I posted this to my late, great wasts0nseblog but forgot to post it to my new rustybolt.info blog. I wound a third or tertiary winding on a Joule Thief to drive another LED. The LED could have been put in parallel with the existing LED IF it was the same color or the same forward voltage. But it wasn’t the same color. If you’ve ever experimentind with a JT, you might know that if you put a red LED for example, in parallel with the blue LED, the red LED will hog all the current and the blue LED will get none, just because the red LED’s forward voltage is so much lower than the blue LED.
So to get around this, I wound a third winding on the JT’s core, and it drives the other color LED. If you notice, in the picture there are two blue LEDs and they are connected anti-parallel or cathode to anode. One LED lights on one half of the AC cycle and the other LED lights on the other half of the cycle. But one LED is brighter, and that means that the third winding’s connections are polarity sensitive. It will make a lot of difference in the brightness if you connect the LED the wrong way.
The JT is power limited, so the available power will split between the original LED and the LED on the third winding. The number of turns is not critical, as long as there are enough to get the voltage up to the forward voltage of the LED.
Back to experimenting…