Amplifier to the classic pulse sensor

The proposed amplifier is designed for one of the most classic pulse sensors - optical. That same "clothespins" on the earlobe or finger, like some simulators. Only here the clothespin is not in quotes, but the real one. Wooden. It drilled 3-millimeter holes in which are placed the infrared diode SFH487 and the photo transistor SFH309FA from Siemens. Instead, they fit almost any IR diodes and phototransistors, only the holes will have to drill other diameters. To use the sensor it was comfortable, soft lining with holes was glued to the surfaces of the clothespin adjacent to the ear lobe or the finger. If the pulsating light of the LED or fluorescent lamps interferes, the phototransistor should be closed with a filter that only transmits IR.



The amplifier circuit is shown below:
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The phototransistor is loaded on a resistor, the variable component from which through the electrolytic capacitor enters the first op-amp. The signal from its output through the regulator on a variable resistor and the second electrolytic capacitor is fed to the input of the second op-amp, and from its output, in turn, to an oscilloscope, an analog input of a microcontroller, or a comparator to which a counter is connected. Each op-amp provides a gain of about 100, they may be of other suitable types.

Since the power supply is unipolar, the signal common wire of both op amps had to be “lifted” over the common wire of the entire device with the simplest parametric stabilizer of a resistor and two diodes. The total voltage drop across the diodes is about 1 V. The improvised bipolar source turned out to be asymmetrical, but for the OS even it is better than unipolar.

If you are going to conduct an experiment by connecting the amplifier to a storage oscilloscope, set it up so that it makes 500 records with an interval between them of 20 ms. Wait about 10 seconds for the thermal mode of the microcircuits to be established, and set the variable resistor located between the first and second OU, so that the amplitude of the pulses is about 2 V. Adjust the sensitivity of the oscilloscope so that the pulses are placed on the screen vertically, and start recording. While the oscilloscope is recording, do not move your ear or finger.

If the oscilloscope has a sound indication (as in Picolog, which the author used), it can be configured to sound with an amplitude of more than 1 V. Then you will hear your pulse.

After the end of the recording, to calculate the pulse rate, subtract the absolute time of one of the pulses from the absolute time of the previous one, for example, 2190-1052 = 1138 ms, or 1.138 s. Divide 60 / 1,138, it will be 52.7 beats per minute.

When using an improvised software oscilloscope using the audio input of a smartphone or computer, you will need a sound generator modulated by pulses in amplitude.