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Suggestion on wiring up flashing LEDs

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Newbie level 2
Aug 2, 2009
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hi all.
Wondered if any one has any suggestions on how to wire up lashing leds?
I have hot glued 2 red flashing leds into my house dummy alarm bell box to imatate a live working alarm. The leds are wired in parallel using the power supply 12v dc from the old alarm that no longer works. But I have tryed to use the transformer in the alarm for a constant 12v dc voltage so I do not have to climb up and change batteries for the leds when they run out. When I tested the leds on the old alrm battery they flashed fine but when connected to the alarm wire which is about 5 meters longe they only just light up and do not flash. Is this due to the extra resistance in te 5meters worth of cable??:cry:

12v flashing led

Firstly, do the LEDs run on 12V? Most flashing LEDs run on 5V or need series resistors to limit their current.

Secondly, connecting LEDs in parallel is never a good idea. They are constant voltage devices and whichever has the lower forward voltage will hog the current to the other one. Always use a series resistor in each LED.

The voltage drop in your cable is not responsible for the problem. If the two reasons above are not causing it, the next thing to try would be a capacitor across each LED. 100nF at 10V or more rating should do. The reason: inside flashing LEDs is a timer circuit. If it sees a lot of electrical 'noise' across its terminals it may not run properly. The noise could be coming from the other LEDs timer or form pick up along your cable.


capacitor for flashing leds

I bought the leds from my local electronics store. I asked for 12v flashing leds and they said no problem. The spec says they require 12v 20mA and a V.R Of max 0.5.
So could the lack of resistors be the problem and if so how would I work out the value of the resister I need. An I take it connecting them in series would help??

flashing led current

Can you give me the part number of the LEDs. As you are in Leeds, a store order code will do (Maplin?) I should be able to trace the specification.

The 0.5V reverse voltage would indicate it has a built in timer IC, otherwise it would have been about 4V.

Connecting them in series will not work, the current they draw will rise and fall as the LED lights and goes out, if connected in series the same current has to flow through both devices so the 'off' one will starve the other of current and vice versa. Either they would both flash once or more likely not at all.


voltage rating of flashing leds

These leds are for direct connection to 12v DC. I have used lots of these in the past. The danger is that if the voltage gets much above this, the led with blow. I have always used a 470R resistor in series with each one to limit to a safe current, they still flash just fine. If you are using a remote psu, then follow betwixt advice, I assume he meant 100uF and not nF. You should only need one cap across the psu wires inside the box. Do not connect across each led, just the one across psu lines. Not critical 100 - 2200Uf is fine.

Added after 5 minutes:

Just a further thought, most criminals know that lights, flashing or otherwise fitted to cameras, bell boxes etc, usually indicate a dummy device. Genuine devices do not need to advertise their presence.

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