Need help choosing transistor for a 555 timer lighting 2x1W LEDs

[eester B]

Hi, I have a 555 timer 'one-shot' circuit running off 12V, which lights up 2x 1W UV LEDs (350mA, 3.4V, in parallel). My understanding is that the 555 only puts out 200mA, so I would need transistors to get full potential from the LEDs.
I'm looking at transistor calculators and reading up on it but the math/theory is beyond me at this point without some humanoid intervention.
How do I select a transistor / what transistor should I be using for this application?
Your input is greatly appreciated.
EB

 

[Qaisar Azeemi]

Simply use any general purpose Transistor available in the market, most commonly available general purpose transistors are c828, c1383 and 2n2222 will work fine. drive the base of the transistor using output of the 555timer with around 5v zener diode connected between the base of the transistor and output of the IC. Connect the Led with appropriate Resistor in series with the Transistor; means the series string of LED and Resistor to the Collector of the Transistor and the Emitter is Ground Directly with the top end of LEDs and Resistor string connected to the Supply directly.

Hope this helps.
Regards

 

[godfreyl]

Hi, I have a 555 timer 'one-shot' circuit running off 12V, which lights up 2x 1W UV LEDs (350mA, 3.4V, in parallel).

If you connect the LEDs in series instead of parallel, you only need half the current.

 

[jeffrey samuel]

If you connect the LEDs in series instead of parallel, you only need half the current.

but does not the effective voltage required to drive them increase as each led enables a voltage drop

my qn is that wont the intensity of the leds decrease if they are connected in series

 

[godfreyl]

but does not the effective voltage required to drive them increase as each led enables a voltage drop

Yes, but there's plenty of voltage available as he's using a 12V supply. It just means a different value resistor must be used to get maximum brightness.

 

[jeffrey samuel]

a different value resistor must be used to get maximum brightness.

but the serial leds them self have resistances and so when we add furthur resistance wont the brightness drop

second what rating of led is best suited for serial and parallel applications

third is intensity related to current

 

[godfreyl]

More current gives more brightness, but too much current will destroy the LED so the current must be limited to a safe value. That is why extra resistance must be added. The internal resistance of a LED is very low.