Which battery option to select

I have a battery powered circuit with the following items : - Ultrasonic Sensor : 5V - RF Module : 3.3V - MCU : 1.8 - 5.8V

My battery options (excluding rechargeable batteries) are : A. 3 x 1.5 AA (or AAA) batteries with a 3V tap: Ultrasonic Sensor at 4.5V, MCU at 4.5V, RFM at 3V from 3V tap plus level shifting RF signals.

B. 3 x 1.5 AA (or AAA) batteries with a 3V tap: Ultrasonic Sensor at 4.5V, MCU at 3V, RFM at 3V from 3V tap.

C. 2 x 1.5 AA (or AAA) batteries : Step-up from 3V to 5V for Ultrasonic Sensor, MCU at 3V, RFM at 3V.

D. 3 x 1.5 AA (or AAA) + 1 x 3.3V cell battery : Ultrasonic Sensor at 4.5V, MCU at 4.5V, RFM at 3V plus level shifting of RF signals.

E. 3 x 1.5 AA (or AAA) + 1 x 3.3V cell battery : Ultrasonic Sensor at 4.5V, MCU at 3, RFM at 3V plus.

Which would give me the longest operating time?

Well, I'd go with 'A' then.

Did you also have an Arduino of some kind?

johnwasser: Well, I'd go with 'A' then.

Did you also have an Arduino of some kind?

The MCU is an "Arduino on a breadboard".

Wouldn't the B option give longer battery life than A considering there is no level shifter to power?

aisc: Wouldn't the B option give longer battery life than A considering there is no level shifter to power?

When I looked at the message there was only option 'A'. That's why I picked it. Note that if you run the ATmega328p on less than 3.78V you should not run it at 16 MHz. At 8MHz you can go as low as 2.4V.

Another way to provide a low budget 3.3V source is to feed the 4.5V through a couple silicon diodes. They'll give you a ~0.6V drop each. 4.5V - 0.6V - 0.6V --> ~3.3V

johnwasser: When I looked at the message there was only option 'A'. That's why I picked it. Note that if you run the ATmega328p on less than 3.78V you should not run it at 16 MHz. At 8MHz you can go as low as 2.4V.

Sorry I was in the middle posting writing the posted when it got posted prematurely. Thanks for the heads up on the ATmega328p.

Now that I have listed 5 options, what would you recommend where the MCU is an ATtiny84A?

I just tried my cheap HR-SC04 ultrasonic range sensor on my 8Mhz/3.3V Arduino Mini (clone) and it will not work at that voltage. I get nothing but 0 cm. Switching the power to 5V allows the HR-SC04 to work normally.

Here is a helpful article on mixed-voltage systems: http://www.savagecircuits.com/content.php?85-Mixed-Voltage-Systems-Interfacing-5V-and-3-3V-Devices

Thanks for the article John - an interesting read.

Yes I am aware the HC-SR04 will not work at 3/3.3V which is why each option listed includes 4.5/5V.

Can I take it that when u switched the power for the HC-SR04 to 5V, u were still running the signals (trig/echo) at 3.v through the Mini - and it worked?

Anyway with an ATtiny84 as MCU I am leaning towards option B which will power the HC-SR04 at 4.5V (power only) and everything else (including HC-SR04 signals) at 3V.

Have a look on the US-100 Ultrasonic sensor, they work at 3.3V. Keeping the voltage low will help you to preserve battery.

For the MCU, RF step-up and others circuits, check my board: https://bitbucket.org/talk2/whisper-node-avr/overview. You might get some ideas from there.

Cheers!