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Digispark ATtiny85 Lowest Power : Does Anyone Know?

dubbie

In the past I made a very low power LED flasher circuit using a PIC microcontroller that worked continuously for at least two years on the same set of 2 x AA batteries. I retired before the batteries ran out so I never did find it's maximum life span. I have been wanting to repeat this project using Arduino ever since, but with no success. I have recently purchased some Digispark ATTiny85 PCBs as it seems that these might do the trick.

 

image

 

So far the lowest current, using an unmodified PCB is 4 mA and it seems that most of this is taken by the 5V regulator. I am contemplating removing the 5V regulator to see what the lowest current is then. My questions are:

 

1) Is there any easy method of removing the regulator or shall I just snip the two visible legs?

2) Once the regulator is disconnected will the PCB still programme via the USB connection?

3) How low a voltage will the amended board still execute the programme at? The datasheet indicates 2.7V. I am wanting to use something like a CR2032.

 

I have zapped USB ports on PCs before by having out of specification loads and I want to try and avoid zapping my current laptop.

 

Dubbie

  • in reply to michaelkellett

    Michael,

     

    Thanks for this.

     

    I thought I would try shorting out the regulator input and output before making any physical changes to the Digispark but for some reason this increased the current. I'm not sure why. So I thought, ah well get stuck in (I have several more Digisparks anyway) and snipped the in and out connections on the 7805. If wanted I might even be able to solder blob these connections back. As with shorting out the 7805 this increased the current as well. The only explanation I can think of is that the capacitors might be discharging and the recharging while the LED is ON and then OFF. However, snipping the connections did allow me to reduce the operating voltage below 5V (I connected a variable supply to the 5V pin and turned it down to 3.2V (the voltage supplied by a 2032 battery). This did seem to work. The current seems to be about 1.5 mA when the LED is off and about 3.0 mA when the LED is ON. I assumed that the ATtiny85 wasn't taking much current so that each LED was taking about 1.5mA

     

    My next step was to remove the power LED indicator which should reduce the remaining current significantly. . I could not work out which surface mount resistor was connected to it so remove the LED instead. This is the first time I have removed a SMD and it didn't go too well. I just assumed that if I put my soldering iron on one of the pads, it would melt and then the heat transferred to the other pad, which would also melt and the LED would just 'fall' off. It didn't. I had to put my soldering iron onto the middle of the LED and I think it just melted the whole lot. Anyway, the LED is now gone. The current is now only 0.4 mA when the driven LED is off and about 1.8ma when the LED is OFF. So looking good.

     

    A CR2032 battery has something like a 210 mAh capacity so it might last about 200 hours (8 days) which is not really enough for a Christmas decoration. But maybe a couple of AAA batteries which have a capacity around 1000 mAh and should last about 40 days -  enough for a Christmas decoration.

     

    I might see if there is anything else that can be done to reduce the LED off current  even further. I might try reducing the clock frequency but then I'm not sure I'll be able to then reprogramme it. I might try.

     

    Dubbie

  • If you only ever use a 5V or less external supply removing the 7805 (and shorting its input to output) will be as OK as this design ever is.

    According to the schematic there is a diode which allows the board to put power back into the PC but does not allow the PC to power the board - weird.

     

    From the ATtint85 data sheet:

    image

     

    If you can get the processor into power down mode and use the WDT to wake it occasionally your batteries should last a while.

     

    If you use an ATtiny85-10 it will work down to 1.8V, but max speed 10MHz.

     

    Remove the regulator, link out the regulator footprint, use a 5V supply (current limited to maybe 30mA) and USB for debugging, connect a battery for running.

    Remove the resistor in series with the power-on LED to save its current.

     

    MK