EMRFD Message Archive 1220

Message Date From Subject
1220 2007-12-09 14:14:11 w4zcb77 KA7EXM microWattmeter
Having owned my KA7EXM microWattmeter for several years, I was
giving serious thought to purchasing a commanding share of Duracell
stock to offset the cost of my consumption of 9 Volt batteries. It
seems that it takes a big fat red LED to get my attention to a piece
of equipment that is still turned on. An LCDisplay apparently just
doesn't cut it. I'd keep using the package, and leaving it on, only
to discover a flat battery the next time I tried to use it.

The fact that a big fat red LED just contributes to battery life
reduction didn't help anything, and with the realization that my
short term memory wasn't getting any better and eventually even the
red LED might not do the trick, I embarked on an automatic turn off

First efforts at the use of a triple nickel and a dozen parts for a
timer didn't amount to a lot. Similar problems with the only n-
channel FET in the shop resulted in a 3 mile walk while I cleared my
foggy brain. Came home and hotted up the solder station and while it
was doing that, replaced the toggle switch on the front panel of the
Wattmeter with a NO push button.

Wiring that across a 220 uF tantalum cap (with the negative terminal
to ground), I only had to add two components. A ZVP2106A TO-92 p-
channel FET, ($1.26 from Digi, but only $0.34 each if you want 2,000
of them) and a 2.7 MegOhm resistor. Resistor from the positive wire
from the battery to the gate and hot side of the tantalum, Source
also to the hot side of the battery with the drain going to the
Wattmeter Vdd wire.

Pressing the pushbutton discharges the capacitor (If doing that
gives you goosebumps, you can add a 5th item, a low value resistor
in series with the pushbutton). This turns on the 2106 and applies
8.6 Volts from a 9 Volt battery to the load. It takes 5 minutes to
have the load Voltage drop to 8.0 Volts, and a bit longer before the
Wattmeter gives up. Operating time can be extended by depressing the
pushbutton again for another timing period without discarding any
setup parameters you might have put in, since doing so does not
result in the FET turning off even momentarily during reset. In
about 20 minutes time, the Voltage has decayed as the capacitor
charges fully and the battery discharge rate of the timer circuit
drops to a nice low value of 2 nanoAmperes. The whole thing is
supported on the back of the NO push button, Duracell can go fish.