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I have an assortment of minuature (~2.5mm diameter) light bulbs that I'd like to use for a retro-looking project.

Driving LEDs is fairly easy with a micro controller, if the output is too weak, I'd just use a FET, a dropper resistor, etc.

But if my supply voltage is 5V, what kind of circuit do I need to turn on that light bulb?

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  • \$\begingroup\$ What's the operating voltage of the lightbulbs? \$\endgroup\$ – AndrejaKo Jul 10 '14 at 6:24
  • \$\begingroup\$ @AndrejaKo they're unlabeled, but I believe in the 1.5V to 3V range. I have around 100 of them, pretty much all different. \$\endgroup\$ – polemon Jul 10 '14 at 6:27
  • \$\begingroup\$ Still a transistor. Just a different one. \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 10 '14 at 6:43
  • \$\begingroup\$ @IgnacioVazquez-Abrams Yeah, but do I need a voltage divider or something, so I don't burn the bulb? Does the bulb behave like an LED with a voltage drop, or more like a resistor? \$\endgroup\$ – polemon Jul 10 '14 at 6:57
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    \$\begingroup\$ Sacrifice one to science; put it on a lab PSU at 1V and turn he voltage up slowly, measuring the current at each step. When it gets bright enough for your purpose, stop. If it gets too bright, back off a bit... \$\endgroup\$ – Brian Drummond Jul 10 '14 at 8:28
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IMO you can actually use microcontroller PWM output and ULN2003-like driver in case you don't need to control every single lamp (can organize in groups though). So you can even adjust brightness dynamically by microcontroller.

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  • \$\begingroup\$ They likely use more current than a DIO wants to source \$\endgroup\$ – Scott Seidman Jul 10 '14 at 10:39
  • \$\begingroup\$ @Scott Seidman If DIO stays for "digital input-output", ULN2003 needs ~1mA input current from microcontoller, it's far below most microcontroller's rated output current. Should I have take part in discussion before aswering? I'm completely noob here. \$\endgroup\$ – Timurrr Jul 10 '14 at 10:56
  • \$\begingroup\$ Oops, missed the driver, thought you were using DIO directly \$\endgroup\$ – Scott Seidman Jul 10 '14 at 11:00
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There is no difference between your mini incandescents and an led, except for the specific voltage and current they want. You can drive them just the same.

You can put 3 in series with 5v and a transistor/fet. Or you could use diodes to drop the voltage. Or use a regulator to go from 5v to 1.8V, as needed.

You want to find out the current required at 1.8V, just to have a baseline for what size transistor or regulator to use.

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  • \$\begingroup\$ Thanks. I'll try the dropper resistor solution. I was thinking about the regulator solution, but I thought there should be a simpler solution, too. \$\endgroup\$ – polemon Jul 10 '14 at 11:16

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