LED to indicate garage door is open

Thread Starter

oxicottin

Joined Jul 4, 2008
18
Hello, Im wanting to hook up some sort of gaget that would show me a red LED light if the garage is open. I found the picture on the net here and had a few questions about how I could change a few things.

1) First, Im not sure how bright the LED light is but I dont want a light that would light up a dark room I just want a suttle/dim red LED light so what size LED would I be looking for and if I changed the size would I have to chang the size of the resistor?

2) Second, If I wanted to have a green light to show that the door is all the way closed how could I wire it in to the schematic.

3) Third and final question, If I wanted to wire a second red LED how could I wire it into the schematic. Thanks.....

 

MikeML

Joined Oct 2, 2009
5,444
Leds have a reverse breakdown voltage that is very low, some as low as 5V. You need to add a reverse-connected diode like a 1n4001-7 across the LED to protect it from the reverse voltage that would happen on the half of the AC cycle that would reverse bias the LED.
 

Thread Starter

oxicottin

Joined Jul 4, 2008
18
I have no clue as to what you just said :confused: Im new to this and just needing help on this project.... Can you show me where it would go and can I get it at radio shack? Thanks!
 

bertus

Joined Apr 5, 2008
19,944
Hello,

When using a led on an AC voltage source like a transformer, the led will break when the reverse voltage is higher than 5 - 7 volts.
When you put a diode (or second led) anti parallel to the led the reverse voltage will be limitted and the led will keep on working.

Greetings,
Bertus
 

Thread Starter

oxicottin

Joined Jul 4, 2008
18
Where would I put the diode in the schematic? Also can I get one of these diodes at radio shack and if so which one? Thanks!
 

Thread Starter

oxicottin

Joined Jul 4, 2008
18
thanks Bertus... Now the diode doesn't do anything but help cause the bulb not to break and has nothing to do with the brightness of the bulb right? Now can I use a lower volt LED than the one listed 2.8v? I just want a dim light not a light that would turn the room red so what LED would I be looking for that would work with what I have? Thanks!
 

bertus

Joined Apr 5, 2008
19,944
Hello,

The brightness of the led depends on mainly three things.
1) the voltage of the transformer.
2) the forward voltage of the led (2.8 volts in your case).
3) the resistor for current limiting of the led.

If you use a transformer of 24 Volts (the peak voltage is 1.4 X 24 Volts = 33.6 Volts.
The led has a forward voltage of 2.8 Volts.
The maximum voltage accross the resistor is 33.6 - 2.8 = 30.8 volts.
The peak current in your case will be 30.8 / 4700 = 0.00655 Amp = 6.55 mA.
In the resistor will have a peak dissipation of about 0.2 Watts.

Greetings,
Bertus
 

Thread Starter

oxicottin

Joined Jul 4, 2008
18
In your opinion is this the easiest possible way to do this? I don't know anything about building this project and have solely relied on the net and forums, I have however soldered and added things to already built boards following directions and pictures so I can do a little soldering.

Thanks,
Chad
 

bertus

Joined Apr 5, 2008
19,944
Hello,

I would take a lower voltage from the transformer (say 8 Volts).
The resistor can have a lower value then (about 1K).
This will give about the same brightness and will use a some less power.

Greetings,
Bertus
 

bertus

Joined Apr 5, 2008
19,944
Hello,

Yes, that one will do.
Also change the voltage to 8 Volts !.

Greetings,
Bertus
 

bertus

Joined Apr 5, 2008
19,944
Hello,

The transformer you show has some terminals to connect the low voltage side.
probably there are lines drawn wich voltages can be taken of the transformer.
(on the picture it is not shown well)

Greetings,
Bertus
 

Thread Starter

oxicottin

Joined Jul 4, 2008
18
Bertus Im curious, I have a doorbell transformer in my garage that im using for my doorbell already. It has only 2 screws and im almost positive its a 16 or 24volt transformer. Can I just run wires from it instead of getting another transformer and still use it for my doorbell as well. If so would I just change the resistor back to a bigger one?


Thanks,
Chad
 

Thread Starter

oxicottin

Joined Jul 4, 2008
18
So what your saying is if I use the 8 volt side of the transformer I was going to buy then it wont get as hot but if I connect it with an existing transformer thats using a higher watt then the bulb is going to get super hot. This bulb might be left on for hours at a time and I was planning on putting it in a light switch cover so is this thing going to melt the plastic?

Thanks,
Chad
 

bertus

Joined Apr 5, 2008
19,944
Hello,

A led is a low power device.
It will consume some mili watts.
It will not get hot. it will also not be as bright as a conventional lightbulb.
A conventional lightbulb will start at 25 Watts.

Greetings,
Bertus
 

SgtWookie

Joined Jul 17, 2007
22,201
The LED will not get hot. The current limiting resistor will get somewhat warm, as it will be dissipating most of the power.

Here's a couple of different ways to connect the diode and LED:



The schematic on top will use about 1/2 the power as the one on the bottom, as the reverse current will be blocked instead of shorted out.

If you want the LED brighter, you could change the resistor from 4.7k to 2.4k, but it should be rated for 1/2 Watt.
 

Attachments

Top