Need help with a current detection circuit

[MattTheNoob]

Ok electrical gurus!

I'm setting up my trunion table (heavy, German), and the limit switches on it have me stumped. They are 2 wire, and the signal is <1mA off, and >3mA on. So the 8Vdc + and - get hooked to the two leads and I need some way to detect that tiny current and get it out to a relay contact that I can use.

The other useful part is a marking that says Ri=1K Ohm.

[keebler303]

I doubt you have limit switches with a current output. Where are you measuring this current? Do you have a picture of the switch? I think you might not be connecting it correctly, which is giving you funny results. Post up some more details.

Matt

[MattTheNoob]

I would doubt me too. The switch is a Pepperl+Fuchs 83/2022X. Metal, threaded barrel about 3/4" long and 3/16" dia (smallest prox I've ever seen), with potted cable. Only two wires, blue and brown. On the cable is printed "8V-" and BN+ and BU-.

I tried measuring voltage in series (thinking that it might be a mag switch)... nothing, read the same voltage with or without proximity to target.

Then I did some Google-Fu, and while I couldn't find a spec sheet for the switch itself, I did find it as an option in a flow control valve. If you follow the link and search for 2022x, you'll find the specs...

Inductive limit switches,
Proximity switches SJ 3.5 SN, CENELEC intrinsically safe,
EEx ia IIC T6, PTB No. Ex-83/2022X,
Limits freely adjustable between 0 and 100 % of stroke,
Current circuit to DIN 19234 with 8 V DC control voltage
Control current < 1 mA = logical “0“
Control current > 3 mA = logical “1“

http://www05.abb.com/global/scot/scot211.nsf/veritydisplay/8a255238f1aa2d2dc12574cc0038b263/$File/13_832se.pdf

So, I hooked it up in series with my amp meter, and sure enough got the current change with proximity.

[Al_The_Man]

Generally two wire prox switches are used principally with a low current level logic input to I/O logic devices such as PLC's.
If you want to switch a relay, a 3 wire type with transistor output is usually recommended.
Al.

[keebler303]

That is strange, I have never seen a switch like that.

Do you know about Ohm's Law? I think you need to put a resistor in series with the switch and measure the voltage drop across the resistor. That voltage drop can be fed into an op amp comparator which will give you a 5V logic level output based on the state of the switch. Do a little more google-fu and you should end up with a working circuit for about $1.

Matt

[MattTheNoob]

No doubt, and I'd buy something along those lines if I were to start from scratch. That being said, I know there must be a way to cobble a few transistors and optocouplers together to get me where I want to go.

(I only picked those two parts because that's what I needed to build my gecko monitor circuit which was similar... having to detect the 5v without pulling any current.)

[Al_The_Man]

For 2 wire examples Check out post #252 here.
Electronic home switches made easy!
There many 2 wire that will go up to 100ma, did you try and see if it will support a current burden of ~50ma by trying a 160ohm resistor in series as suggested and check the volt drop across the res? If it does then you could sub the res for a relay.
Al.

[irving2008]

Here's one possible solution that uses minimal components...

This wont drive a relay direct (why do you need to drive one?) but its easily adapted...

[irving2008]

For 2 wire examples Check out post #252 here.
Electronic home switches made easy!
There many 2 wire that will go up to 100ma, did you try and see if it will support a current burden of ~50ma by trying a 160ohm resistor in series as suggested and check the volt drop across the res? If it does then you could sub the res for a relay.
Al.

It'll fry if you do... I've seen those ones before, the internal chip won't handle it...

[MattTheNoob]

COOL!

Now on that graphic, the resistors are shown as 1k0 and 4k7. Are those supposed to be 10k and 47k? Also, a quick check of my electronics store (Ebay) shows 74C14J's, N's, and D's. Do I worry about that?

[Al_The_Man]

It'll fry if you do... I've seen those ones before, the internal chip won't handle it...

Do you have the spec sheet on it? It would help in making an informed decision.
The Pepper & Fuchs catalog I have, most of the low voltage two wire go up to 100ma rating.
I don't see much point in them using such a low rated one in that application?
Al.

[irving2008]

COOL!

Now on that graphic, the resistors are shown as 1k0 and 4k7. Are those supposed to be 10k and 47k? Also, a quick check of my electronics store (Ebay) shows 74C14J's, N's, and D's. Do I worry about that?

They are 1k and 4.7k, that is the international standard way of showing resistances avoiding the issue of different decimal seperators (some countries outside the US use commas instead of decimal points)

The J, N, D usually designate different packaging and temperature standards... N i think is the one you'd probably want - normal 0 - 85degC plastic DIP package, but check.