In the control circuit of Fig.2, the principal
component is the PIC16F876 microcontroller,
IC3. This is clocked at
3·2768MHz as set by crystal X1.
The amplified and a.c.coupled signal
from IC2c is fed to the PIC’s RA0/AN0
analogue-to-digital converter (ADC) input.
The input is biassed to about 2·5V (half the
PIC’s nominal 5V supply), by resistors
When no signal is present, the ADC conversion
value read by the PIC is about decimal
128, out of a possible 8-bit range of 0
to 255 (the PIC’s full 10-bit range is not
needed in thisdesign).
From the incoming values, the PIC calculates
the BPM rate according to an internal
clock. It does not attempt to determine
which pulses are truly heart-generated and
which have beencaused, for example, by
movement of the thumb on the l.d.r. The
twin heart-probe path is less prone to extraneous
The pulse beats are displayed by l.e.d.
D4, which is buffered byresistor R25 and
controlled by PIC output RC4. The calculated
BPM rate is output to the l.c.d. via
lines RB0 to RB5, the display being operated
in conventional 4-bit mode. Preset
VR4adjusts the l.c.d. screen contrast.
The software also outputs graphics data
to the l.c.d. for display on line 1 in cells 1
to 8 as a simple waveform (as shown
earlier and on the final page). Thetechnique
was first used by the author in the
l.c.d. display for his Micro-PICscope.
Readers familiar with alphanumeric l.c.d.s
will be aware that the display has several
internal charactergenerator registers that can
be programmed by the user. Data is written
from the PIC into these registers such that
the pixels show a moving display of the
It has to beemphasised, though, that the
l.c.d. shows less detail than can be displayed
on the PC screen. Nonetheless, for
simple handheld use of the unit, the l.c.d.
can produce informative results.
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