Monday, February 22, 2010

Chapter 4 - Exercise 18 Completed

I finished wiring up the breadboard for Exercise 18... I went slow and double-checked all my wires, resistors, capacitors, etc... the first video just shows the circuit after I've wired up the first 555 timer chip... you can see it counting up... 2nd video shows it counting up beyond 100. Note that in Figure 4-40 that there is an error reported by another book reader - S5 is shown connected to positive voltage but it really needs to be connected to negative - when the button is pressed, THEN you want pin 4 pulled to negative. I had checked the errata page a while back and saw this upcoming error, so I wrote in my book on this page and didn't make the mistake of wiring it up as shown. Even without the errata page, however, I think I would have caught this, as after all the readings on the 555 chip, I remembered that the resistor used (R10) was a "pull-up" resistor and was meant to keep the voltage positive on pin 4... seeing the pushbutton also connected to positive voltage would have set off an alarm (I hope).

After I verified the counting was working as desired, I finished up the wiring based on the schematic on page 178, Figure 4-41. Be careful here and make sure you have all the right values for the components listed there - especially the 330k resistor (R11) and the 68 microfarad capacitor (C2). I didn't play around with different values here.

The 3rd video shows the circuit working. I apply power and the circuit immediately starts counting. I then press S3 to stop the count and S2 to zero it. Then I press S4 but you'll notice a slight pause before the LED lights up and the count begins. It works!

As the author explains, however, the counting isn't accurate like a stopwatch. To do this, I'll need to add a trimmer resistor (see the photo). It's got a small slot screw on the side that you use to tweak its resistance value... I haven't looked up the sheet on this yet so I'm not quite sure how to implement it into the circuit, but I'll figure that out shortly... not sure if I want to spend a lot of time trying to get it to accurately count in synch with a stopwatch.

All in all, I'm VERY happy with Exercise 18 - take a look at my breadboard closeups. Two months ago, I had NEVER used a breadboard correctly. Really didn't understand how one works. And I most certainly would never have put together something as complicated as this circuit... let alone understand how it works. But I do now! I didn't just look at the schematic on page 178 and plug in all the wires and hope it worked. I really understood how it worked - I understand how the LEDs work, how the 4026 chips work, and how the 555s work... I know what the resistors and capacitors are doing... how the pushbuttons are doing their thing, and, most of all, I really do understand how the reflex tester works as a whole.

I hope all of you who own the book and are working through it are having the same successes (and failures - learning from them, that is) that I am... and I'm only halfway through the book.

I'm still enjoying this process... and can't wait to see what's next.


  1. I had a trouble with the start switch at first. I used my logic probe to test both the output pin on IC7 and the reset pin on IC6. I confirmed that the output on IC7 was going high for about 4 seconds, then going back low again. The reset pin on IC6 was high with a quick pulse low (showed up as a "pulse" on the logic probe, not a transition from high to low). This is exactly the behavior expected, but it still wasn't starting the timer back up. Turns out there's a typo in Figured 4-41 on page 178 (already in the errata... gotta learn to check there sooner). There are two resistors labeled R10 that the caption says should be 1K. Both of the R10 resistors should actually be 10K.

    I think the 1K resistors were letting capacitor C4 charge back up too quickly after the positive pulse from IC7. This made the drop in voltage on pin 4 of IC6 too quick for it to register. With the 10K resistors, C4 will take a little longer to charge, dropping the voltage on pin 4 of IC6 for a little bit longer. I think that tiny bit of extra time is what let the reset start working.

  2. .on chapter 18 page 180 under Enhancements, bulletpoint 1 says that with a 555timer you can power in and have the on ready mode, tried connecting the Timmer on everyway the book show, using output 3 to pin 15 of IC1 but to send a negative pulse to pin 2 IC6 I connected to pin 6,..didnt work. Could you show me a schematic for this connection, thank you