I'm interested in putting the MOSFET back on a Weathershield and I'm trying to piece together the values of the resistors to use here: VIN > R1 * R2 > A3, where * goes to the gate of the P-channel MOSFET.
Looking at this original post:
https://lowpowerlab.com/2015/01/30/weathershield-is-here/In the schematic it looks like it originally had R1=10k and R2=1.5k, but it shows 10k crossed out and replaced by 1k. Assuming VIN is at 5V, to me it looks like R1=10k would give an output of 0.652V coming out of the divider at * and going to the gate of the MOSFET gate. With R1=1k I calculate 3V going to the MOSFET gate.
Then when I look at the spec of the MOSFET, it looks like the gate-source threshold voltage is from -0.47 to -0.9 volts. So both the 10k and the 1k resistor would provide more than that difference. So, why the switch to the 1k? It seems like the 10k would provide more difference and draw less current. This is probably my lack of understanding of MOSFETs.
My questions are:
1) Why switch to 1k?
2) Why get rid of the MOSFET? Seems like a great way to save power.
3) I've been running the sketch that is setting A3 low when checking battery voltage, then sets it high again. But I have a 220ohm resistor where the MOSFET was. So I'm curious what is happening when A3 comes on and the battery readings start, and why I still seem to be getting accurate battery readings even though I'm feeding voltage from A3 into the measuring circuit, right? That is making my head hurt a bit.
Thanks anyone who can help.