We have so much to do at home and on the bus that we might have skipped the SLAARC (ham radio club) breakfast in South Lyon this morning, but I had agreed to meet Chuck at his shop (bus garage) at 10 AM in Novi and to bring Mike (W8XH) along with his oscilloscope to look at the tachometer signal, or lack thereof.  We had a nice chat with our ham radio friends, discussed having dinner in a week or so with Bruce and Linda, and then headed to Chuck’s shop.

We had two different opinions as to what signal we might find, if any, at the end of the wires that connect to Chuck’s tachometer.  Matt, from Bob’s Speedometer, told me that the signal to both the VDO tachometer and speedometer were variable frequency square waves at 3 to 5 volts peak and that the electronics in the gauge moved the needle in proportion to the frequency.  Mike (W8XH) had talked to Jim (N8KUE), who works in the research lab at Ford Motor Company, and Jim was of the opinion that the input to these gauges was a pulse width modulated signal.  With pulse-width modulation the frequency and amplitude of the waveform are constant but the width of the “pulse” (the “on time” of non-zero voltage) varies from zero to some maximum percentage of the half cycle, up to 100%.  If it is on for the entire half cycle it becomes a square wave.  The longer the pulse (on time percentage) the more energy is transmitted.  The gauge electronics can convert that to a needle position or run a motor faster or slower, such as might drive an odometer.

So which was it?  Well…neither.  What we saw was an alternating current signal that appeared to simply be an impulse (sudden spike in the voltage), one positive and one negative per cycle, with the frequency responding in direct proportion to the engine RPM.  The impulse had a rapid but noticeable decay time that appeared to me to exponential, but we did not have the wires connected to a load and that may have affected the signal. The voltage we were seeing appeared to be in 300 mV range, a far cry from the 3 – 5 volts we expected.

We loaded the cardboard in my car before going to breakfast, so when we were done at Chuck’s we headed directly to Recycle Livingston.  From there we went to pet Supplies Plus for some cat litter and then to Lowe’s for four more sheets of drywall (Sheetrock) and a large tub of better drywall compound.  After fighting with the back wall of the garage recently and having trouble with using the patching and repair compound yesterday, I wanted a drywall compound that would go on easier and smoother.  It could just be my technique, of course; I wasn’t that good at dry-walling 32 years ago, and feel like I have lost what little technique I once had.

Back home we unloaded everything, changed into our work clothes, and had lunch; grilled “cheese” sandwiches with tomatoes and dark leafy greens and fresh peaches, ripened to perfection.

While Linda sanded the drywall compound I applied yesterday I removed the panel from the library side of the opening for the old window A-C unit.  I insulated the cavity, cut and installed a new piece of drywall, and re-taped the seams.  I helped Linda finish the sanding, wiped off the dust with a wrung out sponge, and then applied another coat of drywall compound.  I then applied a first cost of “mud,” as drywall compound is commonly called, to the filler panel in the library.

In preparation for dry-walling the new utility closet we had to do some carpentry to box around the flue and gas pipe where they pass through the west wall.  We also had to box around the supply air duct where it passes above the utility closet door.  Finally, we added some backer boards along the edge of the platform by the west wall.  The purpose of all of this carpentry was to provide backing along all drywall edges so it will be supported and can be secured.  Our final task for the day was to trim a piece of 2×4 to block off the top of the wall cavity where the return air duct is connected next to the door between the library and the garage.

For dinner we had leftovers from Thursday:  Koshary and pita bread with vegan garlic “butter.”  Linda read somewhere recently that drier white wines are generally considered (by someone) to go better with Middle Eastern food, but we thought our 2009 Egri Merlot went quite well with dinner.  Of course, Koshary is an Egyptian dish, and so perhaps more Mediterranean than Middle Eastern.  All of that reminded me that there really are no rules about these things; drink what you like and enjoy life.