Monday, January 18, 2010

Chapter 2 - Exercise 10 Part 1

This is Part 1 of 2 for Exercise 10.

Exercise 10 is all about the wonderful little transistor - a simple looking little component that does some amazing things. What kinds of things? Read the text! Just kidding... but there is a LOT of information in Exercise 10, so I've decided to split this one into two parts.

The first circuit I breadboarded is the one found on page 73. I didn't have all of the resistors this called for so I used my notebook to map out the replacements I would use... Instead of the 180ohm, I substituted a 220ohm... I had the 10k, but not the single 680ohm... so I substituted two 470 ohm resistors in series. My hope was that I wasn't adding too much resistance to keep the LED from lighting... and fortunately the resistors I used were fine.

I've included a couple of closeups of the breadboarded circuit - you might see an error I made... the LED at one point wasn't inserted into the proper row as the 2nd 470 ohm resistor (look closely)... a simple fix and the LED lit up when the button was pressed.

Next, I removed R2 and the pushbutton and inserted two short wires with stripped ends. Careful to not use both hands to touch the wires, I used my fingertip and made a simple connection between the two exposed wires... and the LED lit up. Very cool to realize my finger was providing the bridge to let the circuit close and the LED activate.

Be sure to read over the NPN and PNP writeup starting on page 76... read it twice... I did. It's got a LOT of info squeezed into just a few pages.

Tomorrow, Part 2 for Exercise 10.


  1. This circuit didn't work for me at first, and I finally looked up the data sheet on the transistors I bought (P2N2222AG), and it turns out their pins are actually backwards from the way the book shows them. (I think it's because I bought weird transistors, not because of a mistake in the book). Anyway, something to watch out for. Fortunately, I don't think I hurt the transistor, because it worked once I flipped it around.

  2. oops I had a x2220 laying around and I couldn't figure out why the light would always stay on, So I googled for the data sheet and found out it was a PNP not a NPN. Wrong sandwich I guess. lol

    I ended up using a 2n3904 and it did the trick!

  3. I also had the same transistor as Chris and had the same problems. Not sure if it is an errata in the book, or if this particular make of the 2n2222 family is just put together different. As it is the only plastic body 2n2222 variant that Mouser stocks there was nothing to compare it to.

    Oh well, makes for good troubleshooting

    Wishing you all solid connections,

  4. I had the same problem with my 2N2222 facing the opposite direction as suggested in the book. Flipping it around fixed it. In fact, I tried two separate 2N2222 transistors from two separate sources, and both of them were opposite from the arrangement listed in the book.