Tuesday, February 16, 2010

Chapter 4 - Exercise 17

For Exercise 16 I built the circuit towards the bottom of the breadboard... if I had read ahead, I would have seen that Exercise 17 was built towards the bottom. Oh, well... in my pictures, the LED circuit (Ex16) is at the bottom and the speaker circuit (Ex17) is at the top.

For the exercise, I (once again) lacked a .047 microfarad capacitor. But luckily, this circuit is all about flexibility. R1 and C1 are the controlling components that will allow us to modify the sound emitted by the speaker... so, on page 166 I noticed that the author included a few capacitors in there that I did have - namely 4.7 microfarad and 47 microfarad. The chart indicates that I should get some sort of sound from the speaker... so let's see.

First, I installed the circuit in Figure 4-21 using a value of 4.7 microfarads for C1. All other components were as listed. Because I'd already wired up C3 in Exercise 16, there was no need to do that again. Remember, C3 is for smoothing the voltage provided by the 9V power supply and the author recommends always including this for integrated circuits.

The first video below will show you that my speaker is emitting a low but fast sound... and the LED circuit is still working.

Next, I removed the 4.7 microfarad (C1) and replaced it with a 47 microfarad. The second video shows the results of that exercise. As you can hear, the sound is much slower and lower... almost like a fast ticking of a grandfather clock. (That loud clicking sound you often hear in my videos is a foot pedal power switch I have installed... I leave the foot pedal plugged into the wall and plug in my adapter to the pedal... I press the pedal with my foot to turn on and off the power.)

Finally, I shorted the power (pin 8) for the speaker circuit to the output (pin 3) of the LED circuit using the long red wire seen in the photograph. I turn on the power (with my foot pedal, but not heard in the 3rd video) and the speaker will not emit any sound until I press the button on the LED circuit... notice also that the reset (pin 4) button immediately kills the sound coming from the speaker.

There is a LOT of information in Exercise 17... I plan on re-reading it once or twice... I may even go back and read Exercise 16 and 17 together... I think we all need a solid understanding of what's going on here - especially the descriptions provided on pages 158 and 164 where the timer chip is explained with a little more detail. What's working for me now is I'm totally understanding how all the components in these circuits are working together to give me the expected results... I understand WHY that capacitor is where it is... WHY that resistor is necessary... WHY that second capacitor is there and WHAT it's used for...

Also, don't skip the last sections on chaining chips together... I had to read it a few times to sink in and I still need some time to process it all. I think knowing that it's hard to damage the 555 timer once it's installed in the circuit makes it easier to play around with the various components. Be sure to work through some of the modifications mentioned on pages 168-169.

Finally, if anyone tackles Figure 4-29, please let me know. I may try it this weekend when I have some extra time, but I really don't want to slow down on the book and this one looks tricky to implement. I have 10 of the 555 chips, so I've got enough to give it a shot, but if anyone has tried it or plans to do so, let me know.


  1. I've been looking at Figure 4-29 (page 168) and I cannot for the life of me understand the pin layout of the schematic that is shown.

    I know it is not a regular experiment but why was it put in there if we are not able to try it ourselves?

  2. I don't have my book with me, Lew, but if you give it a shot and get it to work, shoot a few pics and maybe a video and let me know... I'll be more than happy to update this post and share your results.


  3. I'd love to give the circuit in 4-29 a go, too, if someone figures out and posts the pin layout, resistor values and capacitor values. I'm hanging in with what the different components DO, but as far as figuring out component values ... that's probably a way off for me.

  4. This circuit is actually the one shown pictured on the front of the book. You can therefore reference the front cover, to get an idea of how to lay out the circuit.

    Note, however, that there is a mistake in the schematic as shown in Figure 4-29. There should be a .1 capacitor wired in series between pin 3 and pin 2 of the next 555, along with a 1k resistor between positive and each 555's pin 2. These capacitors and resistors are present in the photo on the front cover.

    As far as the capacitor and resistor values to choose, reference the table on page 157. Simply decide how long you want each LED to be on for, and then choose a resistor and capacitor that matches up to the desired time in the table. I chose a 100k resistor and 4.7 electrolytic capacitor to get a 0.52 second on time.


  5. Thank you! Will have to give it a whirl now!