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
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 solar cell shown here is a thin film on glass type available from imagesco.com (#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
Step 4: Low Light Operation
Step 5: Solar Whirligig
Step 6: Walker the Robot
Step 7: Modified Circuit
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.