BEP Application: Turbot

Dave Hrynkiw
July 12, 2007


The Turbot is a member of the Scophthalmidae family of flatfish and is almost completely circular. Turbot is often found partially buried in the seabed in sand, gravel, rocks and sediment. It is an active predator, as adult turbots live almost exclusively off other fish. Turbot is a good source of protein and is also rich in selenium. Its fat content varies, but it usually contains roughly 1 gram of omega-3 fatty acids per 100g filletBarring advances in genetic engineering, we will not be building that kind of Turbot. However, this kind of Turbot can be built with a soldering iron:


The original name was coined by Mark Tilden, as his original experiments in this form of robotics were inspired by Turing Machines.

Turbots are a unique robot, they move around by flipping themselves over. An interesting method of locomotion but somewhat tricky to make phototropic. Every time the Turbot flips over, the motors are effectively reversed, and a different side will be facing towards the light.

Turbots are well known as being the velociraptors of the BEAM park tearing into wires, flipping BEAMants over and organizing themselves into military units. OK maybe not quite "Jurassic park", but you get the idea.

So How does it Work?

This turbot design uses four eyes, one pair determines what way is up and the other pair looks to the sides. To date, this has proven to be very effective. For simplicity with this project, the motors run in only one direction.

The electronic control for this device is deceivingly simple. The core circuit consists of four photodiodes, a 74AC240 chip and a resistor. This signal is then amplified to drive the motors with a transistor driver.

Vision: A voltage divider is created using two reverse biased photodiodes. Why reverse biased? If they are forward biased, they would short out the power supply and that usually ends up being a bad thing. Basically, a reverse biased photodiode acts like a photoresistor, meaning that the resistance varies with light intensity. The voltage at the center of the voltage divider will vary with changes in light level.

Brain: The voltage divider output is fed into the input of an inverter, specifically a 74AC240 inverter. These inverters have the property of switching at close to 1/2 +Vcc (1/2 the power supply voltage). This combined with the tight tolerances of photodiodes usually make tuning unnecessary, especially with a design as noisy as a Turbot.

Brawn: Since the output signal from the 74AC240 is from only a single output, it will need to be boosted to drive a GM3 motor with sufficient power to make it move. It is convenient that in this particular Turbot design the motors need be driven in only one direction which can be simply done by using a single transistor and a bias resistor. A bias resistor size of 1K was used to drive a 2N2222 transistor, which should give a drive current of close to 500mA.


Circuit Diagram:


Wiring Diagram:


Here's a collection of the circuit diagrams in a nice, high-quality 25kB PDF file.

Assembly procedure:


1. Collect all your parts:

Click here to have these parts added to your cart.

Parts List Electrical:

1 - BC1 BEP board
1 - LMP BEP board
2 - GM3 style gearmotors
1 - 74AC240
1 - Power switch
4 - Photo diodes
2 - 2N2222
2 - 1K resistors
1 - 47K resistor
4 - AAA rechargeable batteries
2 - Dual AAA battery holder

Parts List Mechanical:

1 - 10ga Leg Wire 24'' long, need about 16''
2 - LMP BEP board
1 - Sintra 3mm, Isosceles triangle 10cm by 7cm by 7cm
1 - Sintra 6mm, Same measurements as above
8 - Sheet metal screws (for wheel and leg mounting pad)
2 - Mounting screws (3/4" long - we do not carry these, you may have to scrounge)


2. Soldering the 240 chip

Start with the 74AC240, and mount it into the BC1 board. Please take care to solder it in the right way, as these can be sizably obstinate to desolder (ummm, we mean "they're a pain the butt to remove").


3. Solder bridges on the BC1

Install a solder bridge that connects pin 1 to ground. This permanently enables the inputs on that side of the chip.

The three solder bridges on the left side are grounding unused inputs. Not absolutely necessary but can keep unwanted noise from entering the circuit.

Solder bridge at top left connects output 1 to the enable, that way the photo diode bridge directly control the enable pin.

Lastly there are a pair of bridges that run between O2, I6 and O6, I3

With proper solder bridges you can save yourself a lot of time otherwise spent running wires.


4. Jumper on the BC1

It's a bit hard to run a solder bridge this far across the PCB, so a short blue wire is used to connect I4 to O3.


5. 47K resistor

The 47K resistor runs between the center bridges of O2, I6 and O6, I3. This is the pass-through resistor. When the inverter is disabled, the signal gets passed through this resistor, but when the inverter is active the resistors are over-ridden by the inverted signal.

BEP05-2N2222-DSCN4311.jpg BEP05-2N2222_B-DSCN4309.jpg

6. Driver transistors

The 2N2222 transistors are installed in free pads with the emitter going to ground, with the collector ready to connect to the motors, and the base sitting ready for future soldering.


7. Cutting Traces

Three cuts need to be made.

First, cut the trace connecting the two enable pins together.

Next cut the trace connecting pin 19 to ground.

Lastly cut the trace that runs from ground to that little group of two pads.


8. The 1K resistors

The two 1K resistors are used to limit the current from the output of the 74AC240 chip to the base of the driver transistors.

One resistor runs from O3 to the base of the transistor. The other 1K resistors run from O4 to the base of the other transistor. This should limit the driving current to about 500mA, which is more than enough to run these motors.


9. Top Eye

Photo diode cathode connects to I1 on the BC1 board. Connect the anode to the nearest ground, which (in this case) is conveniently located right beside I1. This will be the eye that looks towards the "top" of the Turbot.

Remember! Reversed bias... reeeeverrrrsed biaaased! Otherwise, SMOKE! (bad)


10. Battery pack

The battery packs are put side by side, with the center leads bent over and soldered together. This wires the battery packs in series making a 4.8V pack. The positive pin is also clipped so that the pack can be mounted flat on the sintra base.


11. Solder motor wires

Solder the motor wires onto the motors right now, as the next step will cover up where they need to be soldered. A length of 6 inches of wire gives lots of room for error (not like you would make an error, right?).


12. Motor mounting to sintra

Yes, the picture is white sintra on white background with white motors, but the only 6mm sintra we have is white (sorta looks like a picture of a polar bear in a snowstorm, hmmm?). Alternately, you could glue two pieces of colored 3mm together to make a 6mm sandwich.

The mounting screws go through the motor holes into the sintra about 1/4" inch. Which adds a this mechanical bonding which isn't absolutely necessary, but provides much greater strength than just glue.


13. Battery pack and side looking eyes

The battery pack should fit nicely between the motors. The pins on the other side of the battery pack will need to be cut off to fit on the sintra as shown.

The eyes are mounted on the inside edge of the motors, and make sure to use the plastic motor nub to help protect the eyes from damage when the Turbot is rolling on the ground.


14. Wiring side looking eyes

Left eye Cathode goes to + on the battery pack via white wire.

Right eye Anode gets wired to ground via the black wire.

A close-up of the right eye anode connection


15. Bottom Eye

Bottom eye mounted between the battery pack and the sintra. White wire goes to + on the battery pack. Green wire connected to the photodiode Anode and runs to the other side of the turbot.


16. Green Wire

Green wire from anode of the bottom looking eye, connects to I1 on the BC1 board.


17. Left eye/Right eye wired

Left eye anode gets wired to I2 with an orange wire.

Right eye cathode gets wired to I2 also with another orange wire.


18. Motor connections and testing

Probably the trickiest part of building this is getting the motor connections right, you have a 25% chance of guessing them ALL correctly the first time. Good luck!

See these red and black wires from the battery pack? Use these to temporarily test the operation of the Turbot and figure out the correct motor connections.

The motors share a common positive connection, with the other motor connection are going to the emitters of each of the driver transistors. Each motor activates when ground is switch by the transistor.

Basically it doesn't matter what way the motors rotate as long as they are both the same direction. Get that right, and you have something that will be either phototropic or photophobic.

Note: It's more convenient to test this before attaching the legs as they tend to get in the way and tangle things up.

Next step is figuring out how to make the Turbot phototropic. Try hooking power up to the board temporarily, with topside up. The motor closest to the eye should be rotating so that it will flip the Turbot towards the light. If the other motor moves, then the motor connections to the driver transistor need to be swapped. If the motor goes in the wrong direction, then reverse the connections to that motor. Clear as mud, right? Read it again...

Now on the same top side up, try pointing the other eye towards the light. Again, the motor closest to that eye should be the one turning. Make sure that its rotating trying to flip the turbot towards the light source.

With that in place, the bottom side should be correct also, that being now the opposite motor to the eye seeing light will rotate to move the Turbot parallel to the light source.

It's not easy to wrap your head around a robot brain that has no distinguishing difference between left/right, and up/down. Re-read, experiment, and resolder if need be!


19. Brain mount

Here the wires get trimmed a bit and the BC1 board gets mounted to the sintra. Make sure that the wire connections that were decided on the previous step are the same here.

The black wire is left over because we still need to put in some kind of power switch.


20. Power switch

The power switch switches ground. Shown here a DPDT (double-pole, double-throw) switch used to turn the Turbot on and off. A SPST (single-pole, single throw) switch would also work fine. The switch is mounted on the ground rail on the outside of the BC1 board. The two free pins are attached directly to the negative side of the battery pack. When the switch is flipped, the ground rail of the BC1 is connected to the negative side of the battery pack.

The switch should be buried enough that it will not shut itself off when the robot is in operation. You may want to put a guard rail around the switch to protect it from getting damaged.


21. Soldering the Legs

Cut two lengths of leg wire one 7 inches long and the other 9 inches long. Strip the insulation off 1 inch from the ends.

Solder the leg wire to the mounting pad and screw them down to the GMW.

Mount the legs on the Turbot. Then the last step is bending. The shorter leg gets bent 90 degrees so that it rotates between the body and the longer leg. The longer leg is bent at about 55 degrees so that the legs don't hit each other when the turbot is moving.


22. Finished!



  • When powered up for the first time, check to see if the photodiodes start getting hot. They were? That means they were put in forward biased, and are now toast. Replace them, but installed in reverse bias mode (that means wired backwards). If the 74AC240 chip starts getting hot, then either the system voltage is too high (>6V) or the battery pack polarity is backwards.

  • If you flip the power switch and nothing happens, either the motors are hooked to the wrong polarity (should be to +) or the batteries are not charged. If in doubt, check the voltage across the BC1 power connections with a multimeter. When turned on, you should get at least 4.8V.

  • If the motors move but the light falling on the eyes doesn't seem to affect which motor activates, then check the voltage divider. The connection to the voltage divider may be going to the wrong set of inputs.

  • If the light between the eyes was perfectly balanced, the output may oscillate at high frequency. This is easy to fix by adding a small capacitor providing positive feedback to the photodiode bridge. Do this by putting the capacitor across the input of the photodiode bridge to the associated output gate that the bridge is attached to.


Other Musings and Hints:

  • This device has the ability to run under solar or battery power. All that's needed for solar operation is a suitable solar engine such as the MSE. Remember, you will need to put solar cells on both sides.

  • You can actually put wheels on this, one side will be phototropic and other side photophobic. A Whrbot? Hmmm....

  • Now a sufficient number of Turbots combined with your midget pony army should be able to take over the world!

Copyright © Solarbotics Ltd., 2003, all rights reserved.


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