Nuts and Volts - December 2016

Clock Talk

Bill van Dijk’s September 2016 article, "Build the Numitron, A Six-Digit Clock," is just plain fabulous! I really enjoyed it, and what's more, I have learned something new, and that is important to me. I now have several questions for Bill.

For soldering surface-mount components, you mention that your fine point tip for your soldering iron is almost needle sharp. It looks like our soldering stations might be the same. I have a digital Weller WESD51 powering a PES51 iron. I bought the model "ETH" tip, but have always thought I needed a finer point for my SMD exploits. Have you found a finer tip to fit onto your iron other than the Weller ETH?

When I built a PIC16F877 based clock a few years ago, I used a single 4 MHz crystal. The interrupt routine and Timer1 perform timekeeping using that frequency. In my climate controlled house, I routinely see accuracy better than 10 seconds per year. In your design, you added a second crystal with capacitors to do the timekeeping at a slower frequency. Why? Is there an advantage in using the additional parts?

I really admire your use of a hardware solution for debouncing your pushbutton switches. The link you included for the debouncing discussion ( www.eng.utah.edu/~cs5780/debouncing .pdf) is fantastic. I have never seen a more thorough handling of the topic.

However, I believe that author does make one mistake: As voltage climbs, the high-going transition point for a Schmitt trigger is not 0.9 volts, so I think some of his results are in error. It seems that he has mixed up the two transition point voltages.

After reading that in-depth debouncing discussion and then looking again at your schematic, I became totally befuddled. You do not seem to follow that author's logic in your circuit. Your

15K discharge resistors (R19 and R20) are very similar in value to those employed in the discussion (R2 is 18K), so at first glance it appeared you were on the same track. However, your debounce capacitors are specified as 0.1 µF instead of 1.0 µF, which would certainly be a big change in the math equations. Plus, your charge resistors (R17 and R18) are only 1K compared to the discussion's R1 of 101K. Coupled with the diodes D3 and D4 in your debounce circuit, that is a very fast charge time indeed! Could you tell me how you decided to use the 1K resistor here?

I used the formulas in the discussion to do the math for your debouncing circuit. The port C pins you use on your PIC are Schmitt triggers, so the threshold voltages are one volt low-going and four volts high-going. With your components, that will take care of bouncing contacts when a button is pressed for 2. 4 milliseconds. In the discussion, it was decided that a good target would be 10 to 20 milliseconds. I bet you have good quality pushbutton switches in your clock, and with reduced bouncing in quality contacts I imagine your functionality does not suffer. If another builder puts this circuit together with old or inferior switches, unfortunately, there could be contact bounce problems. I am more interested in the button release, however. I did the math, keeping in mind the bypass diode. When you release your pushbutton, that 1K resistor with the 0.1 µF cap will only suppress bouncing for 0.15 milliseconds. I understand that released contacts do have less bounce than the press-down, but some bounce hash is present in READER FEEDBACK