GPS UART activity LED issue

[kevin8]

Hello guys,

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.

Thanks!

[mvs]

UART is usually active low signal. In your circuit LED will be on in an UART idle state.
You can insert additional common emitter stage to act as an inverter.

You can make also a pulse extender for better visualization, for instance with a 555 timer in a mono-stable configuration.