Making a switch
 

Hi all,

I need some advise on circuitry. I'm trying to create a circuit in which opening a set of contacts will turn on an LED light. For reference i am using a button cell, but will be upgrading to a AAA or AA battery when i get the mechanics right.

I have a few ideas, but nothing that wouldn't require many days of tinkering with a soldering iron and circuit board.

Does anyone have any ideas on how i might be able to do this?


Cheers

This would be fairly easy to do with a NOT gate...

I'll post a schematic soon....

Cool, cheers!

I think the attached image would probably be the simplest way - opening the switch would force the current through the switch (the path of least resistance) - but obviously this would be a complete short when the switch was CLOSED - so you would want to put a resistor of some fashion.

If you're using a battery, this would probably be VERY battery intensive.

This site has a simple door alarm circuit that could probably be used...

Thanks, few things to think about there.

Choose highest value resistor that will saturate transistor.
Try 100,000 ohms. My circuit shows a bulb.
You said LED so use a current limiting resistor in series with the LED.
I have drawn the battery upside down, sorry :-)
Negative polarity should connect to emitter/switch.

http://s3.simpload.com/090248bdb14aacb45.gif

Awesome, cheers.

untested

still untested, but it looks a little better :-)
LED current will be about 6ma. For 15ma R1 should be 67 ohms if using a normal red LED (voltage drop 1.7 volts).

Cool, just need to find a 100k resistor...

Edit: Huzzah for the resistors, boo for needing a new soldering iron.

You can buy a very cheap low wattage iron. Make sure to use rosin core "radio" solder.
The resistor value is not critical at all. However 100k (1/4 watt, 5%) is a standard value.
So is 68 ohms.

Using a MOSFET as a switch
 

Ideally we want very little current to flow when the switch is closed.

For the bipolar transistor, a darlington arangement using two transistors
would work well.
Then the 100k resistor could be increased to at least 1Megohm, I think.

Here is another circuit to play with. I found this circuit here:

Using a MOSFET as a Switch

Using Paint Shop Pro I flipped the positions of R1 and the switch.

I would change the 1M resistor (R1) to 5M or 10M and see if the circuit is still
stable. The project could still be powered by 3 volts but R2 should be a much
smaller value if the LED has to be bright. Perhaps 220 ohms.

http://s1.simpload.com/091748d1060ed3053.gif