An artificial signal that corresponds to an actual ECG signal is needed for the development and servicing of ECG equipment. The simulator described here produces a suitable signal. Since this signal is crystal controlled, it can be used for the calibration of pulse rate displays. In order to make an electrocardiogram, electrodes are attached to specific locations on the body such as the forearm, calf and the breast cage. The electrical potentials produced by the activity of the heart, as measured between these electrodes, and then recorded. The source of the voltage for the heart muscle, the sinus node, a pulse that branches into two main parts. The pulse and the progression of the execution can be measured on the surface of the body. The shapes of the resulting waveform and their progression over time provide doctors with important information regarding deceases of the heart and circulatory system. The ECG can be either continuously displayed on a monitor or traced by a pen on paper for documentation. In the later case several; different versions of the signal measured at different points are often recorded at the same time. with this type of ECG; which is called a surface ECG the measured potentials lie around 1mV. The heart rate can lie between 40Hz and 150Hz.
Medical
specialists use the letters ‘P’ through ‘U’ to refer to the various curves and
spikes of the ECG. Modern ECG recorders
and monitors verify and evaluate the input signal and are able to filter out
artifacts and foreign signals such as pacemaker signals. This means that a simple square wave
generator is not satisfactory as an ECG simulator, since the ECG equipment
would simply ignore such a signal. The
signal produced by the simulator described here has been successfully tested on
several different ECG recorders and monitors.
The micro-controller system is normally used to generate the test signal in
industrial ECG test equipment which is consequently rather expansive. Only two standard logic ICs and a few passive
components are used. IC1 is a 24 stage
binary counter with an integrated oscillator and divider. With the indicated crystal frequency of 41194304Hz,
a 16Hz square wave signal appears at the Q18 output [pin-10]. Switch S1b picks up a second signal [2Hz or
1Hz]. The 16Hz signal clocks IC2 which
is a decimal counter with ten outputs.
The second signal is differentiated by the combination of C3 and
R3. Needle shaped pulses are present at
pin 15 of the decimal counter [IC2], as indicated on the schematic diagram.
These pulses reset the counter to zero at the appropriate times. The job of
diode D2 is to block the negative pulses.
The decimal counter reputedly
reaches a count of ‘9’ and holds this state, since pin 11 is connected
to the /Enable input [pin13]. It is only
reset when the reset pulses occurs. The setting of the switch thus influences
the duration of the ‘U’ interval, which ultimately results in a simulated heart
rate of either 60Hz or 120Hz. If
necessary, a 4MHs crystal can be used.
This will reduce the heart rate of the signal to 57.2Hz or 114.4Hz
respectively. 
The ECG signal is generated in a remarkably simple manner using a dozen discrete components. Time displaced square wave signals appear at the Q1, Q4 and Q6 outputs. The first pulse [from pin number- 2] is converted into the ‘P’ wave by the integrator R6/C4. The value of R6 is chosen such that C4 charges exponentially from ‘0V’ to around ‘1V’. The ‘T’ wave is generated by a second integrator [R7/C4]. Since R7 has less than half the resistance of R6 charges C4 to more than twice the voltage [2.2V] of the ‘P’ wave.
The differenciator C5/R10 inserts the ‘R’ pulse between these two waves. Resistor R8 limits the charge current for C5; while D5 ensures that the peak value of the pulse does not exceed approximately 3.8V. the negative portion of the pulse; on the falling edge of the input pulse; is shorted out by D4, wo that all that remains is a good (- 0.7V) due to the voltage drop of D4. This produces a very pretty ‘S’ component. Diode D3, with its series resistor R9 flashes during the ‘R’ spike.
The signals from both integrators and the differentiator are summed by R11 and R12. Capacitor C7 smoothes out excessively spiked pulse components. The final waveform is also shown on the schematic. The voltage divider provides the output signals with amplitudes of 1mV and 1V.
Insensitive equipment that normally works with signals that have already been amplified, such as secondary monitors can be connected to the second output. A 9V battery can be used as power source. The circuit draws’ only around 2.5mA current; so the battery will last longer. For testing, battery power is recommended.
I’ve assembled and tested this circuit, and working fine. Tested with different brand ECG machines.
A prototype that I’ve assembled is displayed here.
