Powering an Arduino Mini Pro using a CR2032 battery as a remote control

Question

I'm looking into making my own remote control using an Arduino Mini Pro and some RF transceivers. The receiving end won't be a problem, but the remote will. I want to make it as small as possible and consume as little energy as possible.

Is it possible to turn on the Arduino Mini Pro at the push of a button, while also using that same button to trigger a piece of code? That way I can cut the power when it's not being pressed.

I will make sure to code it, so I have to hold the button down for 1-1.5 seconds, so it has time to "boot up" and send the RF signal, but should power off immediately when I let go of the button.

What kind of power should I expect from a single CR2032 battery? I have a couple of NRF24L01 RF transceivers laying around together with an Arduino Mini Pro.

If we assume this, while looking at this sheet:

NRF24L01: 10 mA per second (low-ball)
ATmega328P: 5 mA per second (low-ball)
CR2032: 0.19 mA @ 2V = 2100+ hours

Even if we just say 600 hours and it takes 2 seconds to turn something on, I should be able to run it 600H*60M*60S = 2.160.000 seconds = 1.080.000 times. If my calculation isn't completely wrong, it means I could literally turn it on a million times in the worst case scenario.

I hope that calculation is wrong, because that seems crazy.

EDIT: I could desolder the LED and regular as well, if that won't damage it too much. Just to consume power. I've read I can basically run it for free then.

Answer 1

You should take a look at ATmega328P datasheet

On page 2, it says: Six Sleep Modes: Idle, ADC Noise Reduction, Power-save, Power-down, Standby, and Extended Standby

These are the modes which can be used to put Arduino Pro Mini in stand-by. You can search for them at datasheet and choose the only that better applies to you. To learn more about sleep mode and other methods to save battery, you can take a look at this source: Arduino Low Power - How To Run ATmega328P For a Year On Coin Cell Battery

On the datasheet of battery, you should pay attention at its charge:

Typical Capacity: 240 mAh (to 2.0 volts)

This means that from 3 to 2 Volts, it relases 240 mAh. This is the charge you can expect to use. Using the consumptions rates you posted above

NRF24L01: 10 mA per second (low-ball)
ATmega328P: 5 mA per second (low-ball)
CR2032: 240 mAh
Time: 240mAh/(10mA + 5mA) = 16 hours

Which is not so much. To increase this autonomy, the link I posted above is quite useful because it will show you alternatives like removing the voltage regulator that will reduce drastically your consumption. Now, if you feel really adventurous, you may take a look at how to underclock your board: Arduino pro mini @ 1MHz - 1.8V. This is a very efficient way to save power where clock frequency is not a must.

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EDIT after comments:

The 16 hours were computed using 5mA for the Arduino (which is a value for power consumption that can be expected on active mode) and 10mA for the NRF24L01. You have to estimate a sleep mode consumption for both and a percentage of time in sleep mode if you want to estimate an autonomy. At the link I sent you about sleep mode there are some tables with power consumption in different configurations.

ATmega328P can run with a minumum of 1.8 Volts, while nRF24L01+ can while with a minimum of 1.9V - see datasheet. So probably you are going to have a little bit more than 240mAh to work with. Your battery won't have 3v all the time. It is going to start with 3V (maybe a litte bit more) and gradually reduces it voltage as the charge is provided to the circuit. Once the voltage goes bellow 1.9V, nRF24L01+ won't be guaranteed to work properly.

Definitely, to sleep the Pro Mini, just waking it up when using, is the right way to design this circuit. According to this reference I posted above, power consumption for Arduino should be 0.90 mA. At the item 5.1 of nRF24L01+ datasheet, it is described that it consumes 0.32mA at standby mode II. If you sleep the Pro Mini for, let's say, 90% of the time, without removing leds or voltage regulators, you can estimate that

t = 240mAh * (0.1/(5mA+10mA) + (0.9/(0.9mA+0.32mA)) = 178.65 hours

If you use strategies to reduce Arduino power consumption, like the ones I send you above, you can increase this time. You will also save power if you change nRF24L01+ from standby mode 2 to standby mode 1. Check the database to see which one fit your needs.

Answer 2

If you have a single button, it is useless to use the button to trigger the piece of code. The fact that you are powered on means that the button is pressed.

^{simulate this circuit – Schematic created using CircuitLab}

Of course you need a 3.3V Arduino PRO Mini...

If you have multiple buttons, you can have different circuits to detect this.

Solution 1

Use a set of diodes to power the board, then another to detect the digital pins.

^{simulate this circuit}

The second set of diodes is needed because you should not show to the atmega pins a voltage greater than Vcc+0.5V. Since a pull-down resistor is needed, if you want you can replace the second set of diodes with 2k2 resistors.

Solution 2

Again use a set of diodes to power the board, but then decode the value through an analog reading

^{simulate this circuit}

You will need to experiment a bit to figure out the thresholds. For instance, with the values I wrote, you can expect to read from the ADC a value around 1000 for the button 1, a value around 600 for the button 2, and 300 for the button 3; for this reason, you write

int buttonRawValue = analogRead(A1);
if (buttonRawValue > 800) {
    // Button 1 was pressed
}
else if (buttonRawValue > 450) {
    // Button 2 was pressed
}
else {
    // Button 3 was pressed
}

This will require some tuning, but I think it can work.

Solution 3

You can power the board through a transistor (I usually prefer small MOSFETs)

^{simulate this circuit}

You will need to find a MOS with a very low Vgs-th, sinceit should fully turn on with 1.5V. This solution has the advantage of not wasting the small voltage you have (while the other ones waste around 0.3V for Schottky diodes and 0.6V for PN diodes).

Personally I'd go with solution 3, since it is also easy to let the atmega control the power (simply attach one of the digital outputs to the cathodes of the diodes, and the ATMEGA will be able to force power on until it has finished the operations).

As a side note, I would recommend you not to use a pre-made board, but use a bare ATMEGA328P in this project. You can experiment with another arduino board, but then use the arduino board to flash a bare ATMEGA; you will get rid of all the useless things. You can avoid using a crystal and a voltage regulator, so your circuit will consist only of the components I used in the schematics and a 100nF capacitor between the VCC and GND pins of the microcontroller.

And... Don't flash a bootloader. Use another arduino board to flash the microcontroller, not a serial peripheral. This is because the bootloader is useful, but at boot it wastes some seconds (and you need to be as quick as possible)