From: "R. Lewis"
Subject: Re: Opposite of a photocell?
Date: Mon, 16 Dec 2002 19:50:13 -0000
Organization: Nextra UK
NNTP-Posting-Date: Mon, 16 Dec 2002 19:50:11 +0000 (UTC)
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
"J81" wrote in message
> On Sun, 15 Dec 2002 16:28:20 +0000 (UTC),
> >J81 wrote:
> >> 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.
> >Problem solved.
> 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
> wrong. ;)
Take one NPN, add resistor base to collector, add one photo transistor base