What do you think it will do?

It will delay the trigger by the RC time constant (4.7s) at the input. If the input pulse is short (say 200ms), it will still add the RC time, which is not what I think the requirements state. It should only add the timer timeout upon recovery of the HMI signal.

 

Hence the diode and 100 ohm resistor, which gives a 4.7ms time constant on the start of the pulse.
So, output pulse starts 4.7ms after the start of the trigger and ends 4.7s after the end of the trigger.

 

Isn't 4.7ms soon enough?

 

The cap is 47u or ?

 

It's 47uF.
Switch on resistance is 100ohm, switch off resistance is 100k.

 

1 RC=4.7 seconds
It will take about 1.8s to reach 1/3 Vcc of 5v

The only issue I see is the output will be high on power up since the trigger will be initially low.