This heart rate monitor is designed for communication between man and machine, so that the robot could react to mentor the frequency pulse. The digital output of the circuit makes it also applicable in other applications.
Driving device shown in Figure 1.
sensor itself consists of a conventional photoresistor LDR – light depended resistors (resistance in an illuminated condition 300 Ohm and the resistance in the dark for approximately 10 MOM) and bright LED (D1).
The LED should have output power of the light flux not less 1000 mKD, because the light from it must pass through the finger and light LDR.
When the heart pumps blood through the blood vessels, finger periodically becomes slightly more opaque (with the frequency of the human heart), therefore, less light reaches the photoresistor LDR.
This phenomenon can be converted to an electrical pulse.
LDR resistance varies with each pulse, and, Consequently, the input voltage of the operational amplifier IC1.A.
Amplifier gain potentiometer P1 is set to the feedback path.
The sensitivity of the circuit can be adjusted by the potentiometer P2 at the input to comparator IC1.B.
The transistor T1 produces an output driver, which not only ignites the D2 LED for local indication of the detected pulse, but also nourishes the oscillator circuit on-chip timer IC2 – NE555.
Output is, pulse modulated at a frequency, which can be set in the range of 30 kHz to 40 kHz using potentiometer P3.
Step excitation connects NE555 output infrared emitting diode, which can send the modulated signal to the infrared receiver module IC3.
The more power is used to control the IR-diode, the greater the communication range: resistor R11 can be changed to achieve the desired LED current.
The demodulated output of the receiver module may be fed directly to the microcontroller.
The center frequency used by the receiver module will determine the correct setting P3.
The heart rate monitor can be made of simple plastic tube length 40 mm, closed at one end, selected as, so that it fits close to the tip of the finger (rice. 2).
Holes for mounting the LED and LDR are made at a distance 15 mm from the closed end, and the elements are glued to the hole so, so that they are directed toward the center of the tube.
The connecting wires are insulated from each other, and the entire sensor in the shrink tube is isolated from the external light entering from LDR.
The presented project should be used in robotic other programmable control systems.