Simple counter

So I've got a wire sending serial data, and what I'd like to do is have an LED turn on and stay high for about a half second whenever there is serial data being transmitted. I would like this to be done without the use of a microcontroller, but I unfortunately don't have much knowledge in electronics.

Just to be clear though, lets say that only 1 byte of data was sent - that should be enough to turn on the LED for a half second. If there is a chronic stream of data being sent for 1 full second, the LED should stay on for 1.5 seconds.

I'm sure this is simple and cheap to set up, I just don't know where to begin.

More details needed:
Is the wire at RS232 levels (+/-10V)?
TTL (0-5V)?
Something else?
What speed?
Does the LED turn on As Soon as data starts flowing, and then stay on?

The search phrase you need is "pulse-stretching circuit".

CrossRoads:
More details needed:
Is the wire at RS232 levels (+/-10V)?
TTL (0-5V)?
Something else?

TTL

What speed?
Does the LED turn on As Soon as data starts flowing, and then stay on?

It should happen as soon as possible. So, waiting for a capacitor to charge is probably going to take too long. The LED should stay on as long as data is flowing, and remain on for another half second when data stops flowing. It doesn't have to be exactly a half second either.

@MarkT
I'll look into that in a moment and see if that suits my needs.

So pulse-stretching circuits appear to be what I'm looking for but the ones I've encountered in my google search deal with pulses within nanosecond ranges - I'm looking for something within a millisecond range. I did come across things like the HCF4098BE (a monostable multivibrator), but I don't understand the datasheet well enough to know if I can make it do what I want, or how. I do like the fact that this particular IC has 2 separate modules, because I am going to want to replicate this for other serial lines (that being said, I prefer the most compact solution possible).

2 methods spring to mind.

  1. "Pulse stretching" as mark pointed out, a 555 timer would be up for the job.

But no.

I'd take the output from the LED and use a transistor to charge a capacitor with a bleed resistor (1.5seconds when fully charged to discharged) from the capacitor we switch on a second transistor which signals high/low and a 1.5 second delay from the last byte of data recived.