# Supply 3.3V to two devices from one battery?

I have an XBee transmitter: XBee 1mW Wire Antenna - Series 1 (802.15.4) - WRL-08665 - SparkFun Electronics specs are:

3.3V @ 50mA
250kbps Max data rate
1mW output (+0dBm)

and a co2 sensor: http://www.co2meter.com/products/cozir-0-2-co2-sensor specs state:

Power Input: 3.3-5.5VDC (3.3V recommended)
33mA peak, 1.5mA avg power requirements

The sensor is running in streaming mode which states in the manual that the power consumption is 3.5mW in this mode.

I have this battery: Lithium Ion Battery - 2Ah - PRT-13855 - SparkFun Electronics specs are:

Each cells outputs a nominal 3.7V at 2000mAh!

Three questions:

Can i just wire a splitter to both my devices from this one battery to power both?

How do I step the voltage down to 3.3 from 3.7?

How do I calculate how long I would expect this battery to keep things powered based on the power consumptions listed above?

Can i just wire a splitter to both my devices from this one battery to power both?

Yes

How do I step the voltage down to 3.3 from 3.7?

I would use a buck converter. The voltage difference is too small for a conventional linear regulator.

How do I calculate how long I would expect this battery to keep things powered based on the power consumptions listed above?

A bit more tricky as the mAh figure does not specify a discharge rate. But for a rough idea, add up the current divide the mAh figure by this current, and then take about 80% of that.

And more like 50% after a year of use, probably. I reckon you should recharge once a day with that
battery as it probably won't last 2 days.

Grumpy_Mike:
Yes
I would use a buck converter. The voltage difference is too small for a conventional linear regulator.
A bit more tricky as the mAh figure does not specify a discharge rate. But for a rough idea, add up the current divide the mAh figure by this current, and then take about 80% of that.

what do I buy to build the circuit for the converter? And does that need power too or is it passive with some resistors/capacitors or something? Would this work? https://www.adafruit.com/products/1066

I have a similar set up but with a sensor that requires:

Voltage: 4.5 - 5.5 VDC
Current: < 6mA (streaming @ 1 Sample/sec), < 17mA Peak

How can I power it and the XBee board from the same battery (assuming I bought a dedicated one just like the one above?) I think I will need something like this: https://www.adafruit.com/products/2465 but want to be sure before I buy stuff.

MarkT:
And more like 50% after a year of use, probably. I reckon you should recharge once a day with that
battery as it probably won't last 2 days.

They really only need to last 6-8hrs max, then I can recharge them overnight.

I would not use a buck converter there - at least not a cheap one. At low power consumption, the overhead of a cheap buck converter will exceed it's efficiency benefits. Why not just get a good LDO? The AP2114 has low enough dropout that you'll be in safe waters running on a LiPo (per fig 32), and it's as easy to use as a 1117-series regulator!

DrAzzy:
I would not use a buck converter there - at least not a cheap one. At low power consumption, the overhead of a cheap buck converter will exceed it's efficiency benefits. Why not just get a good LDO? The AP2114 has low enough dropout that you'll be in safe waters running on a LiPo (per fig 32), and it's as easy to use as a 1117-series regulator!

What's an LDO?

I ended up buying one of these to try for my 5V case: PowerBoost 500 Basic - 5V USB Boost @ 500mA from 1.8V+ : ID 1903 : \$9.95 : Adafruit Industries, Unique & fun DIY electronics and kits

and one of these to try for my 3.3V needs: LM3671 3.3V Buck Converter Breakout - 3.3V Output 600mA Max : ID 2745 : \$4.95 : Adafruit Industries, Unique & fun DIY electronics and kits along with a linear regulator just for kicks. 3.3V 250mA Linear Voltage Regulator - L4931-3.3 TO-92 : ID 2166 : \$0.95 : Adafruit Industries, Unique & fun DIY electronics and kits

LDO - Low Drop Out regulator
See the "Voltage - Dropout (Typical)" column here. Some have small d
http://www.digikey.com/product-search/en/integrated-circuits-ics/pmic-voltage-regulators-linear/2556290?k=regulator

See page 3 of this datasheet for example.

The linear regulator you linked is sufficiently low dropout that it should work. It's got less head-room than the one I suggested, but it should work.

DrAzzy:
The linear regulator you linked is sufficiently low dropout that it should work. It's got less head-room than the one I suggested, but it should work.

Awesome! Yea i just need to breadboard this system up and then I can hand it off to a real EE person and they will build PCBs, housing, etc for me. Just gotta proof of concept this now.