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The arduino mini consumes 5V. The Voltage input is min. 7V.

Is it possible to ignore that and attach 3 AA batteries (4.5V) or 4AA Batteries 6V between the GND and 5V pin of an arduino mini for long term wireless use?

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You can connect 2 or 3 1.5V cells or 2 to 4 1.2V cells between 5V and GND. You will have to limit your speed though, depending on exactly what you want to run it off of. Section 29.3, "Speed Grades", of the ATmega48A/PA/88A/PA/168A/PA/328/P datasheet shows the relation between supply voltage and maximum clock rate (note that the device may run above the speed at a given voltage, but this is not guaranteed).

It is possible to start at, say, 2MHz with the 16MHz crystal and the CLKDIV8 fuse, and then ramp up (or down) the prescaler based on the voltage available, but check that the libraries you're using can handle the clock rate being changed under them first.

And I recommend you take advantage of the power reduction features available; see section 10, "Power Management and Sleep Modes", of the datasheet for details.

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  • \$\begingroup\$ I do not need a lot of speed on this unit. It gets the Data from an DHT22, the Level of and LDR and the state of 2 Switches and sends them using some cheap 433MHz Transmitter with the virtualwire library. I think the most processing power will be needed to calculate the checksum. But the device will send the data every 1-10 minutes, so more than enough time ;-) \$\endgroup\$
    – Tobi
    Mar 23 '14 at 1:11
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    \$\begingroup\$ Well good, because the slower you run the longer your supply will last. \$\endgroup\$ Mar 23 '14 at 1:12
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I've done exactly the thing you mention in your comment - although with a different processor than the Arduino. You are looking for "long term usage", and if you're thinking of on the order of years then you probably can't get away with using AA batteries and a linear regulator.

Typically a voltage regulator wastes some power, firstly by its very nature, and also due to the fact that electronics isn't perfect. The regulator on the mini board is a LP2985-N (datasheet is here http://www.ti.com/lit/ds/symlink/lp2985-n.pdf). This regulator will waste of the order of 100uA while your microcontroller is sleeping, and probably 200uA or so while it's operating and calculating the checksum etc. See the "Ground pin current" section of this datasheet. So, don't use the regulator at all, just run from two AAs and go to sleep as much as possible.

BUT - now the problem becomes, how do you provide enough voltage to run the transmitter and the DHT-22? My answer when I built one was to use a step up converter that I only turned on when I woke my chip up to do the measurement and the transmission. So, while your circuit is asleep the chips runs directly from the battery voltage with the step-up converter turned off. And then, when you wake up you turn on the converter, up goes the voltage to 5v, you wait for that to stabilise (a couple of milliseconds should do it), get the job done and go right back to sleep.

This is perhaps more work that you were thinking of getting into, and it all depends on what you really mean by "Long term". In terms of low power consumption though, this is really the way to go. Because the ATmega will run right down to 1.8v, and there are fixed voltage switchmode regulators that'll run down to this voltage too, you'll really be able to get the absolute most out of your AA batteries.

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