Here is how to make 3 voltage meters using a PC, resistors, capacitors and an IC.
The following circuit is powered and read by a serial port. SeriCon, a universal PC
interface program can pulsate the output pins RTS/DTR manually or auotmatically and
can measure other pulses at the input pins CTS/DSR/RLSD.
You can easily customize the outputs and inputs by dialog
boxes to do the following works and save the specifications into a file. If you
don't want to read the SeriCon manual now, try this file: vMeter.scs
Right after the file loaded (by menus or double-clicking), SeriCon starts
pulsating DTR with RTS kept low.
The period is 2 seconds consisting of the two 1 second high and low
level durations. The input pin CTS level is low during the latter half period, since
DTR is low like RTS and the circuit is not powered. When the new period starts, the
circuit gets powered by the high level of DTR, and CTS becomes high. CTS falls when
the capacitor is charged enough for the [+] input to the comparator IC to be higher
than the [-] input connected to Vi the voltage to measure.
The high level duration of CTS is tranformed by an exponential function, which
corresponds to the transient voltage effect characteristized by RC = 100 ms
( = 100kOhm * 1 uF ):[V+][1-exp(-tH/RC)] = Vi
where V+ is the high level voltage of DTR and tH is the measured high level
duration of CTS. The display format of CTS input pulse count and the durations
(or the transformed values) can be also customized to have suitable number
expressions and additional texts for names and units.
Although the quantities also can be included
in the display or excluded, the pulse count is included in the voltage meter
application, since the increment indicates that the meter is working: You should
adjust the variable resistor for the zero Vi to see the count increasing before
measuring the real Vi, and should see it also increasing during the measurement.
The display window becomes white when CTS is high, so that it shines longer for
higher Vi. But, emphasizing once again, be sure to see the CTS pulse "alive"
during the measurement,
by seeing the count increasing, not only by the window color
flickering between gray and white since you cannot recognize very short white colors
for very low Vi.
High Vi is not suitable,
since it makes tH comparable to the first half period
of DTR so that the capacitor gets charged much and discharges insufficiently
during the second half period, which leads to large fluctuations in tH.
The screenshot above shows the result for a mobile phone battery, with V+
set 10V. If V+ of DTR is measured by other instrument and inserted into the dialog
box popped up by the [Bit]->[Cts] menu replacing 10, the result can be more
Generally, one comparator IC have 4 comparators, so that it is sufficient to make
3 meters, ustilizing the other input pins DSR and RLSD, with 2 more sets of
resistors and capacitors added to the circuit.
SeriCon is a
shareware without evaluation period limits but with function limits.
Just reset the pulse count by the Cts button or restart the Dtr automation each time
they stop due to the limitations (30 counts and 60 pulsations), unless you don't
purchase the full license. Do you need a GUI terminal to your microcontroller
project, instead of old text terminal programs? SeriCon will satisfy you also in
such serial communications (RxD, TxD).