Thanks for your help. Could you please explain me the following
I've understood that whenever there is a step pulse Q4 turns on and set the reference voltage to 4.3V using the D1 and R4, R5? please correct me if I am wrong

You are right.
Reference voltage 4.3V always presents on base Q4 with help of D1, R4 and R5.
Step pulse (its value may be from 3.3V to 12V) turns on Q2. Q2 turns on Q4.
Q4 maintains voltage V_load equal reference voltage plus drop voltage on Base-Emitter of Q4.
So V_load(step) = 4.3V + 0.7V = 5V.

 

You are right.
Reference voltage 4.3V always presents on base Q4 with help of D1, R4 and R5.
Step pulse (its value may be from 3.3V to 12V) turns on Q2. Q2 turns on Q4.
Q4 maintains voltage V_load equal reference voltage plus drop voltage on Base-Emitter of Q4.
So V_load(step) = 4.3V + 0.7V = 5V.

That's excellent. So the regulator will see 5V pulse when there is a pulse.What kind of diode do you recommend? I am using 1.8V step pulse now. what is the purpose of C1?

 

That's excellent. So the regulator will see 5V pulse when there is a pulse.What kind of diode do you recommend? I am using 1.8V step pulse now. what is the purpose of C1?

You can use Zener diode 4.3V, 300mW, price $0.23 https://www.digikey.com/products/en?mpart=BZT52C4V3T-7&v=31
C1 eliminates very short (part of us) negative going pulse in moment of transition from 12V to 5V.

 

You can use Zener diode 4.3V, 300mW, price $0.23 https://www.digikey.com/products/en?mpart=BZT52C4V3T-7&v=31
C1 eliminates very short (part of us) negative going pulse in moment of transition from 12V to 5V and from 5V to 12V.

Great. What kind of cap suitable for this? it is a bit tricky to find 1000pF

 

Great. What kind of cap suitable for this? it is a bit tricky to find 1000pF

Any kind of small cap for example ceramic is suitable. See capacitor, price $0.1 https://www.digikey.com/products/en...t=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25.
This transition pulse may or may not affect on your schematic functionality. Check it, if device will work fine, then you do not need this cap.

 

Any kind of small cap for example ceramic is suitable. See capacitor, price $0.1 https://www.digikey.com/products/en...t=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25.
This transition pulse may or may not affect on your schematic functionality. Check it, if device will work fine, then you do not need this cap.

Excellent!!! ordered.

 

I am using 1.8V step pulse now.

Because of 1.8V step pulse, transistor Q4 BS170 replaced with 2N2222, R1 changed to 10k.
For fine tuning 5V OUT voltage (if needed) used R6 33k*. Lower resistance - higher voltage and vice versa.

 

Because of 1.8V step pulse, transistor Q4 BS170 replaced with 2N2222, R1 changed to 10k.
For fine tuning 5V OUT voltage (if needed) used R6 33k*. Lower resistance - higher voltage and vice versa.
View attachment 143062

Thank you so much for your help

 

Thank you so much for your help

You are welcome.