This question is continuation of this question.

I did some research on previous question and made a circuit as a starting point. Here's a picture: image of the circuit as displayed in a simulator

Now my problem is that TS555 device I plan to use can't drive those LEDs, so I'll most likely have to use a transistor for switching. My problem is that I have no idea where to start looking for a suitable transistor and that I'm generally confused about transistor naming schemes.

Also, general comments are welcome.

  • \$\begingroup\$ You are wasting a lot of power in the 150 ohm resistors. What is the 555 for? \$\endgroup\$ – markrages Nov 30 '10 at 18:55
  • \$\begingroup\$ @markrages Well, idea was to have it run PWM in order to conserve power. I'm not quite sure which resistors to use, so I'll probably get some weaker after I take into account transistor's voltage drop. The 150Ω value was given to me by an LED calculation program. There was some discussion about this on the question I linked. \$\endgroup\$ – AndrejaKo Nov 30 '10 at 18:59
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    \$\begingroup\$ small suggestion: It looks like you are using multisim, you can create hierarchical blocks to simplify your schematic some. For this design you could put the resistor and LED in a block, you would then past 10 copies of the block into your schematic. It may not seem like it would help for something like this, but when you start having cases where you want to change out the footprint of all of the LEDs and then add a single mosfet to each led set, or whatever may happen, then it becomes very nice to change it once and have it repeat for all of them. \$\endgroup\$ – Kellenjb Nov 30 '10 at 19:32
  • \$\begingroup\$ No problem, I teach Multisim and Ultiboard as part of our capstone class so I have taught plenty of people about that. It's been a huge help for a lot of people. \$\endgroup\$ – Kellenjb Nov 30 '10 at 19:41

Short Answer

Transistor part numbers are totally arbitrary and seldom have anything to do with a particular parameter. The best way to find one you need is 1) look through what you have, and if that fails 2) use parametric searches on distributors (e.g. Digi-Key BJTs or MOSFETs)

A 2N2222 (very common NPN) should be able to power 10x LEDs well.

Long Answer

You're using a moderate amount of current, so I might use an N-channel MOSFET instead of a BJT, it's the easiest to control and doesn't have a fixed voltage drop. A ZVN4206A would be a TO-92 that might work. Regardless between that and a BJT, you need a transistor with:

  • A voltage rating, Vds (or Vce for a BJT), greater than your supply (1.5x or more recommended)
  • A current rating, Id (or Ic), greater than the current that your LEDs will take (this depends on temperature; if you will be running it outside of room temperature you will have to derate this)
  • A threshold voltage, Vgs(th), that will allow it to be fully turned on with your input voltage ("logic-level" is something to look for)
    • For a BJT you need to make sure you can drive enough current into the base to saturate the transistor against the base-emitter saturation voltage, Vbe(sat).

Connect it like so (image from Electronics Tutorials)

alt text

(For a BJT, it would be similar, but Collector instead of D(rain), Base for G(ate), and Emitter--S(ource))

The "lamp" would be your string of LEDs, however you want to configure them. The diode isn't required for non-inductive loads (generally "things that aren't motors").

One of the advantages of this low-side switch configuration is your logic can be at a fairly low voltage (3.3-5 V), while your load supply voltage can be whatever your transistor can withstand (500-1000 V, whatever).


You mention this is for a specific LED flashlight project. If your main focus is powering LEDs, you could look at something more advanced: an LED driver. The Linear Technology LT1618 can step up voltage from 1.8 V allowing you to supply it with 2 or 3 alkaline cells, and provides current regulation, so you can eliminate all (or at least most of) the LED ballast resistors.

Example from the datasheet:

alt text

The LT1618 or a similar step-up regulator would provide you with the most efficient driving method, both from an electrical point of view and mechanical, as you can use more space-efficient cells (D > C > AA >> AAA >>> 9V)

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    \$\begingroup\$ Would 2N2222A be a suitable replacement for 2N2222? \$\endgroup\$ – AndrejaKo Nov 30 '10 at 19:55
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    \$\begingroup\$ Yeah, the letters behind part numbers typically indicate variants (of minor parameters or packages) or different grades of parts. I think the only difference between the basic and -A part is a lower saturation voltage, which is a useful thing. \$\endgroup\$ – Nick T Nov 30 '10 at 19:58
  • \$\begingroup\$ @Nick T OK then. Thanks a lot! I'll pick 2N2222A then. \$\endgroup\$ – AndrejaKo Nov 30 '10 at 20:00
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    \$\begingroup\$ @AndrejaKo: That's a different kind of LED driver, one that deals more with logic, not the power. \$\endgroup\$ – Nick T Nov 30 '10 at 20:39
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    \$\begingroup\$ @AndrejaKo: There's no point in using it then; just use a single transistor if you want something that basic. Your "local shop" restriction is somewhat odd; you will never have a decent selection of special parts \$\endgroup\$ – Nick T Nov 30 '10 at 21:01

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