Nuts and Volts - November 2010

QUESTIONS & ANSWERS

Mailbag Continued ...

detect a complete cycle from push to release could be used before entering the code to be acted upon.

Contact bounce, of course, brings in its own little problems that can be solved either with hardware or in software. For those not familiar with contact bounce, it is the mechanical “bounce” of the metal contacts as they first hit until they settle down in a continuous closed contact. Computers are fast enough to see this as several closing and opening cycles of the switch. If not compensated for, software will be all over the place when one intention is a single event. In software, one would detect the first contact closure, wait a short time for bounce to stop, then if desired, wait for the first nonclosure, and again a short debounce delay, and then continue on with code for a momentary pushed button as desired in this project. Other projects may need to operate only as long as the button is closed. Hope this helps someone out there.

Sorry if this got a little long, but there are issues here in real time code that programmers should be aware of if they want their systems to behave as they intended and to interact with humans in a user friendly manner.

— Evan Wasserman

Thanks very much for taking the time to analyze the code. Looking at it now, I don’t understand why it worked (just as I don’t understand why it doesn’t work sometimes). Your suggestion for the HOLD loop is a good one. I should have thought of that. I am aware of the contact bounce problem but figured that the PAUSE 1000 would take care of it. The code could have been better, as you pointed out, but I am still learning. The program that I use (Microcode Studio) automatically puts reserved words in bold.

Dear Russell: Re: Irrigation Timer August ‘ 10, Page 22.

I always read your column with great interest. I was wondering whether the circuit needs to be protected from indirect surges. My sprinkler timer (the Champion PR-90) has protection (a MOV) at each terminal block. A ribbon cable runs from each terminal block to the PCB in the controller (a surge will hit the MOV before it hits the solid-state components). There could also be some additional protection on the PCB. Also, the PR-90 sprinkler timer allows the times to be digitally adjusted from 10% to 300% to change watering times depending upon the temperature, humidity, etc. The PR-90 sprinkler timer allows each zone time to be adjusted from 10 seconds to one minute in five second increments; one minute to one hour in one minute increments; and one hour to 12 hours in 10 minute increments which seems adequate to fulfill the needs for the drip irrigation system without any additional circuitry.