Wednesday, January 13, 2010

Chapter 2 - Exercise 7 Part 1

I'm breaking Exercise 7 into 3 parts - Part 1 will cover preparing your AC Adapter, Part 2 will cover taking apart a relay, and Part 3 will cover the circuit you'll put together.

For this part, follow the instructions carefully on page 56 - cut off the end of the adapter and strip the two ends (one shorter than the other) and twist the copper strands together.

I set my adapter to 6V but feel free to set yours in the 3V-12V range. Set your multimeter for voltage and touch touch the red probe to one wire end and the black probe to the other. If the voltage reading has a negative sign (-) then reverse the probes until you see a positive voltage. Make note of which wire the red probe is touching and use red tape or a red Sharpie to mark that wire so you'll know it's the positive wire.


  1. My son and I were finally able to dive back into the book's projects after recovering from summer travel and getting settled back into school. However, we've run into a stumbling block with the power adapter that came with our Maker Shed kit:

    1) We cut off the adapter tip and stripped the wires, attached the multimeter, put it on the 12-volt setting and plugged it in. Got ~10V. Put it in a lower position. Stayed at 10V. All the way down to the 3V position - we're still getting 10V. Very confusing.

    2) Unplugged the adapter. STILL getting 10V, though it's dropping slowly.

    Took a little head-scratching, but here's what we came up with:

    1) There's a switch on the BACK of the adapter - a black switch with black text on the black wall-wart, which I just didn't see, but which switches from 110 to 220V. Guess what position it was in? 220, of course.

    2) Per a little online research (including your Jan 13 2010 post), it seems there's a capacitor in the adapter that's holding a charge. That's why it wasn't dropping to lower readings immediately upon flipping the voltage switch, as we'd expected.

    So, after we watched a couple YouTube videos on discharging capacitors, we touched the wires to a light bulb and pulled the charge down to near-zero (the video indicated there'd be a little residual charge, e.g., 1/10V, which we did observe). Feeling pretty clever, we set the AC voltage switch to 110V and the DC switch to 3V and plugged it in and got ... 7.5V?!? Tried it in the 12V position and got 21V.

    At this point, I'm thinking we probably permanently goofed up our AC-DC adapter by having it in the 220V position when we initially plugged it in. We opted not to risk any of our components with further experimentation tonight prior to a trip to Radio Shack tomorrow to buy a replacement adapter.

    I think it a pretty significant oversight in the book not to point out that AC-DC adapters can have capacitors and store charges. I don't know how much of a shock touching the wires of the unplugged adapter would cause, but it makes me dicey working on this with my son. Also, I figure you could easily burn up components by plugging the adapter in while the switch is in a higher position and then moving it down to a lower position for a project, thinking the voltage would change immediately. The kit included no documentation for the power adapter, so I don't know how I was supposed to know this (what with being a guy reading a beginning electronics book and all).

  2. Thanks for the great writeup on your issue with the adapter. I agree that working with the adapter could be a little risky if you're dealing with exposed wires, but I've had zero issue with accidental touching... instead, I make sure to keep the exposed ends to a minimum size (about 1/4" inch exposed)... if you've cut more, consider using shrink wrap or electrical tape to reduce the exposed area. But it really does sound like you need to invest in another adapter... the one I purchased form Radio Shack is still working great.

    And now that things have returned to normal (after my 2nd son's birth), I'm also trying to arrange things to get back to this book and finish the remaining few exercises.

  3. New AC-DC adapter from Radio Shack solved all -- gives consistent voltage, switches voltage up/down immediately upon resetting the switch, doesn't hold a charge after being unplugged. Best of all, it doesn't have a 110/220 switch to get into the wrong position!

  4. Two things.

    First Radio Shack has a multi-voltage AC-DC adapter by the name brand of Enercell that accepts different tips. You can buy a tip to plug in the adapter that has two tinned wires so you do not have to cut the end off your adapter. That is what I used for this experiment and it works like a charm. One note, though, the tip can be plugged in reversed thus reversing your polarity, always check which wire is carrying positive and negative voltage each time you use it.

    Secondly, I highly recommend the jumper wire set you can get at RadioShack. It consists of a jumper wire with an alligator clip on each end, you can get a set of ten of them for around $7.00us and they make wiring this circuit much easier, although you will need a bit of space to allow for the spaghetti

    Wishing you all solid connections,

  5. I'm an unfortunate second to Thrall's issue with the 110/220V switch. I took the switching adapter straight out of the Make Components kit and didn't notice the switch or that it was set on 220V. EVen after discharging the capacitor, the voltage settings read about double what they are labeled as, and the unit does not discharge properly.