NNTP-Posting-Date: Sun, 15 Dec 2002 21:03:37 -0600
Subject: Re: Opposite of a photocell?
Date: Sun, 15 Dec 2002 19:03:30 -0800
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On Sun, 15 Dec 2002 16:28:20 +0000 (UTC),
>> I've got a little lighting project that I want to be activated upon
>> the presence of darkness. 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.
>> I know that something like this gets used in various
>> darkness-activated 110VAC incandescent night lights, but I want
>> something that I can easily integrate into a DC system, to activate a
>> transistor or relay when there is no light available.
>A relay has both normally closed and normally open contacts.
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."
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