identical in function and operation.
When selector switch S1 is at the 200 mV setting, the
input voltage is connected to the input of U1A via R1.
U1A is used as a unity gain buffer. R1 limits the input
current to a safe level that can be clamped by the internal
MCP6024 protection diodes (± 2 mA). Therefore, any
voltage less than 2,000 volts will be safely handled by the
The output of U1A passes through R6 and R10 (not
shown) to the 1/8” ( 3. 18 mm) stereo output jack. This
limits the output current to 6 mA if both outputs were
shorted. That is well below the 20 mA rated output
current. S1 connects the input of U1A to a standard 10
megohm divider network. The nine meg, 900K, 90K, 10K
1% resistors scale higher input voltages into a manageable
200 mV range. I had trouble finding the resistors for the
divider network as described, but was able to find these
values which provide the same .001, .01, .1 divide ratios:
9.09 meg, 909K, 90.9K, 10K + 100 ohms. This alternate
divider version is shown in Figure 2.
By setting the Soundcard Scope
to 200 mV per division, the divisions
will now be equal to the setting of
switch S1. The full range of the scope
display will be plus or minus 1V,
10V, 100V, or 1,000V. This is useful
for measuring higher voltage signals.
There is a frequency limit to the
Soundcard Scope. This is due to the
sampling frequency of the 16-bit or
24-bit (depending on your computer
hardware) A/D used in the sound
card, which is 44.1 kHz. It is good to
only about 20 kHz, so it is good for
most audio signals and lower
frequency signals like the garage
door sensors. It works well with
PWM motor drive signals below 20
kHz, LED and fluorescent lighting ballasts, and other lower
I tried it out with my homebrew function generator
(see Figures 3 and 4) to see how well it performed over a
frequency range from 1 Hz to 40 kHz. The results showed
it worked well for sine waveforms up to 5 kHz, and up to
20 kHz for square and triangle waveforms. The other
limitation of the PC o-scope is that the PC mic inputs are
capacitively coupled, thus limiting low frequencies below
8 Hz and no DC voltages to be measured.
Op-amp U1C acts as a Vdd/2 voltage reference that is
connected to the bottom of the input divider network and
the output common. This centers the output of U1A and
U1B at Vdd/2 and 0V with respect to the common
output. The interface is powered by three AAA batteries.
They last a long time due to the very low current that’s
being drawn by the MCP6024. It is less than the current
drawn by the power indicator LED D1.
The entire interface circuit draws only about 10 mA
when on. The unused op-amp U1D is connected as a unity
gain buffer, and the + input is connected to the output of
U1C, or Com.
Building the PC O-Scope
The intention was to use inexpensive readily available
parts to build the PC two-channel o-scope. All of the
resistors are metal film 1% for the input resistive divider, as
well as the R15, R16 reference divider. J1, J2 are standard
RCA female chassis mount jacks. J3 is a 1/8” stereo jack.
Nearly any type of case could be used to house the circuit. I
chose a SERPAC Model 032 plastic case left over from a
The board was sized to use the mounting bosses in the
case. The three jacks were mounted to the side walls of the
case. The AAA battery holder lays atop of the PCB and is
24 August 2016
■ FIGURE 3. Sine, square, and triangle function generator.
■ FIGURE 4. 1 kHz at .67% THD.