I've been trying to make an activity LED for my GPS module.
Main idea is to trigger the LED whenever the GPS module is sending data thru TXD pin.
Circuit works well when the LED is powered from 5V supply (fig 1), but when supplied from 12V (fig 2) the LED is on all the time :/ just some slight dimming occurs.
GPS module is supplied from 7805, and 7805 is attached to the same 12V supply as the LED.
Please check the attached images for more details.