Volt free contact

[jimmythefool]

Hi all

I need to interface a PIC Input to a Volt Free Contact in a building. Would it be wise to use an opto-isolator? I Have 24v available on my board too, and I was considering using that through the Opto-isolator's led, as it is possible there may be some distance to the VFC, and I was worried about picking up noise, and also the possible voltage drop..

Thanks
John

 

[crutschow]

Not sure what a "Volt Free" contact is. Does that mean it has no external voltage available? I assume you're just trying to determine the position of the switch?

An opto-isolator requires an isolated voltage to provide isolation. But if you are just want to sense the postion of an isolated contact you don't really need an isolator.

If speed is not a consideration then you can suppress noise with an RC filter (low value resistor in series with the wire to the switch, and large capacitor to circuit ground at the PIC input). If you are sensing the position with the PIC input then the current will be low (say from a 5k ohm pullup to 5V with the switch going to ground) and voltage drop should be negligible for any reasonable wire size.

 

[Hero999]

A volt free contact it just a switch like a relay or toggle switch - an opto-isolater isn't required.

 

[jimmythefool]

Hi Crutschow, Hero999..

Like Hero999 said, a Volt Free Contact, is just a pair of contacts supplied, for example, a building's Alarm/ Fire Alarm or Swipecard authorisation. They're used a lot on revolving doors, automatic doors etc. There is no voltage at the contacts, you supply your own.
I want to use it to pull a PortB pin low (Pullups on) on a PIC18F4550. I was just concerned, about stray interference etc. Worst case, the cable would be approx 20 metres long. I have done this before with PLC's, but never with a PIC.

Regards
John

 

[Hero999]

You can use a pul-up resitor or pull down resistor - it doesn't matter.

If you're worried about interferance, then add a 100nF capacitor across the switch and use debouncing code.

 

[JimB]

If this is for a serious application, at the very leasy I would include a simple RC filter at the input to remove any noise and spikes.
I would only use an opto isolator if the cable to the contact ran through a very harsh electrical environment.
Also consider how you "wet" the contact, be sure that a short to ground will not kill the PIC power supply.

JimB

 

[gerty]

In the states we call them "dry contacts"...

 

[jimmythefool]

Thanks Jim..

What I intend to do, is use the no volt contacts to close the circuit between logic 0v and the PIC input pin (which is using the internal pullups on portB to pull it high). What I was concerned about, was if it was wrongly connected, and a voltage was applied to the input.

The power supply is another question. I have copious amounts of 24V DC available, and hoped to use a 7805 to power the PIC. Mainly because it keeps the circuit simple and cheap. The current draw is low, as it is only switching signals, not driving any significant loads. Although the 7805 is dropping 19V, I have been running a GLCD with backlight, this way, with no probs. Is this a reliable solution?

Thanks
John

 

[olly_k]

In the states we call them "dry contacts"...

That's much better - 'Volt free' is plain wrong. Plumbers use this term for some thermostats and I think they really believe it is voltage free!

 

[Centretek]

"Wetted" contacts have some current flowing, when closed, to prevent contact fouling.

 

[Centretek]

In the states we call them "dry contacts"...

Just goes to prove something or other.