LED specification

[JohnD]

I have a "real" alarm at the front of the house, and a dummy box at the back (where it can be seen from another road)

I was thinking of putting a flashing LED in the dummy. I know nothing about these things.

I have looked at some LEDs that say
"REVERSE VOLTAGE: 5.0 V
DC FORWARD VOLTAGE: 4.5V TYPICAL
DC FORWARD CURRENT :20~30mA "

What is the significance of the Forward and Reverse voltages?

I would like to put a big, long-life battery in it that will hopefully last for years (it will need a ladder to change) with soldered joints that will resist tarnishing or going wrong

I have not seen any kits ready assembled to suit but surely they exist?

 

[rssteve]

i think the flashing led will make it look even more fake, like the old cameras you used to see with the flashing red led.

 

[breezer]

It depends what your current front bell box has now.

if it has no led then neither should the back bell box. (and they should both be the same colour / style bell box.

if you want a flashing led clicky ony problem is which colour?

To wire them is simple, you need to put a resistor in series with it, a 1k will be fine. (i presume this battery you mention is 12v)

 

[joe-90]

Everyone knows that the back bellbox is usually a dummy by the placement of it. You'd have a cable right in the middle of the bedroom wall if it was a real one. All it means is go to the front and have a look at the real one. Therefore it doesn't really matter whether it is the right one or not.

If you really want it to look genuine then use a real live box and wire it in to the panel placing it where the wiring would really go.

 

[mikeyd]

To work out the value of your resistor subtract the LED forward voltage from your supply voltage, then divide by the current in amps.

Assuming a 12v supply, using your figures above this gives

(12-4.5)/0.025 = 300 ohms

Power used by the resistor would be 0.18 Watts so a normal 1/4 watt resistor would be fine.

Power used as in total 0.3 Watts

edit: because it's flashing it probably uses about half the power I quoted above.

 

[JohnD]

What does "forward voltage" "reverse voltage" and "forward current" mean?

 

[mikeyd]

It's the voltage above which you will fry the LED. Forward in is normal use, and reverse is if you put in the circuit the wrong way round.

The forward current determines how bright it will glow.

An LED always has a fixed voltage requirement and if you exceed it it will be damaged. This is why you need the resistor to drop the rest of your supply voltage. The current through the resistor will be the same as the current through the LED (which is how you decide the resistor value).

 

[JohnD]

can you put LEDs in series, like Christmas Tree lamps, instead of using a resistor? If so, can three 4v LEDs run off 12v?

 

[breezer]

LEDS are not made with "voltage ratings" they mostly require 3v to light, hence the current limiting resistor.

I would not recomend 4 leds in series on 12v , i would however suggest you use 5.

here is a uselless fact.

Did you know if you wire a single flashing LED in series with 4 other non flashing LEDS they will all flash at the same time.

View media item 3439
here is somethiing i did just now. (for speed)

The LED on the left has 2 x 2k2 resistors (making abouk 1.1k)

The LED on the right has 1 x 2k2 resistor

You can see there is not that much difference in brightness.

Oh, and i only had 2k2 resistors sitting around

 

[mattylad]

Just buy a flashing 12v led from maplins?

See here and with one of these you can supply it directly from the panels 12v supply.

 

[breezer]

Just buy a flashing 12v led from maplins?

See here and with one of these you can supply it directly from the panels 12v supply.

bit slow

...........if you want a flashing led clicky ony problem is which colour? .................

 

[plugwash]

can you put LEDs in series, like Christmas Tree lamps, instead of using a resistor? If so, can three 4v LEDs run off 12v?

No, you can put them in series but the resistor is important.

The voltage/current characteristic of an LED is unsurprisingly that of a diode. This means that a small change in voltage leads to a very large change in current. Worse the voltage at which the desired current is achived varies a between LEDs even of the same make and model. Worse still the voltage at which the desired current is achived varies with temperature.

Ideally we would drive LEDs from a constant current source but that is too complex for most indicator applications. So we use the inefficiant but simple technique of a series resistor from a higher voltage source to keep the current under control.

note: you can get LEDs with a series resistor built in. Theese can be handy sometimes if space is limited.