LED indicator for signal line help

Thread Starter

Longymod

Joined Jan 29, 2018
13
Afternoon I hope I am in the right section.

Looking for advice on the most efficient way to a problem.

I need an LED indicator to monitor a signal line from an output pin on a MAX232 chip. Currently I am connected to this line using an NPN transistor which is driving an LED. Everything works as it should. All the data comes through fine and the LED blinks.

The issue I have is the MAX232 output pins start up HIGH when first powered on with no input signal being received. Which in turn drives the Transistor on to illuminate the LED.

I need the LED to stay off on power up to not cause any confusion incase the income source is not connected.

Appreciate any simple tips.

Thanks in advance
 

dendad

Joined Feb 20, 2016
3,261
You could capacitivly couple the signal to the transistor so it just responds to the changing signal. It will still flash on power up but then go out.
Also, on the incoming signal line to the MAX232, add a pullup or pulldown resistor, whichever works, so it is not just open with nothing connected.
 

Thread Starter

Longymod

Joined Jan 29, 2018
13
Thanks for the swift reply dendad, I will try the cap. I currently have pull up resistors on the input pins.

All working, Thank you for the help
 
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