common reference ground, and full gain of the heart signals
is obtained. R13 and C6 provide for a second low pass filter
at 48 Hz prior to the signal passing to the display device.
In addition to R10 (CMR2) which is used to balance
the input divider network, an additional CMR1 adjustment
is made via R8. Potentiometer R8 adjusts the CMR of U1A
Op-amp U1B is used as a comparator to flash the red
LED D2 whenever the QRS complex (heartbeat signal) is
observed. Potentiometer R16 adjusts the threshold
reference of U1B; the -input of U1B connects via R15 to
the output of U1D. Adjusting R16 to a couple of tenths of
a volt above the common causes U1B’s output to behigh
and LED D2 off. Whenever the heartbeat is detected, the
QR portion of the signal goes to nearly one volt
depending on the amount of gain, etc., causing U1B to go
low and flash LED D2.
Author's Note: If a smoother waveform is desired, C5
could be increased to 4. 7 nF or 10 nF, to provide a break
frequency of 20 Hz or 10 Hz, respectively, to reduce
noise. Likewise, increasing R13 to 33K
would reduce the output break
frequency to 22 Hz, further smoothing
the waveform noise.
The Parts List includes the numbers
and sources for all of the parts needed
to build the unit. Also provided (at the
article link) are the Gerber files for the
printed circuit board (PCB). I built my unit
using a RadioShack 276-147 general-purpose PCB. The Gerber file check
print (Figure 2) was used to place the
components and wire wrap wire to
connect any parts that were too far
apart to use the part leads to make the
Also, since the PCB I designed was
two-sided, the wire wrap allowed me to
cross over wires and make connections
that wouldn’t have been possible on
the single side of the board (see
Figures 3 and 4).
Resistors R1- R3 and R5-R9 need to
be matched in value. It doesn’t matter if
the value is lower or higher than one
meg, just that they match within one
digit, i.e., 2@994K, or 2@1003K.Th e
same applies for R2-R4 — 2@ 3.92Meg or
2@ 3.89Meg — just so they match. The
26 February 2017
■ FIGURE 2. PCB layout.
■ FIGURE 3. Top side of PCB.
■ FIGURE 4. Bottom side of PCB.