resistance ?--for dual blinking LEDs

[garydalef]

I have two 12v blue 'blinking' LEDs, 20mA, from car alarms. I beleive each has a 470 ohm?? built in resistor. I want to take these leds and hook them together, so they blink opposite one another.

I want to use wall plug in, ac to dc power supply.

1- how do I figure the voltge and resistance to power them both, in series and in parrallel?........... .........I know how to figure it if only using ' non blinking LEDs', but using two leds with each having its own 'built in resister' confuses me.

2- will wiring them in series or parrallel make them automatically blink opposite each other, or will they both blink at same time?

3-I have a box of various ac dc power supplies, and was hoping to figure out which one to use with proper voltage, without adding a third resistor, since both leds already have one....thanks

 

[garydalef]

Would I need 24v if I connect them in series, and 12 v if I connect them in parallel??

 

[Bunk3r]

The Alarm LEDs I have seen didnt have the blink circuit in them but the alarm itself turned them on/off (ie blinked them). unless these are Dummy alarm LEDs, in which case they have a little flashing circuit built in, and they will just flash of their own accord you cant sync them.

to flash LEDs alternatively like you want then there is a nice simple circuit thats easy to DIY, google 'transistor astable multivibrator' the circuit I am thinking of has 2 transistors, 2 capacitors, and a number of resistors, the schematic has wires crossing forming an X in the middle, I dont have time right now to search for a good example.

 

[Noctilucent]

I have two 12v blue 'blinking' LEDs, 20mA, from car alarms. I beleive each has a 470 ohm?? built in resistor. I want to take these leds and hook them together, so they blink opposite one another. I want to use wall plug in, ac to dc power supply.

Okidoke.

1- how do I figure the voltge

Well, most simply, see the specs of your LEDs... If it says 12V, then 12V is what you'll need. Adapters do come in 12V variants aplenty and cheap, so you should be all set there.

and resistance

As they have built-in resistors.. none.

to power them both, in series

You should test that configuration first. Very often the blinking LEDs driven by internal chips will block current flow when they are off; meaning the second led cannot be on when the first is off, and vice-versa.

and in parrallel?

That's more like it

I know how to figure it if only using ' non blinking LEDs', but using two leds with each having its own 'built in resister' confuses me.

In a parallel LED configuration, each parallel strand gets its own resistor. Resistors add up when put in series.
As mentioned above, if you have a 12V adapter, you should be all set. But for kicks, let's say you have an 18V one from a laptop that died or whatever. Your LED is supposedly running at 20mA (are you sure about that one?). R = (Vs-Vl) / I = (18V-12V) / 0.02A = 300Ohm. Next highest R12 series = 330Ohm. quarter watt will do just dandy.

2- will wiring them in series or parrallel make them automatically blink opposite each other, or will they both blink at same time?

In series: see above for quite possible pitfall.
In parallel: These LEDs never have accurate timing circuitry. They will probably blink out of sync.
If you absolutely must have the blink opposite eachother... well, ditch them, get regular LEDs and look up a cheap double-led blinking circuit (it's just a few resistors, 2 transistors, 2 elec.capacitors and of course the LEDs).

3-I have a box of various ac dc power supplies, and was hoping to figure out which one to use with proper voltage, without adding a third resistor, since both leds already have one....thanks

12V if you've got it Don't forget to check the polarity!

 

[garydalef]

thanks----they are both 'dummy' car alarm, 12v blue blinking leds with built in (470 ohm?)resistor.

..........I was figuring if I run both of them in parallel, I wont need a 'third' resistor with ac to 12v dc power.

..........when using the 'resistence'.....in series, formulas,.....was having trouble figuring out what to do ( how to figure in), ......the 'constant' resister values (470 ohm),built into each LED .....i.e. how to program that into the formulas, so I'd only have to add one resister.--------especially if using 18 or 24v.

Thanks for the info.