www.nutsvolts.com/index.php?/magazine/article/february2011_Goodsell
switch so if the monitor was not going to be used for
an extended period, the battery could be totally
disconnected.
However, the “continuous” feature presented a
■ FIGURE 3. Schematic of the monitor features, auto-turnoff,
battery monitor, and LED flasher circuits.
new problem. The monitor could inadvertently be left
on and I would never know it except for the dim LCD
readout. So, I added one of the “desired” features: an
LED. But wait! LEDs draw too much current, you say!
To counter that, I changed the design to
a low duty-cycle blinking LED. I
considered using a 2N6028 PUT or a
CMOS LMC555 to generate the
pulses. They both consumed about the
same amount of current — 200 µA —
but the 555 would be easier to control.
R6 and R7 were selected so the rate
would be ~1 Hz with a duty cycle of
2%. The average current for the LED
would be around 100 µA. Perfect.
The next challenge was to add a
battery monitor so the user would
know when to change the battery. I
wired up a low-power ( 4 µA) LTC1440
comparator on the test breadboard
and selected R9 and R12 to switch at
~ 7.0V. The formula is Vthresh =
1.182*((R9+R12)/R12). Now, here’s the
really cool part. Instead of adding yet
another LED, I tried to make the
comparator increase the flash rate of
the existing LED to indicate a low
battery.
Again, more additions — R10, Q2,
and R11. When the battery drops
below 7.0V, the LTC1440 output goes
low, turning on the P-channel FET
which puts the 100K (R11) in parallel
with R6, and raises the rate up to
about 5 Hz. Bottom line: 1 Hz = Unit
on; 5 Hz = Change the battery.
Even with all these bells and
whistles added to the breadboard, the
final measured average drain on the
battery was only 1,250 µA.
Acceptable!
Lastly, I added a “range” switch
(SW1) to handle wall warts over 20V.
SW1 changes the voltage divider — R5
and R3 — from 100:1 to 1,000:1. Now,
the meter reads either 0-19.99V or 0-
199.9V. Changing the decimal points
was easy too; just add an extra pole to
SW1 and SW2.
There was still a question in my
mind regarding the best way to label
the new range switch. Should it be
20V and 200V, like a conventional
digital multimeter, or something else?
I hesitated to use the 200V label
because the monitor was designed to
measure a maximum of 24V — not
44
February 2011
