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USB Reading Lamp Circuit

Light your table top with this cool White LED Lamp. It is powered from the USB port and is ideal to take notes while browsing internet. The USB port can provide 5 volts and 100 mA current which is sufficient to light the LED lamp.

Power to the circuit is derived from the USB port using the standard A type plug. To give continuous regulated power, Zener regulation is employed. Resistor R1 restricts base current to T1 so that output will be current regulated. Since White LED requires minimum 3.6 volts, 4.7 volt Zener is used to provide constant 4.7 volts to LEDs. Resistor R2 restricts LED current to around 20 mA, so that they will not be damaged. Connect the power lines to USB plug observing correct polarity. Use a reflector for LEDs to get maximum illumination.

USB Reading Lamp Circuit diagram

Seetharaman observed that the current thru LEDs is too small, around 2 mA, so I have simulated this circuit and indeed there is only 4 mA for each LED. You may want to decrease the R2 value to 22Ω. I will build this on a

Pin out of SL 100

19 Comments

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  • Frank1996

    Hi can someone please tell me what the purpose of C1 and C2 are ? 🙂

  • Frank1996

    what is the purpose for C1 and C2 ? Can someone please help me 🙂

  • Seetharaman

    Hi Thilan the maximum current you can draw is just 500mA from USB port. That too when the PC is on mains operation, with battery operation it will be heavy drain on the batteries. For 20 LEDs of 20mA you will be drawing 400mA will be a heavy drain on batteries. Do not exceed 4 or 5 LEDs.

  • Thilan

    Can’t we increase the number of led bulbs used to 20 or more than that. Pls can u post that circuit

  • Oshine

    I would kindly like to know about the working of the circuit.. Especially the working of the capacitor c1.. Thank u..

  • arpeggio

    Hi Guys, Thank you for this and all the interesting comments. I will be trying out this design incorporating Seetharaman’s mods.

    I have one question and pardon me if it’s a dumb one… but why does the circuit use a transistor? (from my very basic understanding transistors are employed as either amplifiers or switches.)
    I cannot see how it performing any of these functions in the LED Lamp Circuit above.

    Thank you and looking fwd to your reply.

  • Sebastian Kushero

    Just an observation it would be preferable to use it when the laptop is plugged in and not running on battery. as it would accelerate the drain on the battery.

    Limiting it to two LED would be ideal

  • Akila pramod

    thank you seetharaman.iam asking for you to please write more circuits for this article.plese write fm transmeter circuit.

  • Seetharaman

    Hi Akila In the above circuit T1 base is kept at 4.7 volt a drop of 0.7 volts across Base and Emitter. The output at T1 emitter is 4 volts. Specification says white LED hence you require 3 volts across the LED hence balance 1 volt will be dropped by R2. 1 / 100 ohms that is 10mA. Since all 5 LEDs are in parallel, each will get 2mA ?!!. If R2 is 10 ohms then each LED will get 20mA glow will be OK. Please do not try this circuit without these modifications. Please see my May 6th 2010 comment. I request author to change the above circuit suitably. Thanks.

    • P. Marian

      Thanks Seetharaman, I just wrote your conclusion in the article and soon I will test it and do all the measurement. I just need to buy the zenner diode.

  • Akila pramod

    this circuit is very nice.sl100,220r,100r,100uf 25v,4.7v 400mw,ingreadiens.this circuit make easialy on dot boards.i am start to make this circuit.
    thank yoy,

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