HEART RATE MONITOR

The simple but very reliable monitor shown in Fig will be an asset to those who have difficulty finding their pulse in their wrist. It is also useful for checking the pulse rate immediately after exercise, which should be well above the normal rate of 60-80 beats per minute if any benefit from the exercise is to be derived.

Light Finger The device depends for its operation on variations in light intensity. When a finger is placed on a light dependent resistor R2, the l.d.r. detects the minute changes in light level caused by variations in blood flow as the heart pumps. These light changes are translated into minute voltage fluctuations that are subsequently amplified through a two-stage amplifier, a non-inverting op.amp (IC1a) and an inverting op.amp (IClb), by a gain of approximately 800 as determined by resistors R5, R7 and R10. At the output (pin 7 of IC1b), each heartbeat is reflected in the rhythmical swing of a meter needle across the dial of a milliammeter (ME1) or other suitable panel meter. No special lighting is needed as the l.d.r. is able to “see” through a finger tip in normal daylight.

 

The gain of the first op.amp is fed into the second and the overall gain is sufficient to obtain a healthy swing of the meter needle. Almost any moving coil meter can be pressed into service because we are not concerned with voltage or current measurement, only the needle deflections across the dial. However, be sure to fit a series limiting resistor R11 to suit the meter and prevent damage. A miniature button-type l.d.r. is preferred to the bulkier ORP12 so that the finger can completely cover the sensor surface and prevent stray lighting from reaching it. Two discrete 741 op.amps could be used in place of the LM358N if more readily available. Although not shown here, the prototype also housed a 30-second timer, using a 555 with an l.e.d. indicator.

When the timer is initiated, the needle movements are counted during the 30 second period, then doubled to obtain pulses per minute. The circuit could also be adapted as a front-end to more advanced monitoring systems. In use, after the unit is switched on, allow several seconds for the meter needle to stabilise somewhere about mid-scale. Place the fleshy part of the middle finger tip on the l.d.r. and rest the hand comfortably while keeping it still, then monitor the meter needle movement. If the meter needle responds by only a small amount, it is probably because your hand is excessively cold and the circulation is sluggish.