Title:
Heart Rate Monitor
Circuit:
Components:
Resistors:
100 k x 2
3.3 M x 2
15 k x 2
220 k x 1
10 M
OPamps: x 2
Capacitor:
0.1 uf
4.7 uf
10 uf
Battery:
A 9v battery
Miscellaneous:
Multimeter:
LDR:(Z-4801)
Switch:
Description:
The secret is a high gain circuit, based on a dual op amp IC
which can be either the low power LM358 or the JFET TL072. The LDR is connected
in series across the 9V battery supply via a 100kO resistor (R1) and the minute
signal caused by the blood pulsing under the skin is fed to the non-inverting
(+) input, pin 3, of IC1a via a 0.µF capacitor.
Pin 3 is biased by a high impedance voltage divider
consisting of two 3.3MO resistors. The feedback resistors to pin 2 set the gain
to 11 times. The output of IC1a is fed via a 0.47µF capacitor and 220kO
resistor to IC1b. This is configured as an inverting op amp with a gain of 45
so that the total circuit gain is about 500. The output of IC1b is used to
drive an analog meter which may be a multimeter set to the 10V DC range or any
panel meter in series with a resistor to limit the current to less than its
full-scale deflection. The prototype used an old VU meter with a 47kO resistor
fitted in series.
A light source such
as a high brightness LED is not required. All that is needed is a reasonably
well-lit room, preferably natural daylight, to produce a healthy swing of the
needle. Only when the hands are very cold does it make it a little more
difficult to accurately count the pulses. To check your heart rate, carefully
position your thumb or finger over the LDR and count the meter fluctuations for
a period of 15 seconds. Then multiply the result by four to obtain your pulse
rate. The circuit can not be used if you are walking or running, etc.
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