First, I totally rewired the entire thing. I found two errors in my wiring on the first try. This time around, I wired up each chip, double and triple-checked the wiring before moving on to the next chip. This worked well, and I was able to get the * button working and lighting up the LED.
After completing the wiring, the # sign would work and light up the LED... but this time I couldn't get the * button to light up - I took a voltage reading - I was only reading 1.0 volt across the LED - I tried different LEDs but no luck. I also tested my LEDs as the # button LED and they worked, so they obviously weren't burned out. My only thought is that they're simply not getting enough voltage. I think I'm using low voltage LEDs but I may be wrong. Either way, why would it initially light up but now now? And it doesn't matter anyway - even if the LED doesn't light up, pressing the * sends current to pin 8 on the 7408 chip, turning it on. The LED is just a visual cue.
Okay, so scratch that... next thing I wanted to figure out what whether the 555 chip was sending any voltage to the LED when the * button is pressed... remember, you have to hold down the * button while entering the code... when I pressed the * button I got a 0.47 volts reading on my multimeter. But isn't pin 3 only supposed to send a pulse if voltage is dropped on pin 2? If I'm detecting voltage on pin 3, this would mean that the 7408 is sending a 1 to pin 3 on the 7404 (a NOT operation) which in turn sends a 0 (no voltage) to pin 2 on the 555 chip. Which means I'm entering the code correctly... so why isn't the LED lighting up? One of the pics shows me using my multimeter and getting a voltage of 0 on pin 3... I didn't have enough hands to hold down *, enter code, snap picture, and hold the probes... argh... so you'll just have to trust me that I got the .47 volts on pin 3...
Oh, well... I gave it a second try, but I think it's time to move on to Exercise 21.
Oh, and there's an unlabeled resistor in Figure 4-84 (I'll submit to errata if it hasn't already been done) - I think it's supposed to be a 10K to match the 10K resistors for the 1 keypad button and the 4 keypad button... I may be wrong.
So, this concludes my attempt at Exercise 20. When I'm done with the book, I may come back and give it one more try... because I hate to leave an exercise with doubts. Is it working? I think so... but I can't get an LED to verify... but I do get positive voltage on pin 3 when I hold down * and press 1, 4, then 7.
So... this leads me to my next contest. I'd like to reward a Maker's Notebook to the first reader who can upload a video showing this circuit working in all its glory... it doesn't have to be mounted to a computer, but it should show all the LEDs working as desired - it should show an improper code being entered as well as the correct code and all the LEDs lighting up as they should - especially the * sign as you hold it down and the UNLOCK LED when the code is entered. First person to post a video that clearly demonstrates the circuit working gets the Maker's Notebook. (Sorry, Charles - as the author, you're prohibited from entering the contest...
I've got a few videos below to finish up and then it's on to Exercise 21... thanks for sticking with me folks - got a bit behind in "real world" work and had some family business that had to be attended to... but I'm caught up and ready to get back on track.