Alt-BEAM Archive
Message #01955
To: Evaristo Westplate evaristo@qctechnology.nl
From: Steven Bolt sbolt@xs4all.nl
Date: Thu, 1 Apr 1999 14:13:05 +0200 (CEST)
Subject: [alt-beam] Re: Phtotovore senors
On Thu, 1 Apr 1999, Evaristo Westplate wrote:
> Looking at Suneater 3, it seems to me that the photo diodes are used as a
> voltage divider.
More or less, but not quite. With Vcc at for instance 2.2V,
the connection between the two BPW41s swings from
0V (or even a negative voltage) to 2.17V (or better) within about 5
degrees of turn. You get that performance in good daylight, looking
out the window towards a clouded white sky, with 100K or 2K7 as
series resistor; and also when looking at low indoor light levels.
The large active area of the photodiodes (7.5mm^2, same as BPW34)
is an important factor here.
> If you substitute the photo diodes for photocells you would get
> the same result, a voltage divider. The voltage swing won't be
> large for small angels but still worth a try.
There will indeed be a moderate voltage swing, and you might reduce
schmitt-trigger hysteresis to try to deal with that (larger
resistor between N6 output and N5 input).
However, in good light the resistance of these light-dependent
resistors drops to very low values. That results in too much
current consumption, and a single series resistor would unbalance
the midpoint. You might use two resistors, but the size of the
voltage swing would still strongly depend on the overall light
level. As there isn't much difference in price, photodiodes like
the BPW41 are imho the better `eyes' in this case.
Best,
Steve
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# sbolt@xs4all.nl # Steven Bolt # popular science monthly KIJK #
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