The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: "J. Hill"
Subject: Re: Opposite of a photocell?
Date: Wed, 25 Dec 2002 19:15:36 -0800
Organization: MindSpring Enterprises
X-Server-Date: 26 Dec 2002 03:20:27 GMT
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.0.1) Gecko/20020823 Netscape/7.0 (nscd2)
X-Accept-Language: en-us, en
Other folks have given you good practical solutions to your problem, but
they haven't really addressed what I see as your core question, "Is
there a substance that is a better conductor in darkness than it is when
exposed to light?"
So I'll take a shot at answering THAT question. There's a reason some
materials are photoconductive, and it is basically this - when they are
exposed to light, the energy of the light causes the electrons of the
material to become more energetic and these energetic excited electrons
are responsible for the photo-conductivity.
Now we're asking if there is a material that is in a highly excited
state when not exposed to energy and which also tends to become LESS
energetic when exposed to a source of energy.
I don't know of any such material, but maybe something exists in the
realm of the superconductors that is ON at absolute zero and OFF above
It would be fun to learn my hunch about this is wrong.
> 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
> wrong. ;)
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup