Interfacing 5V to unknown 10-30V signal

I'm trying to interface a bunch of industrial sensors (that I don't
physically have) to a piece of 5V logic. I don't have specs on these
sensors, but I am _told_ that they range between 10-30V supply voltage,
and they drive low when sensing whatever it is they key on.

So when the sensor is inactive, it will either be driving its output at
somewhere between 10-30V, or the output will be open circuit. When the
sensor goes active, it will pull the output low.

Does this seem like a reasonable approach to the problem:
http://www.larwe.com/oif.jpg

(The optoisolator is a speced requirement).

Sorry this question is rather vague, but I don't have sufficient
information about the topic myself yet to be specific

 

Hi Joe,

There are numerous rails (+5, +10, +12, +24, +48, +90 and 95VAC)
running around the appliance, because it is put together out of a lot
of modules that weren't originally intended to be integrated. I didn't
design the appliance, I merely have to add intelligence to it. I was
planning to run that +5V along with the actuator signals, purely to
power the interface (which would be on a separate, replaceable PCB).
The main circuit itself has a totally separate power supply.

A few mA sink on the input should be no problem.

I have to firm up what exactly I am talking to here, it's very
open-ended and I hate that. But thank you for the suggestion.

 

Hi Spehro,

1) See other reply, the +5V shown here is a totally separate supply
(it's part of the "actuator" side, not the "micro" side). Micro has its
own supply.

2) -> 4) Thanks 0201 is really too small for me though. I'm pretty
much standardized on 0603, with excursions to larger sizes only for
high power ratings. No kind of space constraints in this system.

The last time I used an optocoupler was in 1994 so I'll look at some
more modern app notes, but for this application I have 500ms to respond
to a change of state on the sensor, so there's no rush.

 

Hi,

Biting on the last sentence of your message: The reason I put in the
transistor was because my thinking goes like this:

* I don't know the exact voltage that will be present when the sensor
is inactive. So I don't know what series R to use for the correct LED
current. If I use an opto with built-in switching transistor, I'm
basically going to wind up with the same circuit I have here
discretely.
* I don't even know if the input is capable of driving the line high.
It might be the equivalent of an open-collector, or possibly even a
mechanical relay output with the other contact connected to ground.
* Therefore I need the transistor

 

[Rich Grise]

On Fri, 17 Dec 2004 12:44:10 -0800, larwe wrote:

Hi Joe,

There are numerous rails (+5, +10, +12, +24, +48, +90 and 95VAC)
running around the appliance, because it is put together out of a lot
of modules that weren't originally intended to be integrated. I didn't
design the appliance, I merely have to add intelligence to it. I was
planning to run that +5V along with the actuator signals, purely to
power the interface (which would be on a separate, replaceable PCB).
The main circuit itself has a totally separate power supply.

A few mA sink on the input should be no problem.

I have to firm up what exactly I am talking to here, it's very
open-ended and I hate that. But thank you for the suggestion.

Don't open-ended specs lead to open-ended fees? ;-)

Cheers!
Rich

 

[YD]

On 17 Dec 2004 07:45:28 -0800, larwe@larwe.com wrote:

I'm trying to interface a bunch of industrial sensors (that I don't
physically have) to a piece of 5V logic. I don't have specs on these
sensors, but I am _told_ that they range between 10-30V supply voltage,
and they drive low when sensing whatever it is they key on.

So when the sensor is inactive, it will either be driving its output at
somewhere between 10-30V, or the output will be open circuit. When the
sensor goes active, it will pull the output low.

Does this seem like a reasonable approach to the problem:
http://www.larwe.com/oif.jpg

(The optoisolator is a speced requirement).

Sorry this question is rather vague, but I don't have sufficient
information about the topic myself yet to be specific

Sounds like what you have is a NO NPN open collector output, like
this:


+----------------------+
| Sensor -----------o + 10 to 30Vdc
| Circuit |
| +--------o Output
| | |
| |/ |
| ----| |
| |\ |
| v |
| | |
| -------+--------o GND
+----------------------+

In this case your circuit will work, but as others have already said,
it's a bad idea. Anyway, before proceeding try to find the specs of
the sensors. There are too many variants of them to take the risk of
getting it wrong from the start, especially as you're making your own
hardware.

At the plant I work in there are hundreds of the beasts interfacing to
PLCs. It's a hassle as the cheapskates before me would use which ever
was cheapest at the moment and alter the programming accordingly. This
makes replacement and program logic an issue so I've now mandated
standardizing with NO PNP outputs so both sensors and input modules
are interchangeable.

- YD.

--
Remove HAT if replying by mail.

 

[Lewin A.R.W. Edwards]

Don't open-ended specs lead to open-ended fees? ;-)

Yes! But tragically my standard terms say that the final payment is due
on project completion, where that in turn is defined as written
certification by the customer that the project wholly or substantially
functions as required. No finish, no[t all the] money.

 

Hi,
I _MAY_ have a NO NPN o/c output,as you illustrated, but I don't
exactly know. Could be a relay contact, could be a TTL output even.
Hence the problem. I've asked for samples of all the sensors to be sent
to me (some of them are legacy devices for which datasheets are
unavailable). But there is no guarantee that they won't change, so I
want to be open-ended.

All I know is:

* Driven low when sensor active
* Not driven low when sensor inactive. Don't know if this means driven
to sensor supply voltage, driven to sensor's logic supply voltage, or
driven to some other arbitrary level.
* Supply voltage of sensor is between 10V and 30V

Assumed:
* Most unlikely they would have a boost converter in there pulling up
the output to higher voltages.
* They don't use -ve signaling voltages (relative to ground).

 

[Rich Grise]

On Sat, 18 Dec 2004 12:40:52 +0000, Lewin A.R.W. Edwards wrote:

Don't open-ended specs lead to open-ended fees? ;-)

Yes! But tragically my standard terms say that the final payment is due
on project completion, where that in turn is defined as written
certification by the customer that the project wholly or substantially
functions as required. No finish, no[t all the] money.

It just now hit me what "larwe" means. Up to now, all I could think of is,
"where the larwe?"

;-)
Rich

(like the Fugawi Indians, you see.)

 

Yes, I am the sole surviving member of the legendary Larwe-buk-Mk'Larwe
tribe. When I die, so will our cultural tradition of the ham, turkey,
marshmallow, hot sauce and chocolate sandwich.

 

If you saw it, your comfort level would be increased, I think. There
are a fairly large number of these things installed around the world,
and the nature of the material they process is such that there are
really no safety issues. As a ballpark example, consider those gumball
machines that drop the ball into a tube, and it runs around triggering
different sound and light effects before dispensing to the consumer.
That isn't the actual application, but it is a very good approximation
of the "danger level".

Note: I would have preferred to spec all the electronic parts in this
myself, but it wasn't possible, and the Customer Is Always Right.

 

[Terry Pinnell]

larwe@larwe.com wrote:

^^^

Who is 'you'? It's conventional and considerate to make it immediately
clear to whom you are replying. Virtually none of your replies do so.
Please use either the 'quote' facilities of your software, or just
copy/paste via the clipboard.

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

Google Groups doesn't automatically quote anything in its new
incarnation, and Mozilla doesn't provide a "paste as quote" option when
pasting into a web form. Hence my laziness, for which I shan't
apologize.

 

A followup to this: I kind of gave up on trying to guess around this
issue. I decided it was just too open-ended for me ever to be able to
say "Finished!", and so I told the customer that I would design the
circuit on the assumption that his sensors can drive the line high with
enough current to light the LED in the opto. The design I eventually
put together is at http://www.larwe.com/optoin.png if you're
interested.

The diode across the input will actually be an FDLL4148, not the power
diode shown in the schematic snippet. And the resistor on the input is
to be chosen according to the expected input signal voltage.