Making a switch - Tilted Forum Project Discussion Community

stevie667 · 07-16-2008, 12:37 PM

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

ScottKuma · 07-16-2008, 05:18 PM

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

I'll post a schematic soon....

stevie667 · 07-17-2008, 03:11 AM

Cool, cheers!

ScottKuma · 07-17-2008, 04:47 AM

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...

stevie667 · 07-18-2008, 06:44 AM

Thanks, few things to think about there.

flat5 · 09-02-2008, 01:34 PM

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

stevie667 · 09-03-2008, 03:04 PM

Awesome, cheers.

flat5 · 09-04-2008, 03:00 AM

untested

flat5 · 09-04-2008, 03:24 AM

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).

stevie667 · 09-07-2008, 06:54 AM

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

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

flat5 · 09-08-2008, 03:27 AM

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.

flat5 · 09-17-2008, 05:33 AM

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.

07-16-2008, 12:37 PM	#1 (permalink)
stevie667 Too Awesome for Aardvarks Location: Angloland	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 __________________ Office hours have changed. Please call during office hours for more information.

07-16-2008, 05:18 PM	#2 (permalink)
ScottKuma Insane Location: Maineville, OH	This would be fairly easy to do with a NOT gate... I'll post a schematic soon.... __________________ A government big enough to give you everything you want is big enough to take from you everything you have. -Gerald R. Ford GoogleMap Me

07-17-2008, 03:11 AM	#3 (permalink)
stevie667 Too Awesome for Aardvarks Location: Angloland	Cool, cheers! __________________ Office hours have changed. Please call during office hours for more information.

07-18-2008, 06:44 AM	#5 (permalink)
stevie667 Too Awesome for Aardvarks Location: Angloland	Thanks, few things to think about there. __________________ Office hours have changed. Please call during office hours for more information.

09-02-2008, 01:34 PM	#6 (permalink)
flat5 Very Insignificant Pawn Location: Amsterdam, NL	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 Last edited by flat5; 10-16-2008 at 03:08 PM..

09-03-2008, 03:04 PM	#7 (permalink)
stevie667 Too Awesome for Aardvarks Location: Angloland	Awesome, cheers. __________________ Office hours have changed. Please call during office hours for more information.

09-07-2008, 06:54 AM	#10 (permalink)
stevie667 Too Awesome for Aardvarks Location: Angloland	Cool, just need to find a 100k resistor... Edit: Huzzah for the resistors, boo for needing a new soldering iron. __________________ Office hours have changed. Please call during office hours for more information. Last edited by stevie667; 09-07-2008 at 06:57 AM..

09-08-2008, 03:27 AM	#11 (permalink)
flat5 Very Insignificant Pawn Location: Amsterdam, NL	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.

09-17-2008, 05:33 AM	#12 (permalink)
flat5 Very Insignificant Pawn Location: Amsterdam, NL	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.