Delayed “hold on” circuit ideas?

[Solder_Junkie]

I’m suffering from brain fog, I’ve built one of these in the past but can’t quite sort a simple solution.

The circuit is to hold a data radio transmitter on transmit for 15 to 20 mS after the keying line drops. The transmit duration can be anywhere from around 100 mS to 20 or 30 seconds. The key line is normally high, and pulled low to transmit.

Your thoughts are appreciated.
SJ

[Solder_Junkie]

I finally found the original board that I used in 2008. I have drawn the circuit and attach it here.