ignorant and needing help - please don't laugh :)

[dogfuel]

I have a signal as process current 24v, 7.5mA - the current does not vary - it is there or not.

To get this to a serial port, can I simply use an inline 22k resistor to drop the voltage and instead of measuring voltage drop just look for 7.5 v or 0 v ... can a serial port (usb to serial converted) handle this?

If so, is there any way to effect a "trigger" input level and ignore source current below 1mA?


Thanks!

 

[Sceadwian]

Can you describe this 24v 7.5ma signal better, and more information about the device itself? It sounds approximately like a current loop signal.

 

[dogfuel]

details ...

it is a current loop. It comes from the power supply of a welder and is used to detect if the welder trigger is depressed. As the signal is basical boolean, it should be 7.5 or nothing..

Thanks!

 

[Sceadwian]

You could use a resistor divider to limit the voltage and feed the output into an RS232 handshake line, then you can use software on the PC to poll the RS232 control lines to wait for the signal. What exactly are you trying to feed the output into? A micro controller, a PC, something else?

 

[dogfuel]

thanks again ...

I'm trying to feed the data to the pc directly in this case - either a physical com port or a rs232-usb adapter

 

[Sceadwian]

You could use an RS232 port then. But when the 7.5ma signal is absent is it grounded or is it floating? If it's floating you could use an outgoing handshake line through a resistor to another incoming handshake line to pull it high (RS232 is reverse logic so that would basically turn the outgoing handshake line into a pulldown resistor for the incoming handshake line) then when your feed your 7.5ma signal in (with appropriate voltage limiting) it will pull the line high.
I'm pretty sure I'm not being very clear but I hope you catch the gist. Then the software on the PC could poll the handshake lines and determine if the switch was pressed. You'd just have to poll it as fast as you could.