[TriEmbed] measuring analog voltages near the battery voltage

Scott Hall scottghall1 at gmail.com
Sat Jan 27 16:57:42 CST 2018


By the way, the ATTiny ADC can handle a 50K divider just fine, but you loose
your noise immunity.  A good trade off I use is 38K.

On 01/27/2018 05:19 PM, Alex Davis via TriEmbed wrote:
> I’m testing out LEDs as a substitute for the expensive UV photodiode in my UVI
> business card. I’ve found a good one, in terms of visible light blindness and
> UV-A response. However, it outputs a bit more voltage than the internal 2.56
> vref of the ATTINY85 above a UVI of 5. It works great with a 3.3v ref, but
> even a fresh coin cell is only 3.2v.
>
> I can knock it down with a divided negative feedback op-amp arrangement, which
> isn’t expensive, but I’m trying to make this as cheap as possible.
>
> I tried a 10K 1/2 voltage divider, but that was too much of a drag on the tiny
> current from the LED. I have a feeling going higher than 10K is not a good
> idea with the ATTINY ADC.
>
> It is possible to have the ATTINY read it’s own voltage, and then use that
> with Vcc connected to AREF, but I know from practice that the bandgap
> measurement has pretty poor accuracy.
>
> Looks like external voltage reference is no-go; they’re way too expensive.
>
> So, op amp it is? 1.0K 1% resistor is cheap, and a 1.2K 1% is a few pennies
> more. That would give me -0.833 gain which would work within the 2.56V AREF.
>
> Alex
>
> |\ |  (¯  \/ |¯\  |V| |\ ¯|¯ |¯) | \/ | | | |¯\ (¯   /¯  /\ |V|
> |-||_ (_  /\ |_/ @| | |-| |  | \ | /\ |^| | |_/ (_ . \_  \/ | |
>

-- 
Scott G. Hall
Raleigh, NC, USA
ScottGHall1 at GMail.Com

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