Secret Project: The BEAMAnt- Light Seeking Reversing Robot

BEAMAnts are one of the oldest autonomous BEAM critters. The original BEAMAnts used the ancient HBS engine of BEAM god lore, and although they did their job well, they were slow and tended to get their long power-generating photodiode necks broken by turbots. The next major improvement in the BEAMAnt genus was the Photopopper, which does essentially the same thing as the original "BanjoBug BEAMAnt", but with newer technology that made it faster and easier to build.

The successor to Photopopper technology is developing out of what is called a Unicore type circuit, allowing the creation of a reversing photovore, which is what powers the BEAMAnt 6.0 series robot and the BEAMAnt we will be building using a 9V battery for power.

Battery powered?

Yes, while it is true that BEAMAnts have traditionally been solar powered, this battery powered device saves the trouble of a solarengine as well as finding suitable capacitors and solarcells. The GM2 and GM3 style motors used have less than stellar performance under solar power. However, with reasonably efficient motors and a few extra components this project can be solar powered.

The first BEAMAnts, the "Banjo Bugs"

The first BEAMAnts were powered by a string of solar cell photodiodes and used a HBS (Happy Birthday Singer) for the "brain".: An inspiring original: Mark Tilden's BEAMAnt 6.0
These use very efficient micro gearmotors, they can run full time using a SC3733!!

F01- Complete BEAMAnt

This project is not for the faint of heart as the free-forming gets somewhat involved. You may want to practice on simpler book projects such as the Symet or Herbie first or simplify construction using the solarbotics BEP boards. If you want to jump in head first, no one is going to stop you (or be able to throw you a life-line either!).

The BEAMAnt Explained:

So before we get knee-deep in building, let's get some background theory which will be helpful later if and when you need to troubleshoot your BEAMAnt.

The core of this circuit is the phototropic Bicore, composed of a pair of inverters, a pair of capacitors, and a pair of photodiodes. We won't go into great detail on the principles of Bicore operation (there's some excellent descriptions on the 'net already - Googlesearch them up!). All that you need to know is that the output duty cycle will vary as the light level between the eyes changes. This behavior is what controls the Headbot project in chapter 10 of the book

For this project we want a bit more control over the signals being sent to the motors, so the output of the phototropic Bicore is fed into another Bicore, called the filter Bicore. The filter Bicore can be used to change the response time of the motors and provide an isolation stage to keep the BEAMAnt from locking up when in reverse. The filter Bicore takes the incoming signal from the phototropic Bicore and will output a logic high or low, depending on the duty cycle received. Like all Bicores, when one side is high the other side must be low, meaning that only one motor will be on at a time.

What these two stages control is the actual phototropism of the BEAMAnt, and if set up properly, it will always turn towards the brightest source of light. We could stop here and have a light seeking robot, which is all fine and dandy until it bumps into something. Perhaps we'll add some kind of obstacle avoidance such as tactile sensors...

Getting the BEAMAnt to reverse is simple. By reversing the polarity to the unused side of the motors, we get reverse motion. It also doesn't do much good to just reverse when the tactile is actually hitting something, so having a "time-out" circuit attached to the sensor greatly increases its effectiveness. For simplicity sake, lets just look at one of the two reverser circuits. Each reverser consists of a inverter, resistor, capacitor and a switch. When the switch closes, the capacitor charges up to a logic "high" level, where the signal is then inverted by the inverter to logic "low", causing the reverse to happen. When the sensor is released, the resistor slowly discharges the capacitor until the inverter reverses to it's original state, making the motor go forward again.

There is only one more issue to address: the BEAMAnt has a tendency to stall if the lighting conditions are right and it tries to reverse. Stalling happens because the circuit will turn on only one motor when turning towards a light source and due to the reverse sensor, it will also try to invert that signal going to the motor. To fix this, the reverser will force the circuit to oscillate giving both motors a chance to move.

So in short:

The phototropic Bicore outputs a duty cycle dependant on light level differences between the eyes.
The filter Bicore will provide a logic high or low to the motor driver depending on the duty cycle received from the phototropic Bicore
If a tactile is hit, this will invert one side of the signal sent to the motor driver reversing the motor, also this will force the duty cycle of the filter Bicore to around 50%
The motor driver receives signals from the reverser as well as the filter Bicore and amplifies these signals going to the motors.

The Circuit Diagrams:

This is the schematic of the original basic BEAMAnt with no reverse capability, which will work as a basic light seeker or line follower.

PDF Version

The basic BEAMAnt circuit with a reverse by adding on additional buffers on the ground side of the motors.

PDF Version

This is an even more advanced BEAMAnt circuit with the "reverse process kill" fix, showing motor buffering via a L293 motor driver chip, pretty much what we'll be building but without the reverse indication LED's.

PDF Version

Need a 5V regulator, and can't find a 7805? This one suits our purposes nicely...

PDF Version

And the star of the show, the wiring schematic we'll be using to build a BEAMAnt with positive phototropism, reverse, and anti-lockup features.

PDF Version

BEAMAnt Parts

F02 - BEAMAnt Parts

Here are the basic components to building a BEAMAnt:

	2 - Motors, Gearmotors are preferable, aim for a gear ratio of 4:1 up to 300:1
	1 - 74AC240, DIP package
	1 - L293D, motor driver chip
	7 - Resistors, Assorted values more on this later
	2 - 0.01µF capacitors, label on capacitor "103"
	2 - 0.1µF capacitors, label on capacitors "104"
	2 - 22µF capacitors, label on capacitors 22uF
	2 - Photodiodes (can also be substituted with CDS cells with minor circuit modifications)
	1 - 9V battery, rechargeable or alkaline (avoid carbon cell batteries!)

The following are for the discrete voltage regulator, you could just use a 7805 voltage regulator:

	1 - 2N3904 or PN2222, NPN type transistor
	3 - LED's, use green or yellow if possible
	1 - 1K resistor

The following are components for mechanical construction (not shown):

	1 - Sintra, wood, lexan, sheet metal or whatever you can use to make a body, we used Sintra, its light easy to work with and glues very well.
	2 - 1" lengths of 1/8" Heatshrink
	2 - 2" lengths of spring (we used Solarbotics TACT1 spring)
	2 - 1" lengths of 1/8" OD diameter brass tube

Got Breadboard?

Before you jump right into soldering you may want to breadboard the circuit to make sure it works. You may also want to run wires out to the motors to see if they have a reasonable speed at whatever voltage you decide to run them at (between 2V to 18V).

We recommend building the circuit up in stages on the breadboard. Start with the motor driver, add the phototropic Bicore, then add the filter Bicore and last add the reverser.

If you have the extra parts, leave the working breadboard circuit together, and use it as a model to build the freeform version.

So what do the resistors do?

The slave resistors are 4.3M by default, as this value goes down the response time of the motors get quicker, this means that it wags back and forth less. Its up to you how much wag there is, if the resistor value is low enough the wag basically disappears. The reverse timing resistors controls how long the BEAMAnt will turn, default values are 47K and 68K. Its a good idea to select slightly different values for backup timing because if both sensors get hit at the same time you don't want the backup to time out at the same time and charge strait forward again..

The Robot Geek on What Go Wrong with Your BEAMAnt circuit

	Be very careful with the voltage regulator circuit. It must be in place before you attach power, and we highly recommend testing it before use. If the 74AC240 chip gets a full 9V it will fry and must be replaced! Don't worry about the L293 chip as it can run up to 36V.
	With high current motors, you may run into problems with the internal resistance of the battery. Alkalines are best and carbon cells are near useless. Signs of this are that the motors take a bit of time to get up to full speed.
	Don't forget to put the resistors in before you test the BEAMAnt - it won't do much without them.
	If you have a multimeter, use it to test the photo Bicore. Set the meter to measure at least 5V DC, and with equal lighting on the photodiodes you should measure half the supply voltage at the photo Bicore output (around 2.5V). As you vary the light level on the eyes you can measure the average voltage as the output.

Start Building It!

F03 - Solder Enables together

Take the 74AC240 chip and fold the two enable pins together and solder, these are pins 1 and 19.

Next add the pair of 0.01µF capacitors and the pair of 0.1µF capacitors. One 0.01µF capacitors soldered between pins 2 and 3 the other 0.01µF capacitors between pins 17 and 18. The 0.1µF capacitor is soldered between pins 4 and 5 the other 0.1µF capacitor goes between pins 15 and 16. For a color reference the blue capacitors are the 0.01µF capacitors.

The 0.01µF capacitors are used for the phototropic Bicore, they need to run at a relatively high frequency as compared to the filter Bicore.

F04 - Solder capacitors on chip

F05 - Solder diodes

This next step is probably the hardest part in free-forming. Its a bit tight but you need to solder the diode anodes (side opposite of bar) to the inputs of the filter Bicore ( pins 4 and 15). Clip the excess lead but leave just enough to solder another lead to.

Next add the 100K resistors between the diodes and the outputs of the reverser.

For ease of free-forming we chose to use the bottom two inverters for the reverse timing. This makes the inputs to the reversers pins 8 and 13 and the outputs pins 12 and 9 respectively.

You could put sockets in place of these resistors but they only play a small part in the overall behavior and aren't really worth the trouble of adding sockets.

F06 - add the 100K resistors

F08 - Adding the 22µF capacitors

Now we add the 22µF capacitors. These capacitor are polarity sensitive, with the negative lead being the the shorter lead or the one nearest the side with the stripe on the capacitor body. Solder this negative lead to ground (pin 11), and the other pin to pin 8. Solder the other 22µF capacitor to pin 13 and ground (stripe going to ground, remember?).

During this step its a good idea to tie those floating inputs to a convenient output. The fastest way of doing this is to run a small solder bridge between pins 12 and 13 and another bridge between pins 5 and 6. This will keep the floating inputs from picking up stray signals and causing erratic behavior later on.

Its a very good idea to add sockets so that the behavior of the BEAMAnt can be "edited" later. Sockets go on pins 3,4,8,10(two go on pin 10),11,15, and 18.

F09 - Adding sockets in place of resistors

F11 - Solder Resistor to transistor

Now that the BEAMAnt circuit is basically finished, lets move on to building the voltage regulator. Before we get too involved in free-forming this regulator, keep in mind that you can substitute in a common 7805 voltage regulator and save yourself a bit of work.

The first step is to solder a 1K resistor between the Base and Collector of the 3904. Looking at the flat side towards you, pins pointing down, those are pins 2 and 3. This resistor limits the current provided to the voltage reference generated by three series LED's.

To provide a voltage reference, we need to solder 3 LEDs in series. We found that green LEDs are a very close 5V reference. Actual measured regulated voltage from a 9V supply was 4.94V.

F12 - soldering LED's together

F13 - Adding the LED assembly to the transistor

Connect the Anode of the LED assembly to the Base of the transistor.

That's it for the voltage regulator.

Pinout is as follows:

	The 3904 Collector is voltage input (6V - 12V)
	The LED cathode is connected to ground
	The 3904 Emitter is the regulated voltage out (5V)

If you have a multimeter I highly recommend giving this circuit a test, as if this fails you could fry your 74AC240 chip. The output voltage can be anywhere between 4 to 6 volts and still drive the 74AC240 chip with no problems.

There is minimal free-forming to do on the motor driver - simply solder all of the ground lines together. Those are pins 4,5,12,13.

You will need to run a spare lead across the middle of the chip to connect pair 4,5 and 12,13.

F14 - Motor driver

F15 - Attaching Voltage reg to 74AC240

Now we need to attach the voltage regulator to the 74AC240 chip.

The 3904 Emitter is connected to pin 20 on the 74AC240.

The Cathode on the LED assemble is connected to the enables (pin 19 and pin 1). For now, the Collector of the 3904 is left connected to nothing.

Now that the voltage regulator is in place we need to attach all of the grounds together on the chip.

The black wire here connects the enables to ground. This means that pins 1,11,19 and the LED's Cathode are all connected together.

F16 - attaching grounds together on the 74AC240

F17, 18 - Sensor construction

Brains are pretty much done, so it's time for some mechanical construction! Let's start by building the tactile sensors for the BEAMAnt

Shown here are two pieces of spring tubing and two pieces of brass tubing. The springs fit loosely inside the brass tubes.

Keep in mind that this is just one of many different tactile sensors that can be used. Use whatever type of switches suit your fancy, or simply order a Solarbotics TACT2 kit. The sensors we're building here are more robust, and will put up with much more abuse than a TACT2 sensor.

Take a bit of heat shrink and fit it over the spring and heat it so it shrinks down. This heat shrink should snugly fit inside the tube, if not either add a layer of heat shrink or squish down the tubing a bit.

Now to start the construction of the body. The 3mm yellow plate here is going to be the bottom. We won't give you dimensions, as the shape will depend on your motors/wheels. Want exact dimensions? Contact us, and let us know - if demand warrants it, we'll put a plan together.

The blue square with the smaller white square are spacers so that the metal motor shaft does not hit the battery. The 6mm white Sintra blocks on top provide space between the top plate and the battery.

F19A - Motors on Sintra

F19B,C - Add battery and top Sintra plate

The top figure shows a small white Sintra block that stops the battery from going too far into the body. Also note how the battery fits nicely between the two motors, this makes it easy to replace when it dies.

The bottom picture shows a 3mm yellow Sintra plate used to cover the top and provide an open spot to mount the BEAMAnt brain.

Here all of the body pieces placed together, this gives a good idea of how it will look when its done. Nothing here is glued down yet, we still need to take it apart and wire it up.

F19D - Front sensor plate

F19E - fitting the brain on the body + switch + tactiles

It's time to put the freeform brain on the body.

You may notice that there is a the black front plate with the sensors and the power switch. These were just placed there to see how everything fit together, nothing is glued down yet. The BEAMAnt freeform is mounted on the red piece of Sintra on top.

F20 - placing and wiring the L293D

With the black front plate removed, place the L293D chip between the motors and run a wire between the logic supply Vcc and the power supply Vcc. As you probably guessed that's the red wire connecting pins 8 and 16.

There is also a bit of pin forming here, the outputs to the motors are clipped a bit to try and keep things clear.

Attach the output of the L293D to the motors. Pins 3 and 6 are connected to the right motors and pins 11 and 14 are connected to the left motor. The blue wires connect the L293D outputs to the motors.

F21, 22 - Hooking L293D to motors

F23, 24 - Inputs of motor driver

Attach four wires to the motor drivers and run them back to near the BEAMAnt brains.

The grey wire connects to pin 2, and the purple wire connects to pin 15 on the L293D chip. On the BEAMAnt brains, the grey wire connects to pin 9 and purple wire connects to pin 12, which are the outputs from the reverser portion of the Unicore circuit.

We highly recommend leaving these wires a bit long just in case you need to reverse them later.

Attach wires between the other pair of inputs and outputs of the BEAMAnt circuit.

Its a good thing these wires were a bit long they needed to be switched after testing. It worked out that the tactile fix was holding the wrong side low, this is easy to fix just swap the motor leads. The outermost gray and purple wires needed to be swapped! Look at F33 to see this.

F25, 26 attaching the other motor driver inputs

F27 - adding the battery snap

Now to install the battery pack clip. Feel free to drill holes in the Sintra to make wiring more convenient.

The + on the battery snap goes to the middle pin on the switch. Choose one of the side pins to be where you want to connect all the + connections to.

The red wire trailing to the left of the switch is where the 3904 Collector we left is now attached (the input to the voltage regulator).

The red wire trailing downwards is connected to the red wire on the L293D motor driver which passes a full 9V directly to the motor driver. That small blue Sintra block to the right of the switch is just used to reinforce the switch mount.

F28 - Wire power to switch

F29 - Wire all grounds

The black wire from the battery snap is wired to pin 11 on the 74AC240 chip.

Run a wire from pin 11 on the 74AC240 chip to the ground pins on the L293 chip. This will attach all of the grounds together.

The red wire going to the motor driver goes to the same pin on the switch the "voltage in" on the regulator is connected to. When the switch is flipped the motor driver will get a full 9V.

Don't try and run the motor driver through the voltage regulator!

F30 - Power to the motor driver

F31 - Adding the eyes

Mount the eyes in the body pointed outwards at an eye angle anywhere between 0 and 180 degrees. These eyes are roughly at 150 degrees from each other.

Feel free to experiment with eye angles, as narrow ones will give you great "lock-on" when light is found, and wide ones give a better "search for light" capability.

Solder a wire between the two Cathodes (the side with the notch - green wire)

F32 - Solder a wire between the eyes

F33 - Wiring eyes to the circuit

The two anodes of the eyes get soldered to the inputs to the first Bicore (pins 2 and 17), these are the thin green wires.

This is an important stage: When powered up, the BEAMAnt should be phototropic or photophobic. If your getting a behavior opposite to what you want just reverse where the eyes are connected to the Bicore inputs.

The red wire sticking up was used to test if the tactile sensors worked properly. Do this by touching pin 20 to either pin 8 or pin 11, the motors should reverse. In this picture there is no time limiting resistors for the reverse so it will reverse forever.

If you closely at the purple and gray wire you will notice that they have been swaped as compared to F26.

Back to preparing the sensors!

Solder a wire across the two brass tubes (red wire) and one wire each to the springs inside the tubes (white wires).

Solder a long red wire to the brass tubes, If you haven't caught on already, the basic wiring rule is red wires are connected to positive (in this case +5V) and black wires to ground.

F34, 35 - Wiring the Tactile sensors

F36 - Wire sensors to the Circuit

Wire in the tactile sensors, one wire goes to pin 8 and the other goes to pin 11. Just take a guess at how to hook the white wires up you have a 50% chance of being right...

Unless you want your BEAMAnt to act as a sumo robot, you want it to turn away from the obstacle touched.

If it does charge into object, simply switch where the wires are connected. The red wire we see looping up is from the tactiles, this is connected to the brass tubes and goes to pin 20 on the 74AC240 chip.

That's just about it, at this stage you should have a fully functional BEAMAnt. check to make sure that the tactiles actually turn the BEAMAnt away from obstacles and not into them.

F37 - Almost done

F38 - Cleanup

Now that your sure you have all the sensors and motors going in the right direction its time to clean up the wiring. This step is optional but sure helps with the Aesthetic part of BEAM.

Now just add some stiff wire extensions to the sensors bend them however you like and your BEAMAnt is finished!

F01 - Complete

Care and Feeding of Your BEAMAnt

The BEAMAnt works best with a hard, flat surface with walls high enough to trigger the tactile sensors. It's also not that difficult to incorporate an edge detector (think: trigger both sensors simultaneously). Combine that with crossing the tactile sensors and you have a phototropic mini-sumo (particularly handy if your opponent has a 100 watt bulb strapped to his head).

What about the pair of unused inverters?

You may have noticed that there are still two free inverters that we did not use, these inverters can be used for a variety of extra behaviors such as:

	Random reversing time
	Reverse then turn when a tactile is hit
	A break free pulse to free the BEAMAnt if one motor gets stuck
	Dark illumination using a single inverter so if it gets dark headlights turn on
	Push button turn on/off - Extra LED indicator buffers

Experiment with your devices—since you built it, you’re the best person to make modifications!

Problems or questions? E-mail us!

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