Adding an Activity Led to an IR receiver circuit
You want to indicate activity with a LED.  Activity is defined as a digital signal into a processor going low.

No, you do not want to just light the LED when the line goes low.  It could go low for such short fractions of the time that it will be hard to see some types of activity.  What you want to do is to light the LED for some minimum time when activity is detected.

20 ms is usually good for such things.  That looks like a short but solid blip.  Blips up to about 100 ms don't appear to vary in length, only in brightness.  If the digital line goes low for 1 ms at a time, then the light will appear very dim.

Since the digital signal is already going into a processor, the simplest solution is to drive the activity LED from another processor output, and do the timing in firmware.  I have done this kind of thing many times with a microcontroller.  Activity sets a counter to 20, which is decremented every ms when not zero.  The LED is lit whenever the counter is non-zero.

If you really want to do this electronically, the function you want is called a retriggerable monostable multivibrator or retriggerable one-shot.

However, you don't need really accurate timing, so you can do something yourself that uses a capacitor and resistor for timing, and a diode or transistor to reset to full time when the digital line is low.  A transistor is turned on when the timer is active, which in turn allows current thru the LED.

Added

Here is a example of a RC-based circuit that turns on a LED for a minimum time after a low input level:



When the input signal goes low, C1 is discharged to about 700 mV.  Note that due to the amplification of Q1, most of the current to discharge C1 is carried by Q1 and not sunk by the input signal.

When C1 is discharged, the bottom end of R1 is at low voltage.  This causes current thru R1 and the base of Q2, turning on Q2, which turns on the LED.  The LED will be driven with about 5 mA, which is plenty to make a small indicator LED on a board quite visible.

Eventually C1 will charge up again due to the current thru it.  This raises its voltage, which reduces the current out of the base of Q2, which then can no longer keep the LED on.

The LED will be on at about 5 mA for a good part of the time, but then will fade towards off at the end.  It is therefore impossible to say how long the pulse will appear to last, but the values shown in this circuit should be a good starting point.  Adjust R1 lower to make the pulse shorter.