## Tuesday, January 12, 2010

### Chapter 2 - Exercise 6

Exercise 6 is fairly easy to perform, but don't ignore the reading. There's a LOT of information in these pages, including a detailed discussion on the types of switches and an introduction to schematics.

I'll admit that schematics have always confused me, but so far I'm understanding what I'm reading... we all better understand them because future experiments in the book are only found in schematic form, no full-color pretty drawings showing circuit setups... so be warned.

Now, regarding Exercise 6... I setup the circuit with the two switches starting in the same positions shown on page 44. My photos here will show you how the LED is lit... then when one of the switches is thrown, it turns off... and then when the other switch is thrown, the LED lights up again.

While I was at it, I took voltage readings that you can see in the other photos. The total voltage provided by the batteries was 6.4V. The voltage across the resistor was 4.4V and the voltage across the LED was 2.0V. This tells me that the two switches have negligible resistance and are simply letting the current pass through when in specific positions. If the voltage on the LED and resistor didn't add up to 6.4V, it would be a reasonable guess that the switches have some sort of internal resistance... but that's not the case.

Finally, although I've used my Kronus wire strippers a lot in other projects, here's a closeup of how it works. I've inserted a green 22 gauge solid core wire about 1/4" - when I grip the handles and squeeze, the two top metal pieces come together and bite down, stripping off the end of the wire... very cool. My only complaint is that sometimes the 22 gauge wire gets pulled out of the sheath... I think this is an issue with smaller gauge wire. I think I'm going to use my other plain wire strippers for future exercises that involve 22 gauge wire.

That's about it - having two switches with screw terminals was nice - I only needed to use gator clips to attach the resistor to the LED and the positive battery wire to the resistor... all other wires were screwed down tightly and didn't come apart.

Tomorrow I'll post my experiences with Exercise 7, including a question for my USA readers... the best answer will win the responder a shiny new blue Maker's Notebook. (I'll let me readers plus one or two specialists I rely on help me pick the best answer. Sorry, non-USA readers - postage on this thing is likely to be a little high.)

CLARIFICATION: The question will require use of the book, sorry. If you don't have the book yet, don't sweat it. I'm going to leave the "contest" open until January 31st and pick a winner on Feb 1, 2010. This should give those who have ordered but not yet received their book a chance to respond.

1. Jim:
Where'd you get those nifty wire strippers? I did the Google thing, but the "kronos automatic wire strippers" that came up look nothing like those nifty beasties..!

Keep up the good work! Just got my own copy and am slowly working through too.

2. Aaron,

I got them back in June/July 2009 at Radio Shack (when I was building a CNC machine)... they carry the Kronus name brand... try "kronus" instead of "kronos" and I'll bet you'll get some hits.

They are useful, but like I said, not so good with very thin wire.

3. I found that putting the wire into the strippers the other way around (to what you have in the picture) helps the wire stay in the sheath.

4. Andrew,

I'll give that a shot...

5. Also, putting a bend in the wire increases resistance of the wire getting pulled out of the insulation. It's like pulling a rope around a corner.

6. Sorry for the really long post, but I'd like to provide some detail about a documentation inconsistency I found with this exercise that might be helpful to other readers.

The version of the book that I have is the 'December 2009: First Edition' (listed under 'Printing History' on the 'Credits' page).

Page 43, 'Experiment 6: Very Simple Switching', second-to-last paragraph:
"Assemble parts as shown in Figures 2-17 and 2/18. The long lead on the LED must connect with the resistor, because that is the more positive side of the circuit."

However, Figure 2-17 (a diagram) shows the labeled 'long LED lead' connected to the switch (which is connected to the red wire) and the labeled 'short LED lead' connected to the resistor.
Figures 2-18 and 2-19 are photos and you can't really tell how the LED is oriented. In both photos, the red lead goes to the switch and the black lead goes to the resistor.

The O'Reilly site seems to acknowledge the errata, but doesn't offer a fix: http://oreilly.com/catalog/errata.csp?isbn=9780596153755

Ciochetto's report (paraphrased): Earlier in the book, the convention is established that the red wire leaving the battery pack is more positive than the blue (or black) wire. However, 2-17 shows the blue wire going to the resistor. The long LED is correctly shown going to the red lead.
Author response (paraphrased): 2-17 was modified in second printing of the book. (But he doesn't say what adjustment was made.)

Bogart's report (paraphrased): The connection of battery pack leads to components shown in diagram 2-17 (which wires go to which components) is the opposite of that shown in photos 2-18 and 2-19. (This is NOT the case in my version of the book -- in all three figures, the red lead connects to the switch, and the black to the resistor.)
Author's response (paraphrased): Diagram should be revised to match the photos. (This appears to have been done in my version of the book.)

Parsons' report (paraphrased): Fig 2-35 on page 50 (a schematic) shows positive going to the resistor, and negative going to the switch. This is "reversed from the book text." (The schematic in my book - with red going to the resistor - matches the text on page 43 - "the long lead ... must connect with the resistor, because that is the more positive side of the circuit", but neither of these match figs 2-17/18/19 - where the red (assumedly positive) wire goes to the switch.)
Author response (paraphrased): Diagram was revised for second printing. (Again, doesn't say HOW it was revised.)

All previous pictured experiments (1-45, 1-50, 1-55/56/57, 1-62/63/64, 1-71) show the red wire going to the long lead on the LED. Further, this is supported by the text on page 16: "Always follow these rules: The longer wire protruding from the LED must receive a more positive voltage than the shorter wire."

I thus believe these are the specific corrections that should be made:

>> Figure 2-17 should be revised to indicate the red lead connecting to the resistor, the blue lead connecting to the switch, and the orientation of the LED reversed (long lead connecting to the resistor, short lead to the switch. (Jim, I believe this is how your circuit is set up in the photos above.)
>> Photos 2-18 and 2-19 should be revised to show the red lead connecting to the resistor and the black wire connecting to the switch.
>> The text on page 45 saying that the long lead on the LED should connect to the resistor and the positive side of the circuit is correct and should remain as-is.
>> Schematic 2-35 on page 50, which shows the positive lead connecting to the resistor and then the LED is correct and should remain as-is.

Did I get this right, guys? Thanks --

7. References to 'fig 2-19' in my post above should instead say '2-22'. Sorry!

8. Most of my experiences with this experiment fall in line with others have already stated.

I will add one new experience I had while doing this experiment. I never fully realized before how you measure current with a multimeter. I assumed it should be just the same way you measure voltage. The author does a really good job (Experiment #4) explaining that you need to insert the multimeter into the ciruit to check the current and that a typical multimeter may only be capable of measuring up to 1A of current.

In this experiment I tried (3 times) unsuccessfully to measure the current by touching the multimeter probes to two different parts of the circuit. Each time I successfully blew the fuse in my multimeter. After the 3rd blown fuse I realized that I was effectively bypassing the resistor and trying to run a much higher current through my multimeter. I now truly appreciate the usefulness of fuses :). Instead of costing me a \$25 multimeter I only blew about \$5 in fuses. :)

Just thought I would share my experience. Everybody's got to start learning somewhere :).

Stephen