Page 45 - Robots Androids and Animatrons : 12 Incredible Projects You Can Build

Solar cells can also indirectly power a robot by being used as a
power source for recharging the robot’s batteries. This hybrid power
supply reduces the required capacity of the solar cells needed to
operate the robot directly. However, the robot can only function
for a percentage of the time that it spends recharging its power
supply.
We can also utilize solar cells by combining the technologies of direct
and indirect power. Here we build what is commonly called a solar
engine. The circuit is simple in function. The main components are
a solar cell, main capacitor, and a triggering circuit. The solar cell
when exposed to light begins charging a large capacitor. The solar
cell/capacitor provides electric power to the rest of the circuit. As the
charge on the capacitor increases, the voltage to the circuit also rises
until it reaches a preset level that triggers the circuit. Once the cir-
cuit is triggered, the power stored in the capacitor is dumped
through the main load. The cycle then repeats. The solar engine may
be used in a variety of innovative robotic designs.
Building a solar engine
The solar engine is commonly used as an onboard power plant for
BEAM-type robots, sometimes called living robots (see the dis-
24 cussion about BEAM robots in Chap. 8). The inspiration for this
solar engine originated from Mark Tilden, who originally designed
a solar engine. Another innovator was Dave Hrynkiw from
Canada, who modified the solar engine design to power a solar
ball robot. I liked the electrical function so much that I decided to
design my own solar engine. In doing so I was able to create a new
circuit that improved the efficiency of the original design.

Figure 3.1 is the schematic for the solar engine. Here is how it works.
The solar cell charges the main 4700-microfarad ( F) capacitor. As
the capacitor charges, the voltage level of the circuit increases. The
unijunction transistor (UJT) begins oscillating and sending a trigger
pulse to the silicon controlled rectifier (SCR). When the circuit volt-
age has risen to about 3 V from the main capacitor, the trigger pulse
is sufficient to turn on the SCR. When the SCR turns on, all the stored
power in the main capacitor is dumped through the high-efficiency
(HE) motor. The motor spins momentarily as the capacitor discharges
and then stops. The cycle repeats.
The solar engine circuit is simple and noncritical. It may be con-
structed using point-to-point wiring on a prototyping breadboard.
A printed circuit board (PCB) pattern is shown in Fig. 3.2 for
those who want to make the PCB. The solar engine kit (see parts

Team LRN
Chapter three