Show your creativity and ingenuity with this Valentine's Heart with 12 circling LEDs that are powered by a 'Joule Thief' using a single 1.5v (or 1.2v rechargeable) battery.

Step 1: The Circuit

The circuit is composed of 2 major parts:

First, a 'Joule Thief' to boost the voltage from a penlight battery to about 4 volts to power red LEDs.

Then, 3 identical circuits to make a 'ring-oscillator' which, when connected as shown, will make strings of the LED chase around in a circle (or a heart-shaped ring).

Step 2: The Parts

A unique aspect of this circuit is the coil, which is made by winding 2-wires together (I used wire-wrapping 'Kynek' wire) using a 1-1/2" (35mm) form - like the cap from a small aerosol can. Wind 16 to 20 turns together - this will take about 6' (2m) of each wire. This kind of inductor is known as an air-core coil and DOES NOT need a ferrite core.

The inductance of this coil should be 20uH per side. Make sure you mark the starts and ends - the circuit needs to have them connected a specific way!

Other parts used in the voltage booster are:
1 x 2N4401 or 2N2222 Audio transistor.
1 x 1N4148 or other small signal diode.
1 x 10uF capacitor
1 x .1uF capacitor

The Ring-oscillator has 3 sections, each section is made up of (see 2nd image):
1 x 2N7000 (or BS170) MOSFET
1 x 100K resistor
1 x 2.2M-ohm resistor
1 x 0.1uF (100nF) capacitor

So, you would need 3 times the above parts.

Step 3: Making the Template

Pick a shape that appeals to you and mark out the locations of each LED. You will be using 12 red LEDs, so space them evenly.

Step 4: Begin Assembly

Start with the booster - this takes a few minutes and allows you to test the rest of the circuit should you need to later.

Make sure you note the start and end points of the coil. Remember that only one coil handles the power, which will be our Primary coil. The other is only to provide the timing signal, we will call that the Secondary. If you can, use the thicker wire (AWG# 28 or heavier) for the Primary, while #30 or 32 is enough for the secondary.

In the circuit diagram - terminals 0 and 1 indicate the Primary Start and End, while 2 and 3 are the start and end for the Secondary.

If the boost circuit tests OK, install the LEDs next, They have to be connected in series pairs.

Step 5: Final Assembly

The simplicity of the circuit means you should be able to finish it in an afternoon, once you have all the parts available.

To enhance the effect of the display, I cut out a heart in red paper with the LEDs running around it.