Introduction: CSS555 Solar Engine

About: Emeritus Professor of Mathematics.
A Solar Engine is a circuit that takes in electrical energy from a solar cell, stores it in a capacitor, and after a sufficient amount has been saved up, switches the stored energy over to drive a motor.   When the running motor has used up a set portion of the capacitor's stored energy, the solar engine circuit switches the motor off and goes back to collecting and storing energy.  The  cardinal virtue of a solar engine is that it provides usable mechanical energy when there is not enough light - or not enough cell area - to run a motor directly or continuously from the solar cell.  It is an ideal way to intermittently power tiny robots, models, toys, or other small gadgets on indirect as well as direct sunlight or from artificial room light.

The Easter Solar Engine Instructable describes in some detail how to make a versatile and robust solar engine from common discrete electronic components: just transistors, resistors, diodes, and LEDs.  That Instructable also provides general background information on solar engines and tips on choosing the right motors, cells, and capacitors to use with solar engines, and so it can be a useful reference when making any type of solar engine.  

This Instructable describes an easy to make solar engine which has a special  IC as its heart, namely the CSS555.  This chip is a micropower version of the well known 555 timer IC.  It is pin for pin compatible with the basic 555, and has many quite remarkable additional features, but for a solar engine application, its incredibly low operating current of under 5 microamps is what makes it ideal.

The basic idea to use a 555 IC as the heart of a solar engine originated with Manfred Schaffran in 2003.  He used the  CMOS version of the 555, namely the 7555, which was the lowest power 555 then available.  Shortly after, Wilf Rigter came up with the idea of employing a photodiode as a voltage reference for triggering the 7555.  The circuit described here is basically the same but with the improved CSS555 IC which requires less than a tenth of the current of the 7555 and so gives very efficient operation and excellent low light operation.  This Instructable also includes a simple modification of the circuit which permits the motor to operate at a slightly higher voltage if desired.

Step 1: The Basic Circuit

The solar cell SC charges the storage capacitor CS and powers the CSS555 IC via pins 1 and 8 (the pinout diagram is included below).  Pin 4 is the reset which must be held high to enable the operation of the chip.  PD is a photodiode used as a constant voltage source with a fixed lighting level.

When the voltage in the capacitor CS rises to about 3 times that of the PD, Pin 2 is only 1/3 of the supply voltage to the chip.  In that case, the 555 makes the output Pin 3 of the chip go high.  This turns on the transistor QP and the motor starts.  RB is a 1K resistor just there to limit the current through the base of QP.  As the motor continues to run, the voltage of CS drops - unless the solar cell is large enough or the lighting strong enough to maintain the voltage while the motors runs.  The operation of the 555 is such that Pin 3 will now stay high as long as the voltage also fed to Pin 6 by the photodiode remains below 2/3 of the supply voltage to the chip.  Thus, if and when the voltage across Pins 1 and 8 falls to about 3/2 of the photodiode PD voltage, the chip turns Pin 3 off, the transistor cuts off, and the motor stops.  The capacitor CS then can charge up again and the cycle repeats.

Step 2: Stripboard Construction

The main components of the CSS555 Solar Engine are labeled in this photograph.  The CSS555 chip is available at

The solar cell shown here is a thin film on glass type available from (#SC-02).  Although listed as an outdoor cell, it works very well on indirect and good room light.   It is rated at 5.2V OC and 21mA SC.  Cells rated as indoor type work well under fluorescent lighting, like calculator cells.  Whatever cells are used, they should be capable of putting out a volt or two above the turn-on voltage of the solar engine to ensure an adequate rate of charge for the capacitor.

The motor is a low current model suitable for solar operation.  The static resistance of motors that work well with solar engines is in the neighborhood of 10 Ohms.  A typical "toy" motor made to run on a battery or two is under 2 Ohms which is much too low for this kind of application; the capacitor will discharge through such a motor before it even begins to move.

The capacitor shown is 0.1 Farad rated at 5 Volts.  This solar engine has shown itself to work perfectly with capacitors ranging from 1000uF to 1.0F.  A requirement for any capacitor in any low power solar engine is that it have a low ESR - under 1 Ohm. 

Most ordinary LEDs actually function as photodiodes, that is, when light falls on an unpowered LED, it outputs a voltage and current.   A typical LED will usually put out around 1.2 Volts in bright light, but that varies among LEDs with some being a few tenths higher or lower.  The output current of an LED is extremely small, but enough for the high impedance of the Trigger and Threshold pins of the CSS555.

Now with an LED putting out 1.2V, the turn on voltage theoretically would be 3.6V and the turn off 1.8V.  With 1.4V from the LED, turn on would be at 4.2V and off at 2.1V.  This compares very well with measured voltages using a variety of LEDs: red tinted, clear red, green,yellow, small and large.

The output transistor in this circuit happens to be a ZVNL110A Mosfet, but an ordinary 2N3904 transistor works just fine.

This circuit is made on a small piece of Stripboard.  Some of the details of construction are visible in the accompanying photos.

Step 3: Experimenter Board Version

This photograph shows the circuit made on an "Experimenter's I.C. Protoboard" made by DATAK (# 12-607).  Details of  the  layout are shown in the lower photographs.

Step 4: Low Light Operation

This solar engine works reliably well across the entire range of lighting levels from outdoors to inside. In an experiment to check for dependable operation at the lower end of the lighting range, the engine was set up for a near extreme low-light small-cell test .  The solar cell was a little SC-2422 which gave just 36uA SC and 4.75V OC under the chosen light level.  The engine was first fitted with a 1000uF capacitor.  Near the 3.5V turn-on point, the solar cell was supplying just 15uA to the engine.  The engine cycled on and off perfectly.  Further, a 22000uF capacitor was plugged into the engine and the test repeated.  Charging this larger capacitor with the tiny cell was extremely slow (about 0.3mV/sec near the firing point), but the engine cleanly turned on and off when it was supposed to.  This test confirmed the reliability and efficiency of the CSS555 solar engine.  Hacking a meter into the circuit revealed that the chip itself was only taking 3.5uA during the entire charging process.

Step 5: Solar Whirligig

An amusing application of any  solar engine is to operate a "Solar Whirligig" as pictured here.  This video shows it in action.

Step 6: Walker the Robot

This is "Walker" -  a little robot that soaks up solar energy and every now and then takes off for a short but energetic stroll. His energy comes from a RU6730 polycrystalline cell atop  his head.  You can catch a glimpse of him in action on this short video.

Step 7: Modified Circuit

The maximum operating voltage of the CSS555 IC is 5.5V.  Therefore one cannot string two LEDs in series as input to pins 2 and 6 to run the motor at a higher voltage.  However, the following trick can be used instead, which is to supply power to the chip trough a diode or two or three. The circuit is the same as before, but two 1N914 diodes connect the chip to the supply line.  The drop across the diodes allows the voltage in CS to go higher than the PD would allow.  The Pin3 still clicks on and off at the voltage levels that the chip sees, just as it did before, but the voltage supplied to the motor from the capacitor is higher.

This circuit was tried with the following measured results (in bright light with a LED photodiode giving 1.42V):

# of diodes     Turn on voltage     Turn off voltage
        0                      4.3                        2.3
        1                      4.7                        2.7
        2                      5.0                        3.0
        3                      5.4                        3.3

Note that the voltages listed here are measured at the capacitor, CS.  Also note that the voltage drop across the diodes is less than the usual 0.6V or so because the current taken in by the CSS555 is so meager.
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