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Building a Chloroplast Solar Engine

This great little solar engine is easy to build, and very powerful.  If you would like a free printed circuit board ( the black part ) please email me at and let me know where to mail them.  I'll send you a few! 

If you would like to get the entire kit, click here!

Here's how to solder the solar engine together!

Gather the following tools
bulletSmall wire cutters
bulletSmall needle nose pliers
bulletSafety glasses
bulletSoldering Iron ( Suitable for electronics)
bulletElectronics grade solder
bulletHot Glue Gun (Optional)


Handle solder and the soldering iron very carefully as the tip is very hot!

Always wear safety glasses

Never clip the cut end of a wire toward a person or yourself. The wire end can easily shoot into an eye if carelessly directed.

Wash your hands after handling solder.




Place the two 100K resistors in locations R1 and R2. Polarity does not matter!

Install the 2200uF capacitor

Install the capacitor as shown with the positive pole to the pad marked C+ and the negative pole to the hole marked C-. ( The negative pole side is clearly marked with a white band on the case)

Install the semiconductors

Install the MC34164 and the MPSA12 semiconductors in the locations marked on the circuit board. Be certain that the flat sides of the two packages are facing each other

Install the solar cell

Using the wires included in the kit connect the positive (+) end of the solar cell to the V+ pad on the pcb and the negative (-) end to the V- pad.

Install the motor

Connect the two poles of the motor with the wires included in the kit to the M+ and M- pads on the pcb. The polarity of the motor in unimportant.





The Chloroplast in a living plant is the structure responsible for conversion of energy from the sun to food for the plant ( in the form of starches )  .  Similarly, the CYBUG Chloroplast converts solar energy to electrical energy in a controlled and efficient manner for use in small robotic lifeforms such as the Solarfly.  It is an excellent solar engine for larger robots, walkers, and solar powered robots with on-board microprocessors such as the BASIC STAMP.

I built the Chloroplast circuit because I enjoyed tinkering with BEAM circuits, but didn't appreciate how the conventional solar engines dropped ALL energy across the motors ( leaving none for logic or uP ) and the low voltage they operated at forcing me to use very low torque pager style motors.  I wouldn't be happy until a CYBUG sized robot and motors moved quicker and with more oomph than my smaller BEAM robots.  I really found the circuit that I need in this Chloroplast, and I'm pleased to share it with you here!

bulletMaintains a minimum voltage of around 5 Volts, useful for maintaining digital logic or microprocessor circuits.
bulletHas a hysteresis from 5-7 Volts which is adjustable to suit the designers needs.  This "bonus voltages" is discharged across the motor, but discharging stops when the minimum threshold voltage ( around 5V ) is reached.
bulletHas more punch at a higher voltage than other solar engines, making it suitable for larger, torquier motors, such as those used on the CYBUG.
bulletUses only two three-pin devices and two resistors available from most electronics retailers.
bulletHas no lock-up bugs.
bulletVery simple to free-form.
bulletIs very inexpensive.
bulletExtremely low quiescent current ( about 25 micro amps or less)
bulletWorks the capacitor at closer to capacitor voltage limits for greater efficiency.

Theory of Operation:

The heart of the Chloroplast is the Motorola MC34164-3 Micro power Undervoltage Sensing Circuit ( U1 in the following diagram ).  In normal use, this component monitors the voltage at pin 2, and applies a ground at pin 1 ( out ) when the monitored voltage drops below 3V ( 34164-3 monitors 3 volts, 34164-5 monitors 5 volts ). This low will then be used to assert a RESET on a microprocessor circuit.  The output ( an open collector ) will be open when the output is above the threshold voltage.

Through tinkering, I have discovered that placing a 220K to 270K resistor in series with the input of U1 will produce a significant hysteresis in this sensors output.    For instance, a 220K resistor as R2 will cause the output to ground until the solar voltage hits 6.8V, at that point it will open (float) until the solar voltage drops to 5.5V.  ( A 1.3V hysteresis ).  Different values of R2 will produce different results.

So how does this all work together?  Well,

  1. The solar panel will slowly charge up the storage capacitor C1 towards 6.8V.  U1 will assert a ground ( believing the voltage is too low ) which keeps U2 ( a high gain darlington NPN transistor ) open and the motor OFF.
  2. When 6.8V is reached, U1 will open.  The base of  U2 will then be pulled high ( through R1 ) and U2 will turn on, allowing the solar energy in the capacitor to discharge through the motor.  The motor spins.
  3. The motor will continue spinning and discharge the capacitor until the solar voltage falls to 5.5V.
  4. At 5.5V, U1 will assert a ground at its output (Out), believing that the voltage is too low, and it must apply a RESET.  This ground turns off U2 and the motor stops spinning and the system is ready for another cycle!

Free-form your own Chloroplast Solar Engine.

I picked up all my components from Active Components (  with the exception of the solar cells which you can get from us if you like.    The MPSA12 Darlington and 34164-3 Voltage Sensor are Motorola products.

The following diagram illustrates how you can free-form your own Chloroplast solar engine.    The 34164-3 and the MPSA12 parts are shown as a bottom view ( pins facing up, out of the screen ).  The two 3-pin devices may be held together ( face to face ) using foam double sided tape.  Using two solar cells is optional, but if you use 1 solar cell, be sure to use a lower value of R2 to accommodate the lower voltages.

That's all I wrote!

I hope you try the Chloroplast Solar Engine, and tell me what you think!  I've found it a great alternative to other BEAM style solar engines, and opens the doors to a greater evolutionary diversity in your robots.

Craig Maynard 

Updated July 11, 1999