Okay, so I took some time away from this experiment, hoping that I'd be able to come back, find my mistakes, and get this circuit working. Well... no luck. But I'd like to try and explain a few things I did differently this time and a few things I was able to verify about the circuit - maybe this will help someone else trying to get this crazy thing working.

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.