The Joule Thief

I had it ready for months, but never finalized. This Xmas 2007 we needed a simple way to light the Nativity scene, so I took the chance to get it working.

I used the joule thief in its simplest form, as probably published November 1999 issue of EPE (Everyday Practical Electronics). It's a NPN transistor, one resistor, a bifilar transformer and a LED (any color). I found the diagram on the web and worked immediately.

Lighted using a dead saline battery.

Lighted using a dead saline battery. No flash.

The Nativity scene, with room lights ON.

The Nativity scene, with room lights OFF.

How did I wind the transformer? I saw many questions about it around the web. First of all, you need some luck since you're probably winding it on a core with unknown characteristics. I took some length of enameled copper wire (2 metres), bent in half and wound about 8 turns through the core. Windings are spread about on 80% of the core. The core is a ferrite bead recovered from WhoKnowsWhereLand. Try it first with a fresh battery. If it doesn't lit, add some turns and try again. If at 20 turns you still don't get light, then:

• check your LED is wired properly
• change the core

Don't forget to scrape off the insulation from the enameled wire at solder points!

Before someone asks, no, I have no spare cores to send around.

With the saline battery my joule thief produced noticeable light (in darkness) for 36 uninterrupted hours. I will measure how much current it draws at the next battery change.

Driving a LED from TTL/CMOS

It's the first thing you learn about TTL/CMOS chips: they cannot (should not) drive a LED directly. A transistor driver is needed, and is shown at the right in its simplest form.
When the gate output goes high the BJT saturates and current flows through your LED. A limiting resistor is needed to control the LED current.

The table below shows R values vs supply voltage vs LED current.

Vcc	R ohm	R_ohm	R_ohm
3,6	45	30	22,5
4,8	105	70	52,5
6	165	110	82,5
7,2	225	150	112,5
8,4	285	190	142,5
9,6	345	230	172,5
10,8	405	270	202,5
12	465	310	232,5
13,2	525	350	262,5
I_LED [A]	0,02	0,03	0,04
V_LED [V]	2	2	2
V_CE [V]	0,7	0,7	0,7

I will need these in my hi-brightness red LED optical MCW transmitter.