e005052.pdf

MEASUREMENT &TEST

ECG simulator

an analogue

waveform

generator

Design by J. Holzhauer

Electrocardiograms (ECGs)

are used very often for medical

examinations, research and the

development of biomedical

equipment. An ECG simulator thus has a large variety

of potential applications.

Warning!

The ECG simulator described in this article should be used only for testing and repair pur-

poses. It must never be connected to any equipment that is at the same time connected to

a patient. Neither the author nor Elektor Electronics assumes any responsibility for injury

or damage that may result from the use of the ECG generator.

ECG, which is called a surface ECG,

the measured potentials lie around

1 mV. The heart rate can lie between

40 Hz (rest rate) and 150 Hz (with

strong exertion).

Medical specialists use the letters

P through U to refer to the various

curves and spikes of the ECG, as

shown in Figure 1 . Modern ECG

recorders and monitors verify and

evaluate the input signal and are

able to ﬁlter out artefacts 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 differ-

ent ECG recorders and monitors.

An artificial signal that corresponds to an

actual ECG signal is needed for the develop-

ment and servicing of ECG equipment. This

makes it unnecessary to make measurements

on people, and in particular with research

and repair activities it eliminates a potential

risk to the test subject. The simulator

described here produces a suitable signal.

Since this signal is crystal controlled, it can

also be used for the calibration of pulse rate

displays.

The source of the voltage for the

heart muscle, the sinus node, emits

a pulse that branches into two main

parts (temporal and spatial). The

pulse, and the progression of the

excitation, can be measured on the

surface of the body. The shapes of

the resulting waveforms and their

progression over time provide doc-

tors with important information

regarding diseases of the heart and

circulatory system.

The ECG can be either continu-

ously displayed on a monitor (for

intensive supervision) or traced by a

pen on paper for documentation. In

the later case, several different ver-

sions of the signal, measured at dif-

ferent points, are often recorded at

the same time. With this type of

From the heart

A discrete circuit

In order to make an electrocardiogram, elec-

trodes are attached to specific locations on

the body, such as the forearm, calf, and breast

cage. The electrical potentials produced by

the activity of the heart, as measured

between these electrodes, are then recorded.

A microcontroller system is normally

used to generate the test signal in

industrial ECG test equipment,

which is consequently rather expen-

sive. However, you will look in vain

Elektor Electronics

5/2000

MEASUREMENT &TEST

T wave is generated by a second integrator

(R7/C4). Since R7 has less than half the

resistance of R6, the pulse from Q6 charges

C4 to more than twice the voltage (2.2 V) of

the P wave.

The differentiator 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 3.8 V. The negative

portion of the pulse (on the falling edge of the

input pulse) is shorted out by D4, so that all

that remains is a good –0.7 V due to the volt-

age 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

(with different weightings). Capacitor C7

smoothes out excessively spiky pulse com-

ponents. The final waveform is also shown

on the schematic diagram. The voltage

divider provides output signals with ampli-

tudes of 1 mV (for connection to the input of

ECG equipment) and 1 V. Insensitive equip-

ment that normally works with signals that

have already been amplified, such as sec-

ondary monitors, can be connected to the

second output.

A 9-V battery can be used as the power

source. The circuit draws only around 2.5 mA,

so the battery should last quite a while. The

circuit can be assembled in a few minutes

using the printed circuit board shown in Fig-

ure 3 . The ICs may be mounted in sockets. If

you cannot obtain the slide switch, you can

use two separate, single-pole toggle

switches, one for the operating voltage and

000058 - 12

Figure 1. A heartbeat in detail: the waveform for each contraction can be broken

down into various parts, identiﬁed using the letters P through U.

for a microcontroller in the schematic

diagram of this ECG simulator,

which is shown in Figure 2 . 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 4,194,304 Hz, a 16-Hz squarewave

signal appears at the Q18 output

(pin 10). Switch S1b picks up a sec-

ond signal (2 Hz or 1 Hz). The 16-Hz

signal clocks IC2, which is a decimal

counter with ten outputs. The sec-

ond signal is differentiated by the

combination of C3 and R3. Needle-

shaped pulses are present at pin 15

of the decimal counter (IC2), as indi-

cated on the schematic diagram.

These pulses reset the counter to

zero at the appropriate times. The

job of diode D2 is to block the nega-

tive portion of the pulses.

The decimal counter repeatedly

reaches a count of 9 and holds this

state, since pin 11 is connected to

the /Enable input (pin 13). It is only

reset when the reset pulse occurs.

The setting of the switch thus inﬂu-

ences the duration of the U interval,

which ultimately results in a simu-

lated heart rate of either 60 Hz or

120 Hz. If necessary, a 4-MHz crystal

can be used. This will reduce the

heart rate of the signal to 57.2 Hz or

114.4 Hz, 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 out-

puts. The first pulse (from pin 2) is

converted into the P wave by the

integrator R6/C4. The value of R6 is

chosen such that C4 charges expo-

nentially from 0 V to around 1 V. The

2mA3

1N4148

BT1

S1a

680k

R11

560k

CTRDIV10/

DEC

R12

R13

IC2

VDD'

470n

400mW

100n

OT1

OT2

Q18

3k9

330k

Q19

Q20

Q21

Q22

Q23

Q24

R14

S1b

22p

3k3

IC1

1N4148

4017

4521

CT=0

330n

1mV

IN2

220n

CT ≥ 5

4,194304MHz

82p

IN1

VSS"

RST

R10

R15

D 3

330n

1N4148

000058 - 11a

Figure 2. An analogue arbitrary signal generator using discrete components.

5/2000

Elektor Electronics

MEASUREMENT &TEST

ELEKTOR

ALKALINE

IC2

Figure 3. The ECG tester ﬁts in a small plastic enclosure.

COMPONENTS LIST

R13,R14 = 10k

Ω

D3 = LED, high efficiency

D5 = zener diode 3 V / 400 mW

IC1 = 4521

IC2 = 4017

R15 = 150

Ω

Resistors:

R1 = 1M

Ω

R2 = 3k Ω 9

R3,R4,R12 = 100k

Capacitors:

C1 = 82pF

C2 = 22pF

C3 = 220nF 5mm pitch

C4 = 470nF 5mm pitch

C5,C6 = 330nF 5mm pitch

C7 = 100nF 5mm pitch

Ω

Miscellaneous:

S1 = slide switch, 2-pole chanegover (Con-

rad Electronic o/n 708097)

BT1 = 9-V PP3 battery with clip-on leads

X1 = 4.194304 MHz quartz crystal

Enclosure 60x95x23 mm (Conrad Elec-

tronic o/n 522864)

3 wander (banana) sockets (2 mm or 2.6 mm)

Ω

R7 = 330k Ω

R8 = 3k

R9 = 2k Ω 2

R10 = 47k

Ω

R11 = 560k Ω

Semiconductors:

D1,D2,D4 = 1N4148

the other for switching the pulse rate. The

assembled circuit board ﬁts in the suggested

ABS enclosure, which also provides room for

the battery. Miniature banana jacks

(2.0 mm or 2.5 mm) are an excellent

choice for output connectors.

(000058-1)

Design editing: Karel Walraven.

Elektor Electronics

5/2000

R5 = 18k Ω

R6 = 680k

Ω

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