Please check plan

Discussion in 'The Projects Forum' started by wori1, Mar 6, 2014.

  1. wori1

    Thread Starter New Member

    Mar 3, 2014
    (failed posting with longer title)

    This will be the first time I have built a project like this and would like someone to review it before I wire it up on a breadboard to test. The goal is to control a cluster of LEDs with a Raspberry pi. Idea is to power the LEDs with a 8v 1A wall wort via a constant current circuit switched with a PNP BJT hooked to one of the Pi GPIO ports (3.3v 16ma max). I looked around and didn't notice a circuit that did exactly what I needed. I took an example constant current circuit and modified the component values a little to fit my needs and added an extra BJT transistor to switch the circuit on and off with.

    Falstad (image also attached)

    R1: 100k-ohm 1/4 watt
    R2: 5 ohm 1w
    R3: 550 ohm (4 2200 ohm 1/4 watt)
    Q1: Fairchild FQP50N06L
    Q2: Fairchild 2N5088BU
    Q3: Radio shack MPS2222a
    LED: ~1.5v 100ma cont.
    V1: 8v 1a
    V2: 3.3v 16ma

    Since the LEDs are only running at about half rated constant current, I think it should be fine even if I had an open in one of the pairs. Does the power handling look reasonable on all the transistors?
    Last edited: Mar 6, 2014
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    It looks like you have used a constant current driver consisting of a NPN and a FET, then used another NPN to switch it on and off?

    You can simplify that circuit down to two parts. One NPN darlington, and one resistor;
    * RasPi 3.3v output pin goes directly to base
    * emitter goes to ground through the resistor
    * 8v goes through LED chain, into collector

    The operation works like this;

    * when RasPi pin is 0v, the NPN darlington is OFF

    * when RasPi pin is 3.3v, NPN base is 3.3v, and its emitter will be right around 2.3v. The LED constant current is set by choosing the value of the resistor. For example; 2.3v/23 ohms = 100mA.