Reverse LED On/Off

[Tomas]

Hi all, I've got very little experience in electronics but trying to learn some things. Hoping to get some advice here. Forgive me, but I don't know the correct terminology for power, ground etc..

I was given a GPRS modem and breakout board, and I've connected the RNG output to an LED. The trouble is, this is on when there is nobody calling and off when someone is calling.

So I'm wondering how easy and what components I should be looking at if I wanted to reverse this, so that it is off when nobody is calling and on when someone is.

Basically the setup is power->modem->resistor->led->ground. I'd guess I have to get power to the led and use some component to ensure that when both are powered (PRW and RNG) the led is off and when RNG is not powered, the led is on. I could be way off the mark there though, that was just a guess.

Any advice?

 

[(*steve*)]

You have connected the LED (with a series resistor I hope) between the output and gnd (the -ve end of the battery).

I would suggest connecting the LED and resistor from Vcc (the +ve end of the battery) to the pin.

This presumes that the RNG pin can both sink and source current. If the LED doesn't light at all this way, then you need something a little more complex.

Because LEDs are polarised, you MUST make sure it's the right way around. When you make the change I suggest, the end of the LED that was toward gnd must now be toward the RNG pin, and the lead that was connected to the RNG pin must be connected to Vcc.

If you don't have a series resistor, place one in series with the LED. a 1k resistor will be fine for testing. The LED may be dim, but you shouldn't kill it. You can figure out a better value later.

 

[Tomas]

Hi Steve,

I tried what you described and it works perfectly

Thank you for the help

 

[(*steve*)]

No problems at all.

Thanks for the very well defined problem.