High Power IR Blaster

Hey y'all,

I did build an Arduino IR remote blaster with one 940nm IR LED powered through a BC547 transistor with a limiting resistor to protect the LED.

Now this works okay but I'd like to point the LED(s) at the ceiling to reach everything in the livinroom with a single blaster (now I need to aim the LED directly at the device I want to control).

So I got a couple more IR LEDs (Uf=1.5V, If=60mA / 120mA pulse) and I am really stuck deciding

  1. how many I can / should use lightting up a 25sqm room from one corner
  2. what is the best way to use them (parallel, serial, with what resistor for how many)

I've been checking so many examples but I found none that came sufficiently near to my usecase. If one of you could point me to the right direction with a link or a cirquit I'd really be greatful.

The choice of IR emitter (led) is an important factor. If you are bouncing the beam from the ceiling, you want one with a relatively narrow "viewing angle". I'd say between 10 and 25 degrees.
Maybe look here: farnell

A little trial and error is needed here - but I would suggest that you start with 5 or 7 leds, put them in parallel (you could have each one with it's own driver transistor and resistor so you know you are in the 'safe' zone.

You could even make 5 single led boards up and angle them around a curve radius to get maximum coverage.

  1. how many I can / should use lightting up a 25sqm room from one corner

That's unpredictable. We don't know how reflective your ceiling is, or how much it's going to diffuse the beam and we don't know the light-output of the LED or the sensitivity of the IR receivers, or the ambient light level.

  1. what is the best way to use them (parallel, serial, with what resistor for how many)

In series you need more voltage and in parallel you need more current (and a separate current limiting resistor for each LED). If you have enough voltage available, put them in series, but if you only have 5V don't put more than 2 in series because you need some voltage drop across the resistor. (The more voltage you drop across the resistor the better it works as a current-source.*)

If you put them in parallel, check the current rating on your transistor because you may need a "bigger" transistor and/or a heatsink.

  • The the voltage across the LED is constant and any changes end-up as changes across the resistor... If you have 1V across the resistor and the supply voltage increases by 1V, you'll have 2V across the resistor and twice the current... If you have 10V across the resistor and the power supply increases by 1V, that's a 10% change in voltage & current.

Thanks a lot all! I'll go with the "separate circuit for each LED" solution - starting off with 5 and see where this gets me.

BC547 can't drive more than 100mA, 200mA at a peak, and Ir leds can support 1A in a peak (100mA continuous), so change de transistor and it'll go better.