I got this inquiry from a teacher who teaches high school kids electronics - can we have a cool kit like Aurora mini 18, in a simpler and more affordable format? I suggested Night Joule Thief, but the teacher felt that it wasn't sexy enough for the kids. (ur... you know what I mean.)

So I combined the multi-color LED and Joule Thief in one, and got Colour Night Joule Thief. It's a mood lamp version of Night Joule Thief.

Step 1: Features

Here are the highlights of the Colour Night Joule Thief.

    * Compact & streamlined design
    * Uses only one AA battery (or any 1.5V battery you can hook up to)
    * Easily adaptable to different size batteries - hook up holes to attach home made clips
    * A multicolor/color changing LED
    * Automatic turn on via a light sensor (adjustable sensitivity level)
    * Energy efficient - works even with a run-down battery, down to 0.6V
    * Makes a great little mood light

Step 2: Technical Overview

The circuit is minimally changed from Night Joule Thief. In fact there are only two parts added. You can refer to Night Joule Thief instructable for the theory of operation, etc.

Circuit

The key difference from Night Joule Thief is that the white LEDs are replaced with a self color changing LED. This LED has a little chip inside that control three color LEDs also inside. It's incredible that a circuit like that can fit within a 5mm LED.
However the chip inside requires DC voltage to operate, but the original Joule Thief circuit produces pulsed DC voltage - the voltage swings high and low very quickly. So I added diode (D1) and a capacitor (C2) to rectify the output voltage of the boost circuit. Now the color changing LED gets about 3V of steady voltage to operate.

Parts List

1x CdS Photoresistor (rated 3k - 0.3M ohm) (CDS1)
1x 1k ohm (R1)
1x 100k ohm (R2)
1x 10k ohm (R3)
1x 50k ohm trim pot (VR1)
1x 22pF (C1)
1x 10uF (C2)
1x 470uH (L1) (anywhere between 22 - 470uH would work - might have to reduce the C1 value however)
1x 2N5401 or equivalent (Q1) (or just about any general purpose PNP transistor, such as PN2907, 2N3906, etc...)
2x MPSA06 or equivalent (Q2, Q3) (or just about any general purpose NPN transistor, such as PN2222A, 2N3904, 2N4401, etc...)
1x 1N4148 or equivalent (D1)
1x Color changing LED (D2) (I used "slow changing" type - use anything you want)
2x Battery Clips

ColourNightJouleThief-schematic.pdf
Download

Step 3: Assembly

The assembly is very straight forward. Insert the parts into the PCB, and solder them. Start with smaller/lower profile components, and move on to larger/higher components. Follow the order listed below.

Parts List (in assembly order)

1x 1k ohm (Brown-Black-Red-Gold) (R1) - red line on the tape
1x 100k ohm (Brown-Black-Yellow-Gold) (R2) - green line on the tape
1x 10k ohm (Brown-Black-Orange-Gold) (R3)
1x 1N4148 or equivalent diode (D1)
1x Photoresistor (rated 3k - 0.3M ohm) (CDS1)
1x 50k ohm trim pot (VR1)
1x 22pF (C1)
1x 10uF (C2)
1x 470uH (L1)
1x 2N5401 or equivalent (Q1)
2x MPSA06 or equivalent (Q2, Q3)
1x Color changing LED (D2)
2x Battery Clips - attach from the bottom side

Transistors, diode, capacitor C2, and LED have polarities, so make sure to insert them in the correct orientation. One of the leads of diode D1 (left when you view from the top) sits close to the battery clip. Make sure to clip this lead short to avoid short circuit.
Battery holders need a bit of force to snap into the holes. They attach from the back side of PCB as you can see in the picture.

Once everything is soldered in place, double check the part placement, orientation and solder joints. Then insert a battery. The polarity is marked on the front side of PCB.
If you don't see the LED light up, don't worry. The room is probably too bright. Take a piece of black paper or tape and block the light from hitting CdS light sensor. (and/or darken the room) If the LED still don't come on, turn the trimmer (the little orange thing) with a screw driver, counter clockwise. This makes the sensor less sensitive to light, so the LED will come on by just placing the sensor under shade, or turning off the room light.