Stepping Down Increased Source Voltage

I have a sensor comprising an ATTiny85 MCU, HC -SR501 PIR module and 433MHz transmitter.
The sensor is powered by 3xAAA batteries.

My objective is to reduce size.

One option I am considering is powering the sensor from an A23 12V battery.
The PIR module and transmitter can both handle 12V supply.
The PIR module outputs 3V (need to double check).
The only issue would be supply to the MCU.

Questions...

  1. What would be the simplest form of voltage step-down to supply just the MCU.
  2. Am I better off, from a battery life perspective, to supply only the MCU with 5V or to supply everything with 5V via something like this.

Alternatively, is a suitable 12V tolerant MCU available/practical?

I have never heard of a 12V MCU, but you might need to consider battery lifetime. The A23 cells have much less energy capacity than AAA cells, and a 12V-5V step down converter would waste energy, making the problem worse.

jremington:
I have never heard of a 12V MCU, but you might need to consider battery lifetime. The A23 cells have much less energy capacity than AAA cells, and a 12V-5V step down converter would waste energy, making the problem worse.

+1

aisc:
...
My objective is to reduce size.
...

For highest energy density (MJ/L)

Lithium one time battery (2.1)
Lithium-ion rechargable battery (0.9~2.23)

For match output voltage:

Use high efficiency step up switch power supply.

(alkaline battery's energy density is 0.67, in other word Lithium battery at same size will give your 300% energy)