Countdown Timer

Hello,
I am doing the same project as discussed here.
I have countdown timer with battery source 1.5V.
From your diagram, I change the relay with LED and use 5V DC power supply.

Experiment-1:
Circuit using DC power supply 5V and Timer using battery 1.5V, the circuit works.

Experiment-2:
Circuit using DC power supply 5V and Timer using same power supply (with stepdown module to get 1.5V), the circuit NOT working. The LED always ON (even the alarm goes off).

My question is: Can I use the same power supply to power both Timer and Your Circuit?

Thanks