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(SOLVED) Break circuit = Make circuit? Answered

Basically, what I want to do is activate something when there is no current somewhere else.

Examples of uses of this would include turning an LED on when there is no light on an LDR and turning it off when there is light, rather than the other way around, along with turning an LED on when a circuit is broken by opening a door.

Apologies if I have not been clear enough.

Is this possible? 

EDIT: Thanks to NachoMahma, I have it set up with a transistor. I will post a schematic if anyone is interested.


. It seems the system ate my previous comment.
.  Use a relay with a NC (normally closed) contact.

Thank you for your reply.
But what exactly is a relay? I am quite a beginner in electronics.

Thanks again,

.  A relay, in a nutshell, is just a switch with an electromechanical actuator. Google is your friend (and should have a picture or few).
.  For the low-voltage, low-power example you gave, a transistor (Google is your friend) may be a better option.

Okay, so I would need a Single Pole Single Throw, Normally Closed relay for this?
I do have a transistor, so how would I use it like this?

Thank you again,

. Yes. A SPST/NC would be perfect. You could also use the NC contacts on a SPDT (or DPDT).
.  It depends on the transistor. If you can find a part number (probably looks something like 2Nx..x, eg, 2N414), you should be able to easily Google the specs.

Great, thanks for your help.
I'll have a look tomorrow....

Thanks again for your help,

Thank you for your help, it works. I have my circuit set up on a breadboard and it works for both scenarios I gave as examples.

Thanks again,

Yes, its very easy to do. You can find anynumber of circuits on line for what you want to do.

Thank you too for your reply.
How would I go about finding one of these circuits?

Thanks again,

These all seem to contain relays, so it would seem I would be better off looking into those (I wish to learn and make, not buy premade circuits where possible)

Thank you anyway,

First result: http://www.circuitstoday.com/light-activated-switch-circuit
Has a schematic, shows a relay.

Second result: http://www.coolcircuit.com/circuit/light_switch/

Third result is talking about something different.

Fourth result: http://www.quasarelectronics.com/smart-kit/1004-light-activated-relay-switch.htm
Clearly states that it uses a relay.

Fifth result: http://www.educypedia.be/electronics/circuitssensorslight.htm
Has a list. I Ctrl+F-ed 'switch' and opened all the relevant ones. All used relays.

Sixth result: http://circuitdiagram.net/dark-and-light-activated-relay.html
Has two circuits - one light activated, one dark activated. Both used relays.

Seventh result: http://www.electrokits.com/light/activated-switch
Again, a list. Those relevant used relays.

Eighth result was a sample of results from an image search.
First: http://bit.ly/gvU9GY - uses a relay.
Second: http://bit.ly/ewZVIo - uses a relay.
Third: http://bit.ly/eszgW1 - again, uses a relay.
Fourth: http://bit.ly/gqPlVo - uses a "relay coil" and is titled light-activated relay.

Ninth result: http://www.whatcircuits.com/light-control-circuits/light-activated-switch/
Clearly states its use of a relay.

Tenth result: http://www.maplin.co.uk/searchtemplate.asp?criteria=LIGHT%20ACTIVATED%20SWITCH
Completely unrelated search.

Eleventh result:http://www.electro-tech-online.com/electronic-projects-design-ideas-reviews/94242-light-activated-relay.html
Uses a relay.

I think my point is proven.