one transistor as a ON/OFF switch for eInk display connected on pin 5
It's very simple and in sleep mode is consumes around 0.08mA. I'm running this with 3xAAA alkaline batteries now, but I'm thinking about to use 2xAA batteries so it will be 3V and I would like to engage some voltage regulator to 3.3V (I preffer alkaline batteries for their capacity and better performance in winter)
I would like to use effective boost converter with low quescient current. I found LTC3525D-3.3 ..what do You think about this chip? Second problem is that I will need another 2x SMD capacitors and inductor to complete this.. I'm beginner and I don't want to buy hundred SMD parts for a few devices. I was succesfull to solder some SMD resistors with DHT22 so I hope I can do it by myself, but I found 50pcs packages with SMDs only...it's a lot and I'm not sure if it is good idea at all.
8 MHz Pro Mini: runs just fine on 2xAA. It needs 2.2V or so for 8 MHz.
DHT22: doesn't run fine on 2xAA, needs 3.3V indeed. But it doesn't run fine on just about any current - unless you're in a dry climate so the humidity sensor won't get stuck at 100% after a few nights. Much better alternative: BME280, works at 1.8-3.3V. Gives you air pressure as well, is unfortunately prone to self heating. You should be able to prevent that by keeping it in shutdown mode most of the time.
eink display: post datasheet. Possibly it can work at <3.3V.
jurayk:
I found LTC3525D-3.3 ..what do You think about this chip? Second problem is that I will need another 2x SMD capacitors and inductor to complete this.. I'm beginner and I don't want to buy hundred SMD parts for a few devices.
If you must, get a ready made boost module, unless you know very well how to design high frequency PCBs taking into account stray inductance and capacitance of the traces and the electromagnetic interaction of especially the inductor with the rest. Switch the boost module on/off with your Arduino (which you of course power directly by the batteries).
I don't have exact datasheet for the eInk Display, just this link. From the back side it has a switch to select between 3.3V and 5V mode. It has some converter on a PCB board and it constantly consumes current so I'm switching Vcc On/Off with transistor and it work well like that.
I will look for some ready to go boost converter, but I'm limited with space in my waterproof box. This is another reason why I would like to subtract one battery and use only two.
Maybe... would be possible just replace converter on pro-mini PCB? Or it is a bad idea?
You are definitelly right abot BME280 and I have already some pieces on my table. I just need to figure out some case for it (DHT has nice plastic cases but BME are naked)
And suggestion to switch converter on/off with arduino is excellent. Thank You.
I'm going to design v2 of my thermometer. I have one nice feature, that I'm drawing bar graph and I have 24h history of temperatures plotted on display. It is very usefull for me.
jurayk:
I don't have exact datasheet for the eInk Display, just this link. From the back side it has a switch to select between 3.3V and 5V mode. It has some converter on a PCB board and it constantly consumes current so I'm switching Vcc On/Off with transistor and it work well like that.
Beware of phantom powering the display through the I/O lines.
And do try out the display at 2xAA batteries. Very good chance that it will work. If so, no need for a boost converter at all, and your 2xAA are likely to last longer than your original 3xAA to boot (because lower voltage means lower current).
jurayk:
I think they need to be connected directly to VCC (not RAW) and GND on "pro-mini".
Of course, that is always the case.
Hey, I suggest you completely forget about "Raw" for any purpose!
The on-board regulator is basically useless for any project that uses components external to the Arduino board itself beyond one or two indicator LEDs drawing 20 mA. Also, as it simply wastes power, it is the wrong way to use batteries. If you are powering the board using "Raw", you are almost certainly doing something wrong!
