Materials and Methods

The main objective of this my project is to count the number of heart beats per minute.  First of all the heart beat is detected with a condenser microphone. Then I’ve filtered  the signal by a high pass filter and then it is amplified. A comparator is used to get two voltage levels for the signal.

Since I have to detect the rising edges of the amplified signal, using an interrupter (CCP1 / Capture mode) in the microcontroller to detect and count the rising edges decided to be appropriate. Finally the count obtained for a programmed period of time which is for 1 minute, is displayed with the aid of the three digit seven segment display. The device is also able to clear the count and start counting again when done with one user.

The system consists of four parts. This is discussed in the Design and Construction section. Here I’m going to discuss the methods I’ve adopted and the materials I’ve used to design the circuits.

Methods

High – Pass Filtering

 

An electrical filter reshape/reject unwanted frequencies of a signal. The most basic high pass filter includes a capacitor and a resistor. V_out is taken from                    across the resistor as shown in Figure 2.1. In this kind of arrangement the capacitor acts as an open circuit for low frequencies and blocks any input signal. Above the cut-off frequency it allows the entire input signal to pass directly to the output. Cut -off frequency is calculated by  f =1/(2×π×R×C).

FIGURE 2.1: HIGH PASS FILTER

Comparators

Interrupters (Of the PIC)

Here in my project I’m using the sophisticated microcontroller PIC16F877A. This microcontroller has 40 pins with 5 I/O ports(A, B, C, D, E). For the Heart beat counter only port D, B and RC2 pin in port C is used. RC2 pin (CCP1) is operated in ‘Capture mode’ and it is set to capture every rising edge of the input signal it receives.

FIGURE 2.3: PIC16F877A PIN CONFIGURATION

LM324 is a low power quad op-amp device. Single supply voltage of this device is from 3 V to 32 V. 0 V can be used for (-Vss). It is a 14 pin IC. Here it is used as an amplifier as well as a comparator.

FIGURE 2.4: LM324 PIN CONFIGURATION

I introduced separate functions to do a specific task such as interrupting, counting, displaying the count and setting the time period.

TABLE 2.2:LIST OF FUNCTIONS IN ALGORITHM

   

Design and Construction

The device is most useful when it is portable. Thus it should be light weight and it should have a power supply of its own. Also it would be in its best shape if the size of the whole device is small and when the overall cost is low. The design was created to accomplish all these necessities.

All the circuits were designed using Orcad software package. Schematics of the circuits were drawn with the aid of Capture CIS and the PCBs’ were drawn with the help of Layout Plus in Orcad 9.2. The system consists of four parts. Te following is a diagram of the three main circuits. In addition to this there is power supply unit.

Figure 3.1: BLOCK DIAGRAM

Main Circuit

 

Base of the main circuit is the PIC16F877A microcontroller. Interrupter in the PIC is used to detect the rising edges of the signal by setting the CCP1 pin to capture mode that detects every rising edge. The PIC is programmed to count the number of interruptions. The I/O ports, Port D and Port B are used to select the segments and to select the digits of the SSD separately and to display the counted value. It also contains the basic requirements to drive the PIC such as voltage regulators, oscillator and etc.

 

 

Sound Signal Amplifying and Detecting Circuit

The condenser microphone detects the sound of the beat. It is put inside the tube of a stethoscope in order to reduce background noises. Then the signal is passed through the high pass filter which includes a capacitor and a resister for further reduction of noise. LM324 IC was used as a non-inverting amplifier and also as the comparator as it has 4 op-amps. The output of this part is received by the PIC’s CCP1 pin as an input.

 

FIGURE 3.2: AMPLIFYING CIRCUIT

 

Seven Segment Display Circuit

The three digit SSD is connected to the D port pins to give the counter output through resistors. Cathode of each digit is connected to Port B through a transistor in order to select the digits. The count will start from ‘000’ and increase according to the number of rising edges detected within a minute. At the completion of a minute it will display the final value i.e. ‘072’.

FIGURE 3.3 : DISPLAY CIRCUIT

 

Power Supply

Power for the whole circuit is supplied by two 9 V batteries in series with a center terminal ground. Capacitors are used in the power lines to smooth out the power. Voltage dividers are used to get the desired voltages for the four circuit parts separately. It is very important for the system to have a built in power supply as it is a portable device.

FIGURE 3.4: POWER SUPPLY

Results and Analysis

The signal obtained from the microphone was not clear at first. But by putting it inside a stethoscope helped greatly to overcome this problem. Also the value of the count displayed on the SSD was skipping from 4 counts for each and every pulse. Introducing a delay function to the Interrupter function solved this. Reasons for these problems are discussed in the Discussion and Conclusions.

There is still a problem when starting the device to count. The count rapidly increases in an unusual way and the value skips from 1 to a higher value (3 to 13) at once. But after that it counts the heart beats more accurately and the count increase by one per pulse.

 

TABLE 4.1: RESULTS OF DIFFERENT STAGES

Device	Measurement
Microphone	Output = 0.02 V
High pass filter	Cut off frequency = 159 Hz
Amplifier	Gain = 100 Output = 2 V
Comparator	Vref = 0.45 V Output = 5 V (for peaks) or 0 V
SSD	The counted value i.e 073

Discussion and Conclusions