Need output be grounded only when PIC is OFF

iso9001 · Joined: 02 Dec 2003 Posts: 262

I'm using a 16F for a new project and maybe this is already the case but,

When my pic is off (no power to VDD) I need a pin to be connected to ground.

But then when the pic is on I need it HIGH or Float. Is this possible ?

This may be a dumb question, but
thanks in advance,
-rhet

If it isn't does anyone have a cicuit that would do the same thing. (It cant use a transistor though. Since I am using a relay elseware that I can not add a diode to protect the transistor)

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA

You don't really give enough information. What is connected to this output? What are its requirements? Do you really need ground or can a pull down resistor work Question

iso9001 · Joined: 02 Dec 2003 Posts: 262

No, it needs to be ground.

I have a relay that when the user presses a button connects the coil to this ground (thus turning the relay ON)... I want to interupt it so that when the unit is ON, even if the user presses the button I control if the relay switches. Get ?

Problem is I can not modify any of the systems except my own and can only get to the ground wire of this relay.

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA

Then just stick a normally closed relay in there and when you want to disable it, turn on this relay thus breaking the current path.

iso9001 · Joined: 02 Dec 2003 Posts: 262

This is a 2" x 1.5" pcb. So adding a relay would take up a little more space then i'de like as well as costing a too much.

This original system's relay (I've been told) and pull a spike up to 1 amp when turning on (std 12V relay).

I'de hate to put in a relay just because of cost reasons. I need a cheap switch...

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

Relays are not cheap with prices starting around $3, but how important is the component price related to engineering costs?

A transistor or fet would be cheaper and smaller, but I don't understand the specifics of your circuit as you say that you 'can not add a diode to protect the transistor'?
And if a diode is impossible, than how about an R-C snubber in parallel to the relay?

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

When the PIC is off is there any +DC voltage anywhere available? If so I would have that voltage turn on a FET through a resistor. Connect a PIC I/O pin to the FET gate to turn the FET off under PIC control.

If there is no DC available when the PIC is off you may still be able to use a "depletion mode" FET. These are ON without excitation and get actively turned OFF, as opposed to common "enhancement mode" FETs that are normally OFF and actively turned ON. But depletion mode FETS are harder to find. http://www.supertex.com/pdf/datasheets/DN3525.pdf
is one possibility, but note it requires a negative gate voltage to turn OFF. Maybe you could steal the negative voltage from a RS232 driver?
_________________
The search for better is endless. Instead simply find very good and get the job done.