Some of the fruit and vegetables we eat can be used to make electricity. The electrolytes in many fruit and vegetables, together with electrodes made of various metals can be used to make primary cells. One of the most easily available vegetable, the ubiquitous lemon can be used to make a fruit cell together with copper and zinc electrodes. The terminal voltage produced by such a cell is about 0.9V. The amount of current produced by such a cell depends on the surface area of the electrodes in contact with the electrolyte as well as the quality/type of electrolyte.
The AVR microcontroller is a leading low power microcontroller that has been around for almost a decade now. Recently, new lower power devices have been added to the AVR family, called the PicoPower AVR microcontrollers.
In this instructable, we show how even the regular AVR devices can be set up and programmed to run off a fruit battery.
Step 1: Preparing the Fruit Battery
For the battery, we need a few lemons for the electrolyte and pieces of copper and zinc to form the electrodes. For the copper, we just use a bare PCB and for the zinc, there are a few options: use galvanized nails or zinc strips. We chose to use zinc strips extracted from a 1.5V battery.
Start with a piece of bare PCB. The size of the PCB should be large enough so that you can create 3 or 4 islands on it. Each island will be used to place a half cut lemon on it.
Step 2: Prepare the Zinc Electrode
Next, open up a few 1.5V AA size cells for the zinc strips and clean it up with sand paper and solder wire to each strip.
Step 3: Arrange the Electrodes
On the bare copper PCB, cut islands with a file or hacksaw and solder the other end of the wire from the zinc strip to each copper island. For one cell, you need half a lemon and one island of copper and one zinc strip.
Step 4: Add Lemons to the Electrodes
Place the lemons on each copper island with the cut face down as seen below. Make incisions in the lemons to insert the zinc strips. The photograph below shows three cells being used.
Step 5: Assemble the AVR Tiny MIcrocontroller Circuit
Wire the circuit diagram shown here on a bread board. The choice of V type of AVR is important. For example Tiny13V is very appropriate for such an experiment, since V type of AVR is rated to work down to 1.8V power supply voltage.
Step 6: Program the AVR Tiny Microcontroller
The AVR is programmed using STK500 in High Voltage Serial Programming (HVSP) mode. The fuse settings are as shown here. The C code is short and sweet:
volatile uint8_t i=0;
Step 7: Battery Performance
Only one bit (bit PB3 on Pin 2) is being toggled.
The lemon battery performance (ambient room temperature 30 degrees Celsius) was measured as follows:
Number of Cells: 4
Open Circuit Voltage: 3.2V
Short Circuit Current: 1.2mA
Voltage with AVR TIny13V and LED load: 2.5V
Voltage with AVR TIny13V and LED load after 3 hours of continuous operation: 1.9V
Number of Cells: 3
Open Circuit Voltage: 2.3V
Short Circuit Current: 1.0mA
Voltage with AVR TIny13V and LED load: 1.89V
Voltage with AVR TIny13V and LED load after 3 hours of continuous operation: Not measured
Step 8: Achtung!
A short video of this circuit operated with the lemon battery is available on YouTube.
AVR Microcontrollers are very frugal devices and can operate at voltage down to 1.8V. The current consumption is also very small and the entire circuit including the LED current can be managed with a fruit battery.
Take care to dispose the materials, specially the zinc strips carefully without contaminating your surroundings. Do not reuse the lemons for any purpose after the experiment. Specifically, do not eat the used lemons after the experiment. Although this experiment is harmless and can be performed by children, it is best done under adult supervision. The authors cannot be held responsible for any injury resulting out of such an experiment.