Errata for the book Robot Building for Beginners, First Edition

See the other page for resource files (for both editions) and second edition errata. As a courtesy to all of my loyal first edition buyers, here is the original errata.

Chapter 4: Page 53

The Marlin P Jones parts, 7161-MI and 7163-MI are NOT hook adaptors for test probes. They are simply hooks without wires attached to them. Purchase the RadioShack part instead.

Chapter 12: Page 175 (purposely retained in future printings)

“When designing a robot circuit, mis18takes will be made.“

Yeah, and when laying out a book, mis18takes will also be made. This didn’t appear in the original text or the first layout. So, the error occurred after I submitted my layout corrections.

It almost looks like an intentional joke by me, but it wasn’t. Perhaps it was a joke by someone who prepared the final layout. Actually, I think it’s kind of funny.

Chapter 13: Pages 199 and 208 (corrected in third printing)

This chapter incorrectly refers to 840 tie-point solderless breadboards as “820” tie-point solderless breadboards.

Chapter 14: Page 218, Table 14-1 (corrected in second printing)

The Jameco Electronics assorted trimpots, part number 18075, are not appropriate for and do not match the trimpots used for the experiments in the book. Sorry, I must have misunderstood Jameco’s description or hadn’t seen a picture of their assortment. Instead, purchase the assortments from one of the other suppliers listed.

If you’d like to pick up just a couple, instead of a grab bag, here are photos and part numbers of 20 kilohm trimpots for R2 and R10 in Sandwich.

Chapter 14: Page 230-232, Brightness-Sensing Pairs

Don’t strain yourself matching photoresistors. It doesn’t make an enormous difference.

Measurements of photoresistors will vary considerably based on ambient (room) lighting and temperature. But, even weirder is that the value may not settle in one place, even if you wait a while. Don’t sweat it.

Your primary goal in testing the photoresistors is to knock out any defective ones. These will not change value or will have really low or really high values -- especially compared to the rest of the lot.

Your secondary goal is to roughly match the photoresistors. Stick one up to a light bulb, count to 5, record the value. Stick it under a dark desk, count to 5, record the value. Good enough.

Because of the balancing potentiometer (R1) and the striking contrast of the line on the floor, the photoresistors don’t have to be matched exactly. Truth be told, the four photoresistors for Sandwich were picked at random from a previously defective-removed lot. And Sandwich performs beautifully. The subsequent Sandwich brothers had more carefully matched values, but show no differences in performance.

Chapter 14: Page 232-236, Higher Resistance Photoresistors

This is probably the most valuable tip since the book was written.

As you can see in Figures 14-16 and 14-18, resistors R1 and R2 make up the majority of the voltage. This is vital for proper operation of the comparator chip (introduced in later chapters), since the chip is unable to perform comparisons on voltages in the upper 1.5 V of the power supply (9 V - 1.5 V = voltages at or above 7.5 V).

The photoresistors I bought and used when making my Sandwich robots happened to have very low resistances (100 ohms in the example) when lit and moderate resistances when dark (6,000 ohms in the example). But, what if your particular photoresistors have higher resistances than mine during normal operations?

Higher resistance photoresistors (say 30,000 ohms) divide up a larger portion of the voltage than R1 and R2, reaching the point that the voltage at TP1 and TP2 exceeds 7.5 V when looking at a dark floor. In that case, the comparator chip will fail to drive the motors and LEDs correctly, and the robot will not follow lines. The usual symptom is that both LEDs turn on at the same time or turn off at the same time.

When testing a sample board from the latest batch of Sandwich PCBs, I just happened to use higher resistance photoresistors and I encountered this very problem. For a brief moment, panic struck me as I thought my recent improvements to the PCBs had introduced an error that resulted in 1,000 bad boards! Fortunately, the boards were perfect, and it was only the resistance of the photoresistors causing heartburn for the comparator chip and me.

A simple test can verify if you have this problem. Just measure the voltage at TP1 and TP2 when the robot is powered on and looking at a dark surface. If either voltage is above 6 V, try turning up (R10) the brightness of the headlights (LED9 and LED10).

If the voltage is still above 6 V even after adjusting the brightness of the LEDs, you have two choices:

1. Replace R1 with a higher resistance. Try a 10,000 ohm or 100,000 ohm resistor for R1. This is inexpensive but requires desoldering the old resistor.

2. Switch IC1 from a LM393 comparator chip to a LMC6772BIN chip ($3 at DigiKey). The newer chip doesn’t have the same limitation on top-end voltage measurement.

Chapter 16: Page 268, Table 16-2 (corrected in third printing)

The instructions are confusing. This could be made clearer by eliminating the final sentence of step 2, eliminating step 3 completely, and eliminating the first sentence that follows step 3. A new sentence at the end of step two would simply read “Continue until you’ve tested all lead combinations as shown in the first column of Table 16-2.”

Table 16-2 column 1 should then read:

1 red & 2 black
1 red & 3 black
2 red & 1 black
2 red & 3 black
3 red & 1 black
3 red & 2 black

The third sentence of the paragraph that follows the table should end with “2 & 1”, not “2 & 2”. What a mess!

Chapter 17: Page 299, Table 17-4 (corrected in second printing)

Fifth line down should have the Known Unit as “kgf-cm” not “kgf-m”.

Chapter 17: Page 315, Absolute Gear Ratio

I simplified the gear ratio example to make it easy to understand, but I should have followed up with an explanation as to the actual number of teeth on a gearhead. I suppose it is possible to have 1535 actual teeth and 65 actual teeth, but that’s usually not the case.

Gears on the Hsiang Neng motor, with teeth counts

Gears on the Hsiang Neng motor, with teeth counts

The Hsiang Neng motor used on Sandwich has a gearhead containing the gears pictured above. The gear attached to the motor shaft has 12 teeth and it meshes with a larger gear that has 28 teeth. That is attached to a gear that has 10 teeth, which meshes with a larger gear that has 28 teeth. That is attached to a gear that has 10 teeth, which meshes with the final output gear (right side of picture) that has 40 teeth.

The gear ratios of each stage are therefore:

28/12 ×
28/10 ×
40/10 =

The absolute gear ratio would be described as 31360:1200 even though there really aren’t that many actual teeth. That fraction could be reduced to 392:15.

The simplified gear ratio can be calculated as follows:

31360 / 1200 = 26.13333

The simplified gear ratio would be described as 26:1.

By the way, the datasheet claims the gearhead is 30:1, which (according to my calculations) is not really the case.

Chapter 18: Page 329, Connecting the Diode in the Proper Orientation (corrected in third printing)

“The cathode end of a diode (the end with the band) must be connected to the emitter (output) of the transistor.”

Oops. “emitter” is the wrong word. (Thanks Dustin) The output of the PNP transistor is the collector. The diode should be connected to the collector, not the emitter. Figure 18-4, the schematic, is correct. Figure 18-6, the photo of the breadboard, is correct.

Chapter 20: Page 372, Drilling the Hole for the Setscrew (corrected in second printing)

In the book, a 5/64 drill bit is suggested for the setscrew hole. That bit diameter is actually a bit small for a 4-40 tap. A No.43 drill bit is the officially recommended size drill bit, and should be used for longer holes or more rigid materials. Otherwise, you'll seriously struggle to tap the hole and risk breaking the tap.

However, in a pinch you can continue to use the 5/64 bit for the relatively thin couplers.

Chapter 22: Page 404, Table 22-2 (corrected in third printing)

The third item down should read:

Mouser 538-22-23-2041 $0.44 Four-pin Straight Friction-Lock Header

The part number was wrong.

Chapter 23: Page 422, Figure 23-1 (corrected in second printing)

SW2 is incorrectly labeled “Dual-Pole Dual-Toggle”. It should be labeled “Double-Pole Double-Throw”. I don’t know where I picked up that incorrect definition for DPDT, but I can’t seem to shake it. My technical editor, Tom Gavin, found and corrected all the other references, but we missed this one because it was within an image instead of text.

Chapter 23: Page 436, Painting Photoresistors

Here’s a new tip. Instead of using black paint with a paintbrush, try a black paint marker instead. They’re easier to guide, which reduces the chances of accidental brush strokes on the face of the sensor. You can find paint markers at local hardware stores or McMaster-Carr (part #16625T25, 3 mm black paint marker, $3.09).

Chapter 24: Page 469, Installing the 9 V Battery Holder (corrected in third printing)

The RadioShack battery clip should be part number #270-326, not #27-326. Alternatives: Mouser 534-080 or 534-095.

See the other page for resource files (for both editions) and second edition errata.