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<div class="moz-cite-prefix">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.<br>
<br>
On 01/27/2018 05:19 PM, Alex Davis via TriEmbed wrote:<br>
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cite="mid:E4350A3C-075A-4989-88BF-E6E1899691A4@matrixwide.com">
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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.
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<div class="">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.</div>
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<div class="">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.</div>
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<div class="">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.</div>
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<div class="">Looks like external voltage reference is no-go;
they’re way too expensive.</div>
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<div class="">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.</div>
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<div class="">Alex</div>
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<div style="color: rgb(0, 0, 0); font-family: Monaco;
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-webkit-text-size-adjust: auto; -webkit-text-stroke-width:
0px;" class="">|\ | (¯ \/ |¯\ |V| |\ ¯|¯ |¯) | \/ | | |
|¯\ (¯ /¯ /\ |V|</div>
<div style="color: rgb(0, 0, 0); font-family: Monaco;
font-style: normal; font-variant-caps: normal; font-weight:
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-webkit-text-size-adjust: auto; -webkit-text-stroke-width:
0px;" class="">|-||_ (_ /\ |_/ @| | |-| | | \ | /\ |^| |
|_/ (_ . \_ \/ | |</div>
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<pre class="moz-signature" cols="80">--
Scott G. Hall
Raleigh, NC, USA
<a class="moz-txt-link-abbreviated" href="mailto:ScottGHall1@GMail.Com">ScottGHall1@GMail.Com</a></pre>
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