I want to run a battery powered bread board Arduino on 3xAA batteries (so about 4.5V to 4.0V). I understand the concept of using a voltage divider to do level shifting between the ~4.5V MOSI pin on the Arduino to the radio module. But if your input voltage dips a bit, you won't get a consistent output voltage. I expect the battery voltage to drop over time, to from 4.5V to 4.0V before needing to change the batteries out.
Is the voltage drop a big deal for radio modules like the RFM69?
As an example:
Input voltage: 4.5V
R1:3200 ohm
R2: 10000 ohm
Output voltage = 3.409V
If input voltage drops down to 4.0V, the output becomes 3.03V. Is that a big deal?
I've seen level shifter modules. Partly because I'm not familiar with them, and partly because I want to keep the circuit simple and lower power consumption, I'd like to use a voltage divider if I can get away with it.
Also, regarding which pins I need to put the voltage divider on, is it just the MOSI pin? What about the clock pin and slave select pin? Do those need to be at 3.3V also?
arusr:
I want to run a battery powered bread board Arduino on 3xAA batteries (so about 4.5V to 4.0V). I understand the concept of using a voltage divider to do level shifting between the ~4.5V MOSI pin on the Arduino to the radio module. But if your input voltage dips a bit, you won't get a consistent output voltage. I expect the battery voltage to drop over time, to from 4.5V to 4.0V before needing to change the batteries out.
It will actually drop a lot lower then 4.0V. AA batteries go from 1.6V (new) down to 1.0V (almost dead) so your voltage range from 3xAA is going to be 4.8V to 3.0V over the life of the battery.
arusr:
Is the voltage drop a big deal for radio modules like the RFM69?
I was hoping that since ATMEGAs read 3.3V as a "1", then radio modules would also read 3.0V as a "1". Is that not true?
A while ago I gave up on finding a voltage booster that is efficient. It seem like the only voltage regulator I could find that has uA overhead current were the MCP1702, which lower the voltage to 3.3V from a higher input voltage, "buck" type. The boost regulators I came across all had unacceptably high quiescent current consumption. So when the system is sleeping and consuming very low current, the regulator itself wastes a lot of power.
I haven't come across these PFM type voltage regulators. This one for example has a 30uA "operating current".
Just curious if I'm reading the data sheets correctly. So if the ATMEGA and radio modules are in sleep mode 99% of the time, I can expect only about 30uA wasted overhead from these type of voltage regulators? That would be pretty good.
Buying from ebay is kind of annoying. They don't provide datasheets and sometimes the part number they provide doesn't match any relevant google results, or they don't provide part numbers. I also can't seem to find any DIP packaged PFM regulators for 3.3V. Most of them are SOT.
This digikey article on PFM regulators is really interesting: