Heart Rate Monitor

Strictly speaking, this
simple circuit shouldn’t work! How could anyone expect an ordinary light
dependent resistor photo cell to ‘see’ through a fingertip in natural
daylight and detect the change in blood flow as the heart pulsates? 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 100k 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 3.3M 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 220k
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 47k resistor fitted in series.

Heart Rate Monitor Circuit

Heart Rate Monitor Circuit Diagram

Note that the unit was designed to use the Dick Smith Electronics light dependent resistor (Z-4801). Other LDRs may require a change in the value of resistor R1. 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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