LED switch on off issue with wiring

Discussion in 'Digital Circuit Design' started by John A Bonilla, Mar 11, 2017.

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  1. John A Bonilla

    Thread Starter Member

    Mar 11, 2017
    So my issue is that I am making a circuit that is able to control a light switch remotely. The items I am using are:

    -Arduino Uno Rev 3

    -TIP31c NPN Transistor

    -PS2 wireless controller and dongle

    -100W LED

    -4x 9V batteries (in series)

    I have used the controller and dongle before with no issue, and I am receiving commands from the controller so that is not an issue. The LED does light up when connecting the batteries directly to them so that takes care of any power issues. The arduino does do its job by sending voltage to the designated pin where the supposed base of the transistor is suppose to be. So that leaves me with the issue of whether I am wiring up the transistor wrong, or if the issue lies in another place.

    The link to where I purchased the transistor is; www.amazon.com/gp/product/B01N9N1RZG/ref=ox_sc_act_title_2?ie=UTF8&psc=1&smid=A26TJE9XKYYNGN

    The other part that may be an issue is the grounds but I am unsure, this is the schematic of the current circuit:


    Should it be like this ^^


    or like this^^

    Another theory that I have is, if the arduino is being powered by a computer, would it be able to deliver enough voltage/current to the base to allow the electricity to flow? Any advice would be much appriciated, as I need to complete this project by March 21st, 2017. Thank you for your time!
  2. dl324

    AAC Fanatic!

    Mar 30, 2015
    The second circuit is more correct, but both are wrong. With a BJT, you need a base resistor to limit base current and a current limiting resistor for the LED. And you want the LED on the collector.

    Probably easier to drive from an Arduino if you use a MOSFET.
    Last edited: Mar 11, 2017
  3. LesJones

    Well-Known Member

    Jan 8, 2017
    Your first circuit will not work at all as there is no ground connection between the Arduino and the battery, transistor and LED.
    Your second circuit is using the transistor as an emitter follower so you will only be supplying about 4.3 volts to the LED. You need to connect the ground from the Arduino to the emitter of the transistor. You should also have a resistor between the base of the transitor and the I/O pin on the Arduino. You will need to calculate it's value based on the maximum current output rating of the ATmega328. (This will be on the data sheet.) The transistor is not suitable as a 100 watt LED running at 33 volts is 3 amps which is the maximum current rating of the transistor. The transistor would also need 300 mA base current to turn it on fully which wil be more than the Arduino can supply. You also need some way to limit the current to the LED The simplest way (But very poor method.) would be about a 1 ohm 20 watt resistor in series with it. Using a logic level mosfet rather than a bipolar transistor would probably be a better solution.

  4. DickCappels


    Aug 21, 2008
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