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<p>This handy troubleshooting chart for Herbie-style robots is adapted from Mark Tilden's outstanding book, <em>Junkbots, Bugbots, & Bots on Wheels </em>(definitely essential reading if you're interested in the subject of BEAM robotics):</p>

<p><strong>If your robot...</strong></p>
<ul>
	<li><strong>...does nothing:</strong> First, disconnect the battery and make sure nothing is getting hot. Then search for an accidental "solder blob" short or other wiring error. If everything is wired correctly, make sure the battery isn't dead—you should measure at least 6V across the two packs in series. Finally, check that the switches aren't bad or mis-wired by testing them with a multimeter.</li>
	<li><strong>...spins clockwise in place:</strong> The left motor is running backwards. Reverse its connections and try again.</li>
	<li><strong>...spins counterclockwise in place:</strong> The right motor is running backwards. Reverse its connections and try again.</li>
	<li><strong>...runs in clockwise circles:</strong> Shine a strong light on the robot while it's running. If there is no reaction, you have a wiring problem, perhaps on the motor itself or on one of the light sensors. If there is a change, like the circles getting bigger, try bending the right "eye" stalk further outward.</li>
	<li><strong>...runs in counterclockwise circles:</strong> Same deal, but on the other side. Shine a strong light on the robot and see if it reacts. If not, look for a wiring error. If so, try bending the left "eye" stalk further outward.</li>
	<li><strong>...runs backwards:</strong> Both motors are running in reverse. Swap the connections on both motors and try again.</li>
</ul>

<h3>Going Further</h3>

<p>Using larger AA batteries and holders (instead of AAA) will result in a heavier robot (not necessarily a bad thing) with a longer battery life and more room to "backpack" components on top. This could be a good first step towards adding the relay and the other components needed for a Mousey-style "panic" reflex. See <a href="http://archive.makezine.com/02/">MAKE Volume 02</a>, Mark Tilden's book, or <a href="http://www.instructables.com/id/Mousebot-Revisited/">this project on Instructables</a> for more information.</p>

<p>One of the neat features of the Mousey design is that it uses light-emitting diodes (LEDs) wired in reverse as light sensors. Just as turning an unpowered motor generates electricity at the terminals, shining a light on an LED produces measurable currents across its leads. What's more, LEDs can be used to detect light of particular colors—for instance a blue LED wired in reverse is more responsive to blue light than to other colors. (See Forrest Mims' <em>How to Use LEDs to Detect Light</em> in <a href="http://makezine.com/volume/make-36-boards/">MAKE Vol 36</a>.) What about adding a "bull" reflex that makes your robot "charge" when it sees the color red? What other color-responsive behaviors can you imagine? </p>

<p>What about other components? Reach into your junk-box and pull something out at random. Do you understand how it works? How can you hack it to add interesting behaviors to your own BEAM robot designs? As always, we'd love to hear about your creations in the comments below!</p>