# Best power source for using 5V sensors

• Hi there,

since I don't have to much knowledge about things Electrical Engineering I have a question.

Goal: I want to use up to several I²C 5V gas sensors with my FiPy/LoPy and an Expansion Board 3.0

I would see two possible solutions:

1. Using a 5V battery and a bidirectional logic level shifter.
2. Using a 3.3V battery and a bidirectional logic level shifter and a buck converter to get the 5V for my sensors.

Question: Disregarding the fact that solution 1. could be accomplished without a buck converter in regards to the battery lifetime which option would be most preferable? And what pitfalls might I encounter?

(Bonus: Are there any good readings/introductory books you'd recommend for diving deeper in applied electrical engineering for creating circuits with boards like FiPy/LoPy and similar hardware?)

• @weingami Note that I2C level shifting is particularly easy as it uses an open drain-pullup signaling scheme. Shifting can simply be done with two FETs: https://electronics.stackexchange.com/questions/255607/i2c-level-shifting-mosfet

• @weingami what is your charging source for the batteries. For instance if it's automotive then you might be better off with 12V and down convert. All of our stuff has 12V or 24V supplies in some form or other. So that simplifies things a lot ;-)

If your charging source is Solar for instance then you may have a lot of battery size choice ?

T

• @weingami I probably wasn't clear enough. There aren't any (useful, at least) 5V batteries.

As @robert-hh said, what you think are "5V batteries" (the USB power banks) actually contain batteries with a nominal voltage of 3.2 or 3.7 V and the additional converter/regulator to get to the 5 V required by USB.

So if you use that as a power source, you would have battery -> conversion to 5 V -> conversion to 3.3 V when powering the FiPy/LoPy. And the 5 V input path is not very optimised, you usually end up using more power (possibly a lot more) than if you are powering via the battery connector.

But as always, YMMV, and testing the various options is the only way to know for sure in your specific scenario!

• @weingami There are good Up/down converter modules available, like from Pololu, which convert an input voltage of 2.54-11V into 5V output at a reasonable efficiency. I would use one of these.
The comment of @jcaron about batteries questions, whether 5V batteries exist. I do not know ones myself. But you might consider 5V Battery packs with USB output. These battery packs indeed should have some regulator inside similar to the one I recommended above. So the difference is only one of assembly.
Selecting sufficiently large Li-Ion battery, output voltage 2.8-4.2V, you can supply the xxPy directly from it, using the Vin port, and source a buck converter for the 5V.
Selecting a USB battery pack things seem easier, and they are available in pretty large capacities. But the standby current for the Expansion board might be larger. I have no experience with expansion board 3. So I do not know if the USB/UART bridge is switched off during deep sleep.