Telephone Off-Hook Indicator
QI have several four-line analog (RCA) telephones in my home and my wife's office (located on our property). We do not use the fourth line and I'd like to use it to signal her when I am on my VoIP
work telephone (a separate, PC connected telephone
headset). My VoIP headset puts out 5 VDC (guessing at
about 5-10 mA) when off-hook, so I have a signal to work
with. What I don't know is what kind of a signal do I need
to provide to my analog phone line input to make it show
off-hook/busy?
— Jon Wilbert
AThe telephone has been around since the Scottish born American inventor, Alexander Graham Bell famously spoke to his assistant, Thomas A. Watson on March 10, 1876, "Mr.
Watson. Come here. I want to see you."
10 January 2016
Re: Electronics Around Vehicle Fuel Tanks
#1 I always enjoy reading your knowledgeable and
well-written Q&A section. I particularly appreciated your
May 2015 tips on batteries. Your August issue on
electronics and fuel tanks was very informative, but I
question the accuracy of the Figure 2 schematic. It would
seem that the potentiometer can't affect the Fuel Quantity
Ammeter, as shown, or am I missing something? Maybe
this is not an issue, since you warned the reader not to
implement their own circuits. Good advice!
Arne Berg, Huntington Beach, CA
Mark, I am glad I could help you. Technology today is
moving so rapidly it is sometimes hard to keep up with. So,
the questions of our readers are helping me to stay on top
of the rapidly changing field of electronics.
#2 I just read your description of the fuel pump and
fuel level measurement systems and had to chime in here
as I have fairly extensive experience working on these —
especially the Bosch Jetronic systems widely used on most
European cars since the mid 1960s. Now, granted the fuel
systems I've dealt with have mostly been 1990s and earlier
so some things may have evolved, but your description of
the safety features has no basis in any reality I've ever
seen. There is no external motor or explosion-proof pump
housings, nor is the impeller in the main pump made of
plastic. Injection pumps need to deliver more than 40 PSI
to the fuel rail (and much higher in some systems) while
lasting thousands of hours, so they use roller type pumps
made of steel.
The motor is actually completely open and fully
immersed in fuel which provides cooling and lubrication.
Yes, the windings, commutator, brushes, and all are in a
thin aluminum housing that is completely filled with
gasoline flowing through the pump. This pump is located
either within the fuel tank itself or remotely under the car,
and a small impeller type pre-pump is submerged in the
fuel — again with the motor and wiring — usually with
exposed terminals completely submerged in fuel. The fuel
sender is a similar deal; it is invariably mounted not in the
neck but to the fuel pickup tube in a bung at the top of
the tank, and consists of a float ball attached to a simple
open frame potentiometer immersed directly in the fuel.
This all may sound crazy but that's how it's been done
since the earliest fuel injected cars in the 1950s. It relies on
the fact that fuel vapor in the tank displaces any oxygen
that would otherwise be in there. Without oxygen, the fuel
cannot burn. This is the same reason the common movie
scenario of a car's gas tank exploding is exceedingly rare in
real life. If you want to see all this for yourself, search for
"electric fuel pump teardown" on You Tube and you will
find numerous videos showing all the gory details. I do
agree with the basic advice that one should not attempt to
install DIY devices within the fuel tank. It's just not worth
the risk. Fuel systems can be dangerous to work on. The
combination of pressurized gasoline, electricity, and
potentially hot engine and exhaust components have
caused more than a few car fires.
James Sweet
James, thanks for this information which is quite
correct for the fuel system devices installed by the original
manufacturer; the same standards are required for
automotive aftermarket devices. These devices are
extensively tested and certified as safe for use in
automotive fuel systems before they are sold to consumers.
My remarks and the ensuing warnings were directed at
devices installed by electronics hobbyists who do not have
the knowledge of how to make devices safe for such a
hazardous application and who cannot conduct the
extensive testing required. It kind of seems to defy logic,
but electrical sparks in a liquid fuel will not trigger
combustion. However, the same spark in the presence of
fuel vapor will cause combustion. I am well aware of car
fires because once I had an engine fire that erupted right
after I had fueled the car and was in the station paying
when we saw the smoke. The hood latch would not work
and I was able to release the brakes, push in the clutch,
and get the car away from the pumps before the fire
fighters arrived on scene. (I don't know if I prevented a
major disaster but I was trying to. I guess instead of being
the hero of the day, I atoned for my own klutziness. In the
past 30 or so years, I have never left off another oil cap.)
Automotive experts at our community college said it
was a fuel leak onto the exhaust manifold, but when I
checked out the car later, the oil cap had been left off the
previous evening when I changed the oil. So, I think the
fire was a result of oil pouring onto the hot manifold (a
couple of weeks later, I had reworked the car's engine
compartment and there were NO fuel leaks even after
driving the car for a couple more years). The cashier at the
station is probably still having therapy to return her eyes to
the sockets. We both agree that our readers should be
very careful when working around fuel systems or other
potentially dangerous systems.
MAILBAG
