think 47k would be a reasonable
starting point. Just sharing my thoughts. Comments welcome.

Looks about right ! The 47K could be a bit high though.

 

[Jonathan Kirwan]

Yup.
Regarding pulse level conversion, I think I'll go for the
option of providing +5V supply for the device and use the
circuit to convert the signal level. It's simpler than
using a dedicated level converter IC. There will be phase
reversal of the pulse, but that's not important for the
intended application. Can you see anything wrong with it ?

+5V +12V

| |
| |
\ \
47k / / 10k
\ \
/ / ----- to f-v converter
\ \ |
| |-----|
| | |
From | |/ ----- to counter
photo-tr --------| NPN
collector |\
|
|
_|_ Ground

At first, I thought of inserting a diode in series with the
transistor base to raise the threshold level. But the
low-level output of the phototransistor is specced as 0.35V
max, so inserting the diode seems to be rather superfluous.

The output transistor of that schematic is NOT a phototransistor.

Resistor values are uncritical. Dark current is not specified
for the photo-tr, but I think 47k would be a reasonable
starting point. Just sharing my thoughts. Comments welcome.

I'd expect that anything slower than about 30kHz would look okay. Your
counter is probably okay, but I don't know what you are using for an
f->v. If the f->v is a simple low pass filter, you might not get what
you want as it isn't even close to a balanced output.

The approach you provide is fine. It will treat the 47k as a rough
current source (you could also use the +12V to improve that a little,
but only if the output transistor can handle the voltage impressed on
its collector -- you need to check the spec on it.) Anyway, this
means a vaguely constant current that is switched one way or another.
Looks fine.

Jon

 

[[email protected]]

On 6 Nov 2006 23:19:03 -0800, [email protected] wrote:


The output transistor of that schematic is NOT a phototransistor.

Oops. I do keep referring to it as a phototransistor,
don't I ? Of course, the output transistor is simply the
output device after amplifying and conditioning the signal
from the photosensitive element.

I'd expect that anything slower than about 30kHz would look okay. Your
counter is probably okay, but I don't know what you are using for an
f->v. If the f->v is a simple low pass filter, you might not get what
you want as it isn't even close to a balanced output.

Frequencies are well below 1 kHz. The counter is a 4060
with Schmitt trigger input, so no transition time
limitation. The F-V converter is an LM2907 that can
operate even with ground-referenced sine waves and has a
switching threshold well below +/- 100mV. I intend to use
capacitor coupling to make it swing above and below ground,
and then drop the +/- 6V pulse (duty cycle is approx 50%)
with a 50:1 resistive voltage divider.

The approach you provide is fine. It will treat the 47k as a rough
current source (you could also use the +12V to improve that a little,
but only if the output transistor can handle the voltage impressed on
its collector -- you need to check the spec on it.) Anyway, this
means a vaguely constant current that is switched one way or another.
Looks fine.

I wish I could power the output directly from +12V to
obviate the need for a level converter stage - I like to
keep my circuits as simple as possible as long as they
work reliably. But the spec sheet gives the max supply as
7V. The output transistor alone can probably withstand 12V
or more, but I don't want to risk it.