Once again I have stumbled upon a quite interesting circuit. It might not seem that special, after all, it's just a blinker, but this circuit is a bit more intricate than what it appears at first glance.

It's like a combination of a joule thief and a multivibrator, hence the name. It turns an LED on and off, but it also can work on very low voltages, allowing it to operate on a single AA cell until it's pretty much dead. It also uses a very small amount of current, so it can run for weeks, months or even years, depending on the components chosen.

Step 1: Components:

Two complementary NPN and PNP transistors: (BA547 & BA557, 2N2222 & 2N2907, 2N3904 & 2N3906....)*
Inductor (0.1mH to 10mH)
Electrolytic capacitor (10μF to 220μF)
Resistor (from 10k to 500k)
LED of preferred color and brightness

*I used BA547 and BA557

The value of the components can vary depending on your needs, I'll expand more about this in the next steps.

Step 2: The Circuit

The circuit can be assembled easily, for convenience I made a board on Eagle, this board has the size of an AA battery, so it can be glued at the back of any single AA battery holder.

Step 3: How It Works

The circuit looks quite simple with just 6 components, but the way it works is a bit more intricate. It seems when the capacitor, charged through the resistor, reaches a certain voltage, the NPN transistor activates, this also activates the PNP transistor, charging the negative side of the capacitor, making more current pass through the transistors and through the coil in a vicious cycle, creating an inductive kick, rising up the voltage and lighting up the LED, this process seems to repeat itself around 40000 times inside the duration of a single pulse until the positive side of the capacitor is drained I presume.

Due the high frequency operation, this circuit might not be breadboard-friendly, since breadboards and jumper wires can introduce unwanted capacitances and inductances, leading to different results when using a breadboard compared to the finished circuit board, you can try, nevertheless, and see what results you get.

Step 4: Playing With the Circuit.

There are many variables in this circuit, after some experimentation I've found modifying some components alters the behavior of the circuit, allowing to tune it to get the desired results.

If you change the value of the resistor you'll find the time between the flashes changes accordingly, a 10kΩ resistor makes the circuit blink very fast, almost too fast for the human eye to see, while a 300k resistor makes the LED flash around 2 to 3 seconds. The resistor changes the period at which flashing occurs. I recommend using a 100k potentiometer with a 1k resistor in series to change the resistance and find the desired value easily.

The same happens when changing the value of the capacitor, a bigger capacitor means more time between flashes, but it also means longer flashes of light, since the capacitor takes longer to discharge and end the cycle. The capacitor changes the period too, but it also changes the duration.

Changing the value of the coil also alters the operation, but this is a bit less intuitive. I've found lower value coils will need higher currents in order to work, this means using a bigger capacitor. An inductor with a lower value will probably be able to increase the brightness of the LED, but the duration of the flash will be lower or none if the capacitor value is not increased accordingly. The resistance of the coil is something that needs to be accounted for, coils with the same values can have different internal resistances, in my case, the coil I used had 0.5 to 1 Ohm, using coils with higher internal resistances can lead to the LED being too dim.

The LED choice also affects the circuit, brighter LEDs require more power, and depending on the color the inductor value will need to be higher or lower. For example, a white LED with a forward voltage equal to 3.3 volts will need an inductor value higher than a red LED with 1.8 voltage drop.

There are many combinations, I encourage you to try as many as you can. For my final circuit, I used a 33k resistor in combination with a 68μF capacitor and an inductor value of 5mH, this produced high intensity flashes.

Step 5: Modifying Iductors

If you have troubles finding an appropriate inductor you can always modify an existing one. I've made my inductor by wrapping 0.15mm enameled copper wire around a core. I used a motor to help me wrap all the turns around it, some hot glue keeps the inductor in place until heat is applied.

Step 6: Extra Features

I've recently discovered adding a small value resistor between the capacitor and the base of the NPN transistor can modify the length of the pulse, making the LED stay lit during more time before turning off. If the resistor is too high the LED usually stops blinking and the circuit will act like a normal joule thief.

Using potentiometers to play around and choose the desired resistor values is recommended, since it's way easier to find the best configuration in this way than changing resistor values every time.