## Thursday, February 11, 2010

### Chapter 4 - Exercise 16

Chapter 4! Finally getting into integrated circuits which have always interested me but I've never quite found a good explanation for how to use them or how they work... until now. That said, I did have to read over this first material about 3 times before it really sunk in. But it's now "sunk."

This exercise is all about the 555 Timer chip. My initial thoughts before reading this chapter was that this was a timer that would keep time... right? Like a stopwatch. Wrong... sort of. I guess it can be used that way in certain circuits, but it's really about holding a pulse (current, I think) for a certain amount of time... an interval that we have control of, by the way. By changing the values of different components, we can manipulate a pulse emitted (allowed?) by the chip to last a specified time (with slight variations).

My photos here show the circuit I built using the schematic on page 155 (Figure 4-15). I tried my hand at building the circuit by only reading the circuit and not looking at Figure 4-16 for help. I got it right the very first time... I was happy about that.

A few differences, though - I lacked a 100 microfarad (C3) capacitor so I substituted a 220 microfarad... since this is for voltage smoothing, I was guessing that as long as it was a minimum of 100mf that I'd be okay... wasn't 100% certain, but willing to experiment. The rest of the components I had in my kit... be sure to note that the voltage has been switched on the breadboard to 9V and the sides are changed - positive voltage on right side, negative voltage on left side. I also had to go back and refresh my memory about the symbol for an LED because I couldn't remember if the long wire was where the arrow was pointing to or away from ("away" is the answer).

Also, be VERY careful with the orientation of your electrolytic capacitors and make certain their positive and negative terminals are inserted into the breadboard properly. Luckily I checked over my components before applying power and discovered I'd reversed C4. I also soldered two lead wires to my potentiometer to make it easier to insert into the breadboard and avoid using patch wires.

Below are three videos - for the one with R4 equal to 100K, the LED stays lit for about 5-6 seconds... difficult to measure it accurately at this point. I next more than doubled R4 to 220K and, as expected, the LED stays lit for well over 10 seconds. Finally, I cut R4 in half and substituted a 51K resistor and the LED stays lit for around 3 seconds. I wanted to mess around with different capacitor values, but my capacitor selection is limited and they are also not as easily doubled and halved in values like resistors.

It does bring up a question which I don't think I've seen covered in the book yet - can you add capacitors like resistors? If I add two 47mf capacitors in series, will it behave as a 94mf capacitor (close to 100mf)? I may play around with this but am not certain if I'll be risking my components... just not sure if this is safe or not. And I don't have enough capacitors to play around with right now, so I'll likely pick up a mixed bag some time this weekend.

I enjoyed this chapter - I'll likely go back and read over this material one more time before starting Exercise 17... not all the pins on the 555 chip have been covered in detail so I'm still fuzzy on some of their uses, but I definitely understand how pins 2, 3, and 4 work and I'm getting better at pins 6 and 7...

1. Regarding the capacitors in series/parallel, you should be aware that they work in the opposite fashion to resistors in series/parallel. In other words, combine them in parallel to increase their capacitance, in series to reduce it!

http://www.kpsec.freeuk.com/capacit.htm#series

Has all the appropriate maths for you!

Really enjoying following along with this. Keep it up!

Ian.

2. Ian,

Right after I read your comment, I remembered my physics course where this was mentioned - I knew there was some sort of relationship with resistors and capacitors - just couldn't remember that they were opposites when treated in series and parallel. Thanks for the reminder and the link.

3. The easiest way to think about it is to imagine two capacitor symbols in parallel, so close that they are touching...to make a bigger capacitor! And indeed, if you were using parallel flat metal plates to create the capacitance, widening the plates would increase the capacitance.

4. Regarding the math for series vs. parallel:

- One thing to remember resistors in parallel always have a resistance lower than lowest of the resistors. Why? Because there are multiple paths for the current!

Using a water analogy where there are now multiple pipes for the water to flow through might help solidify this concept.

The math for capacitors works the opposite way.

The math for inductors is the same as with resistors.

There is a simplified formula for two resistors in parallel and a generalized formula for multiple resistors in parallel. For me it was worth only memorizing the generalized formula since I could use it in either case.

Rtotal = 1 / (1/r1 + 1/r2 + 1/r3 ... )

For what it is worth these formulas are on the US Amateur (Ham) Radio exams. After reading through Make: Electronics, you should have a good leg up on being prepared for the radio exams. The exams are much easier than they used to be since there is no longer a morse code test. Prepping for the exams includes electronics and radio theory, so it can be very educational. A large part of my motivation for doing it was learning. A few months ago I was looking deeper into wifi products (antennas, routers, G,N, etc.) and realized how little I knew about radio.

I recommend reading through the ARRL license manuals (available at amazon for under \$20 each) for the learning experience.

5. Regarding the 555:

There is an article that describes using the LTspice circuit simulation software to simulate a 555 based circuit and view the output waveforms (the changes in voltages over time)

http://www.arrl.org/tis/info/pdf/1003059.pdf

If nothing else, you might want to look at Figure 2 on the second page to see the relationships between the signals. The green shows the nice square output pulses from the 555. It's a bit hard to see, but there are white space on the top & bottom axis, it's not just green bars, it's a square wave.

The magenta (an unfortunate color choice) shows the sawtooth pattern of the capacitor charging and discharging.

Note: this circuit is a bit different than the 555 overview in Make: Electronics.

The article is by Ward Silver. It's part of a series called "Hands On Radio" that appears in QST magazine. The articles are available on-line. The simulating articles begin with experiment #83 from December.

I know that simulation is the antithesis of the books "learn by doing & blowing things up" approach, still, I thought it would be interesting to see.

Hope This Helps,
--Rob

6. Regarding potentiometers:

There are potentiometers that you can put directly on the breadboard which might make some circuits easier. I wouldn't use them for all of the things in the book (I don't remember what the wattage rating on the ones specified in the book are) but for circuits where there isn't current over 1/2 watt look at these:

http://www.elexp.com/cmp_sts.htm
http://www.elexp.com/cmp_stx.htm

These are made by Cermet. You can probably get them from digikey, mouser, etc. Electronics Express has some good deals on tools and kits. I've picked up a bunch of stuff from them.

Hope This Helps,
--Rob