From: "Sir Charles W. Shults III"
Subject: Re: Opposite of a photocell?
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Date: Mon, 16 Dec 2002 04:19:48 GMT
NNTP-Posting-Date: Sun, 15 Dec 2002 23:19:48 EST
Organization: RoadRunner - Central Florida
Actually, you just pick the opposite transistor to do this in many cases.
Or, you can swap the photocell and the resistor that form a voltage divider and
change it from light to dark sensing. instead of referencing a high level you
reference the low. In other words, a photocell that runs from the positive rail
to the base of a transistor will switch one way, but if you run the photocell
from the ground rail to the base it will do the opposite job.
Let's pick an NPN transistor and try this out. Since a high voltage level
on the transistor's base will make it conduct, and you often use the photocell
as part of a voltage divider wired to the base, the circuit is usually just a
photocell to the plus rail, the other leg to a resistor to the ground rail
(forming your voltage divider) and the junction of the two leads to the base
When the cell is dark, the ground leg has precedence and the transistor is
off. When light falls on the cell, the transistor conducts.
But if you swap the photocell and the resistor, now darkness lets the
transistor conduct, and the light turns it off by pulling it to ground.
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