I'm surprised the buzzer works at all on 1.5V but you can fix that by adding one or perhaps two diodes in series with the buzzer. Each diode will drop about 0.6V so reducing the buzzer voltage.

 

You're a genius! I didn't have a diode, but I did have an LED which did the job, and I can incorporate that into the device. Thankyou!

 

The output of the '555 that the timer uses doesn't go completely to the supply rails (either high ot low) and that's where that 1.5V comes from.

 

No idea what that means, but thankyou once again!