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SunEater V

SunEater_V This photovore is based on SunEater_IV and uses a single 74HC14 to do all his "thinking." Two "tiny pager motors" (found here) provide the muscle. Together with the SMT components, they allow SunEater_V to fit in a 3.5cm x 2.7cm x 2.5cm block, with just his wheels and feelers sticking out. He weighs less than 14 grams.

Performance

The SunEater_V behaviour is based on three rules:

  1. If no feeler switches are closed, the motors will obey the `eyes'. SunEater_V moves towards the best light, while trying to avoid shadow patches.
  2. If one of the feelers touches an obstacle, SunEater_V "follows the wall" in the direction of the better light. Both this and the first behaviour are illustrated here. Thanks to the `mechanical memory' - the robot keeps a feeler in contact with an obstacle during evasive action - the motion remains deliberate, even if the voltage drops to zero between steps.
  3. With both feeler switches closed, the robot will push against one of them, trying to get free.

My other solar-powered `bots - Photovore, SunEater_III, Son of Photovore, and Smiley - show exactly the same behaviour. But SunEater_V is the smallest and most active of the lot. With the sun about 30 degrees above the horizon, its horizontal BP243318 (24x33mm 5-cell panel) supplies enough power for two or three `steps' of about 0.5cm per second. When the sun gets higher, the robot achieves several cm/s. And even in the shade there is motion every few seconds.

This performance has little to do with efficiency. SunEater_V doesn't charge its storage capacitor any faster than its predecessors, and the tiny pager motors are easily outperformed by the small recorder and servo motors used in SunEater_III. But the little SunEater_V has a much better power to weight ratio; it weighs six times less than number III, and eats light with the same solar panel.
Being small has disadvantages as well. My larger photovores have no trouble with inclines and uneven floors. Both little guys need to be afraid even of dust. Son of Photovore because of its open lady's watch mechanisms. Suneater_V's very small diameter drive shafts tend to slip if its wheels get dusty. So why build tiny robots? Because they want to be there :)

Circuit

SunEater_V schematic

The solar panel charges the storage capacitor (47000uF). Its rising voltage is monitored by the schmitt-trigger/inverter with the two diodes in series and a 1M variable resistor on its input. But not continuously; that would increase current consumption. Instead, The pulse generator (two ST inverters upper right) supplies very brief `sampling' pulses. As long as the voltage is lower than the switch-on trigger level, the monitor looks at the voltage about 4 times a second, each time for about 1 millisecond. When the voltage is high enough to switch on a motor, the monostable activates the sensors and motors for a length of time determined by the 100N cap and 1M variable on the output of the monitor.

The `brain' is mounted with the solar panel on top and the components facing downward. A second PCB carries the motors, the shaft and wheels, the feelers and the storage cap. SunEater_V PCB

Because SunEater_V is very light, little energy is required to move it. The switch-on voltage is a mere 1.7V, and the duration of a `step' is only about 40ms - enough to make it jump about 0.5cm. During the `step', the voltage drops just 0.1V, to 1.6V. So the recharge takes very little time, even in bad light.
Which motor actually runs is determined by the sensors and the final two STinverters. They need a little help from `diode logic' to make it, but use of a single (74HC14) IC means a lot when size reduction is important.

 

PCB layout

PCB layout PCB film

Click on the layout in black to download a version in Postscript. Print it using a laserprinter which understands Postscript (or use Ghostscript) on transparant paper. That makes it very easy to transfer the layout to UV-sensitive PCB material.

Not that you need this layout. Jeff did a much more artistic job than I ever could, using an entirely different construction method. Have a look at his Homer III

Adjustment

The physically small `supercap' used for energy storage has a large capacitance, so between `steps', the voltage hardly drops. That allows the motors to provide relatively high torque at a low voltage. To determine the lowest practical trigger voltage, first set the monostable for maximum time - variable at 1M. Turn the set screw of the other variable resistor half way and see if the robot moves briskly. Turn towards higher resistor values until it fails to properly move, then back again to find a good compromise between fast recharge and good acceleration. Finally, reduce the `step' time until the distance covered is about 5mm. Longer steps will merely make the feelers less effective.
If necessary, the BPW41 `eyes' can be balanced by covering part of the front of one of them with a bit of black tape. The eyes are facing about 45 degrees downward. That makes SunEater_V avoid shadow patches as well as look for better light further away.

Mechanics

SunEater_V front view

The shafts of the motors rest on rubber rollers found in micro-cassette recorders. These share a common shaft, kept in position by the 0.3mm steel wire `castors'. The same thin wire was used for the feelers.

I hope that the above information is sufficient for you to build your own Suneater_V. If not, you can ask me specific questions. Enjoy!