Evening, having fun with new projects, and have implemented the new power shields. However, I have two questions, one when reading voltage on A7 what math formula are people using with a 3.7 LIPO (A7 * (5.0 / 1024))?
Second, I have a sensor needing 5V, and I used the vout, but realized that I have a constant drain even when in low power mode. Any thoughts on how to direct the 5V when I need it, and kill it when I don't?
Thanks.
If you look at the PowerShield features page (http://lowpowerlab.com/powershield/#features), you can see the resistors used for the voltage divider are vbat > 470k < [A7] > 1m > GND. So A7 will read a divided voltage from the battery. Then you can use whatever formula will derive the actual battery voltage from that divided voltage when you know what the divided ratio is.
Here's an example sketch (https://github.com/LowPowerLab/RFM69/blob/master/Examples/WeatherNode/WeatherNode.ino#L71) that uses a formula to read the battery:
#define BATT_FORMULA(reading) reading * 0.00322 * 1.475
The calculated ratios will assume a range of 5V.
Also see this post (https://lowpowerlab.com/forum/index.php/topic,464.msg2680.html#msg2680) for an example formula.
You can use a transistor or mosfet to control power to a 5V sensor, look on the web for many examples how you can do this or search in this forum, i think there are several discussions about it. SonarMote (http://lowpowerlab.com/sonarmote) has this feature where it needs to provide 5V to the sonar sensor, it does this via the control feature of the LTC3525 booster without the need of an extra mosfet.
OK, so the power shield only provides constant 5V with an 3.x V input. I had that different in my head, thanks.