Using NI 9403 as voltage source for its own inputs

[nlammens84]

I need to have some diagnostics in a circuit I'm building. I am planning on using some relays to throw a switch when some events happen. That switch would be connected to a NI 9403 digital input, as a means of detecting the event.

To make this work, I need to power the switch itself, so it switches a voltage that the NI 9403 considers as a digital "high".

I can buy a 5V supply, but it's expensive compared to what it only has to do. I was wondering if it is allowed / safe / sane ... to use one of the NI 9403 pins as a digital out, and use that as a voltage source for the switches? According to the datasheets I should be fine concerning power. I'm just not sure if this is a decent way to do this. 

[ThiCop]

Hello,

Do you have some information about the switches you're planning to use and a schematic of how you would like to connect everything?

[Riccardo-99]

how did you solved this problem?

[ThiCop]

hello Riccardo,

Are you trying to do the same thing?

If yes, can you then provide us with some extra info?

[Riccardo-99]

I'm trying to find a way to measure if a switch is open or closed; the switch it's a reed contact, the goal is to check if a door is open or closed.

I'm planning to buy a cdaq 9304 (32 DI) , because I have to measure the state of 12 switches, and I also need to adquire a pulse signal from an anemometer.

is that a good idea?

do I have to buy separate 5 volts alim?

can I use a Digital Output DO and create a simple circuit DO--> switch --> DI

are there any other solutions in order to know if a switch is open or closed?

thanks

[Buechsenschuetz]

I am not familiar with the board you are going to use. But almost all digital inputs on NI cards have pull-up resistors, so you do not need any voltage source for detecting the state of a switch. Simply connect the switch between a digital input and digital GND. 

[Riccardo-99]

thanks,

so I think I will need a sourcing digital input as described here:

http://digital.ni.com/public.nsf/allkb/A10122C63A7F5CFE86256B4C007491DD

[ThiCop]

Hello  everyone,

In  this case (9403) pull-down resistors are being used.

So you'd better work the "other way around" in this case.

One thing you could do is to use one of the pins as Digital Output and to set it to 5V.

Connect this pin to one side of the switch.

At the other side you can just use one of the other DIO-pins as Digital Input.

This (in combination with the pull-down resistor) should allow you detect if a switch is open (eg. 0V) or if it's closed (eg. 5V).

Of course this still depends a bit of the type of switch you're using, but in general this should be a possible solution.

[nlammens84 · Accepted solution]

My project ended up requiring a current amplifier board, which I had to solder. I then decided I could as well just add some voltage regulator to that board and some resistors to simplify the switching. I never tried using the NI 9403 as a voltage source.

[ThiCop]

Hello nlammens,

Thanks for the feedback!