BENDER - A Light Seeking Bicore "Head"

This is my effort to build a light seeking "head" based on the Phototropic Bicore schematic at BEAM Online. I started by reading the Bicore and Suspended Bicore sections of the BEAM Robotics Tek Advanced Nervous Networks document. I then whipped up a schematic of how everything should fit together:

Note: My photodiodes are connected in series

--|>|---|<|--

as shown in the BEAM Tek document instead of parallel like the circuit from BEAM Online. Apparently both ways work just fine!

My Drawing From BEAM Online. Circuit © Mark Tilden

Testing: I set up the circuit on a bread board and tested it thoroughly before construction. I wanted to to test a few different sets of photo diodes to see which worked best. I ended up using some that were much smaller than the ones in the photo.

Construction: I first covered the motor "body" with black electrical tape for insulation. I used epoxy to attach the '240 and a "head" built out of a terminal block with LEDs inserted for eyes. I built Bender with the '240 already glued on so I could form fit things around the shape of the gear motor. Here's how he looked:

Before I glued on the '240, I pre-bent the pins in/out to align them for connection as shown in the above photo. I then trimmed back the pins and tinned them with solder to insure a good connection.

I then constructed the Bicore, using left over leads from the capacitors to gang the Bicore buffers together, color coded wire for positive (red), negative (blue) and photo diodes (orange), and machine socket pins to create removable connection points for the battery and motor leads. Here is a close-up of the completed Bicore showing how everything wraps around the motor:

To finish things up I built a stand from a terminal block connector, heat shrink tubing and a few pieces of brass wire. Here are some views of the completed robot:


REAR	RIGHT

Notice how handy the socket pins are for plugging in the motor wires! The empty sockets on the sides are for the battery.

To power Bender I'm using a 9 volt NiCad battery regulated to 5 Volts by a 78L05 voltage regulator. To make the connector as small as possible, I soldered the regulator onto it and covered the whole thing in epoxy for strength and insulation as shown below:


Original 9v connector	Stripped with 78L05 attached. Pins (left to right) are +5V, Gnd, Input	After gooping :-)

From an aesthetics standpoint I think he's pretty cool! Here is one final picture of Bender on his stand with the battery and voltage regulator connected. Please pardon the rubber band -- I haven't decided on a final battery mounting method yet!

LIGHT/DARK BIAS RESISTORS: The BEAM Robotics Tek Advanced Nervous Networks shows how to use a resistor in series with the photo diodes to provide at least some minimum resistance when the robot is in extremely bright light, and a "dark" resistor in parallel with the photo diodes to provide a maximum resistance so the head doesn't just spin endlessly in one direction in very low light conditions. I chose not to use either on Bender to cut down on the number of parts, so he does spin one way constantly in very low light, and oscillates very rapidly when a bright light is shined at him, to the point where it looks like he is locked onto the light source.

Of course, shortly after I built Bender a new circuit called the "Power Smart Head" was published by Wilf Rigter. This circuit is very cool in that the motor only gets power when there is a substantial difference in the light falling on the photodiodes. Although the Bicore itself continues to oscillate, the motor is in effect cut off and doesn't use any battery power! Bender II will use this circuit.

ABOUT SOCKET PINS: Socket pins are a must if you want to change the configuration of your robot after you build it! On Bender, they are used for the motor and battery connectors. The cheapest way I've found to get these pins is to buy plastic IC mounts with machined pins in them (All Electronics sells one with 64 pins for $2.00) To remove the pins from the socket just melt (carefully!) the pins out with a soldering iron. If you rest the iron on the tip of the pins for a bit, you can wiggle them free. After a few minutes and a little wanton destruction you'll have a nice pile of sockets to plug your changeable components into!

'240 CHIP FLAVORS: The Nihon gear motor works very well with the current output (6 ma per buffer) of the 74HCT240. If you need more power for your motor then think about using an 74AC240 instead because it can provide a lot more current (24 ma per buffer).

SOLDERING/GLUING TIP: I clean all the parts I'll be soldering or gluing with a cotton swab and rubbing alcohol. This helps a great deal toward making a smooth, clean connection.


FRONT	LEFT