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How to Build a Solar Ball Robot

Once the glue has dried on the first shaft we must glue the other plastic rod on the opposite side of the shaft. Position the glued rod into the half sphere. Place the other plastic rod on the opposite shaft. Place the other half sphere together with the first. Gauge the depth the gear box shaft must be inserted in the plastic rod, then add another 1/8" of depth for error. Glue and let set.

Check your work while the glue is setting on the second shaft to insure that you can close the sphere properly.


The electronic circuit is a light activated on-off switch. When the light level is too low the circuit shuts off power to the gear box. The user adjusts the sensitivity of the circuit using potentiometer V1.

How it Works:

The circuit configures a CMOS op-amp as a voltage comparator. A comparator monitors two input voltages. One voltage is set up as a reference voltage called "Vref". The other voltage is the input voltage called "Vin", which is the voltage to be compared. When the Vin voltage falls above or below the Vref, the output of the comparator (pin 6) changes states.

The two input voltages are applied to pins 2 and 3. Pin 2 (inverting input) is connected a reference voltage of approximately 1.5V, using a simple voltage divider made of resistors R1 and R2. Photosensitive resistor R3 makes up another voltage divider in conjunction with potentiometer V1, that is connected to the non-inverting input (Pin 3) of the op-amp.

There is no feedback resistor between the output (pin 6) and either of the inputs (pins 2 and 3). This forces the op-amp to operate at its open loop gain.

The CdS photoresistor is the light sensor. A photoresistor as it name implies changes it resistance in relation to the intensity of the light that falls on its surface. The CdS produces its greatest resistance in total darkness. As the light intensity increases its resistance decreases. In the circuit, the CdS cell is part of a voltage divider. The changing resistance of the CdS cell changes the voltage drop across the potentiometer V1, that is connected to pin 3. As the light intensity increases the resistance of the CdS cell decreases which increases the voltage drop across the potentiometer. This increased voltage drop is seen as a rising voltage. The trigger voltage can be set for different light levels using the potentiometer.

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