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From: firstname.lastname@example.org (Roy McCammon)
Subject: Re: Opposite of a photocell?
Date: Tue, 17 Dec 2002 12:39:19 -0600
X-Mailer: Mozilla 4.5 [en]C-CCK-MCD 3M/NCP 4.5 (WinNT; I)
> Yes, this is true of relays. This doesn't however answer my question
> of "Is there a simple all-in-one device I can buy, which has the
> opposite effect of a photocell? That is, I want high resistance in the
> presence of light, and low resistance when there is no light."
there are devices to turn on lights when it gets dark that
you can buy.
and with regard to making one, an enhancement mode mosfet
and a photovoltaic cell might work.
> I agree that you have found a solution for my application (thanks!),
> but I'm still curious about the existence of a device which has high
> resistance in light and low resistance in darkness. Response time
> isn't an issue, it doesn't matter to me if it takes anywhere up to a
> minute for the resistance to change; this won't be a mission-critical
> or life-sustaining device.
> Still looking for an answer on this one; any help would be great. I
> wanted to use this darkness-activated switch to turn on a transistor
> instead of a relay because this application involves about 500ma of
> current at 5V DC. It seems like a waste to involve a relay in such a
> low-power circuit. But I'm just a novice with all of this, so I may be
> wrong. ;)
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