A. Analog amplifier.
1. Ramsey
Electronics, Inc. Model No. ECG 1.
This is available as either a Kitset for amateur
assembly, or as a built-up unit. It should be noted that -
Ramsey Electronics Inc. have
not authorised this use I am describing for their equipment; it is described
as for hobby use. Some material on this site is reproduced by their kind
permission. Some parts included in this
design are not needed for our purposes, and may be omitted or ignored.
Also, for safety reasons applicable to this use*, 2 components are slightly
altered, and will need substitution in the assembly. And two small additional
modules will be required, for the calibration network and lead selection
switching, (parts and assembly diagrams follow).
2. For those wishing to work 'from scratch', a list of parts is included, together with a design for a suitable printed circuit board to include both the amplifier and calibrating network. In addition some means of switching between the 12 ECG leads provides convenience in use, and a simple switchbox design is included for this purpose which uses readily available parts, and avoids the need to obtain rare multi-way switching devices; suitable small transparent plastic containers for these parts may be found on supermarket shelves! Please note that C1 and C2 capacitors should be robust; 600 volts working or higher, for safety reasons (see note *).care should be taken to ensure that correct polarity for the patient leads is observed. I have found it convenient to use a 9 volts NiMH rechargeable battery but an ordinary dry battery could be used. I incorporate facilities for charging in situ. If this facility is desired, a switched 2.1 VDC socket may be installed on the front panel, and wired into the battery leads so that it cuts out the supply to the unit when a 2.1 V plug is inserted; this socket allows a suitable charger to be connected, and also a convenient point to check on the battery state of charge, using a meter. I use 2 AAA size dry cells as a power source for the calibrating network; the drain on these is negligible and so they should last a very long time before replacement is needed.
The 1 MV pulse generator network is built in to
the left side at the back of the PCB.
Construction follows the usual procedures, soldering
in the parts according to the PCB overlay pattern. It is usual to start
with wire links, resistors and capacitors, and ending with semiconductors,
etc. A second small PCB design is included on this site, which is designed
for installing an 8-way DIL switch for patient-lead selection; however
this PCB is equally appropriate for use with 8 separate SPST switches,
and the larger size of these makes operation easier than using the miniature
DIL switches.
Anyone who regards the making of the PCBs too
arduous a task, may prefer to have them made professionally, and I can
supply suitable CAD files which most manufacturers can work to.
B. Digital sampler.
NB. Dataq Instruments in no
way endorse the use of their device for this purpose and accept no liability
for any consequences of using it.
I have used a small proprietary device (source details below), which as of August 2003, is available at small cost from its manufacturer. This device is ideal for our needs, and comes complete with software on a CD which also includes a very full instruction manual for its use. The unit fits snugly into my box, with its 9 pin connector protruding through the side, ready to accept a serial cable to connect to the PC. Using the included software, it is possible to configure this unit to display properly calibrated ECG traces, one at a time, on screen, and print them out, complete with an accurate (but not conventional) timebase/voltage grid. It is also possible to export the resulting files in various formats, including one suitable for import to a spreadsheet program, should you wish to use this for display purposes. I have written my own display/print program for this purpose, which displays 3 leads per screen, + lead/patient details, and it is thus possihle to print out a full 12 lead ECG, complete with (conventional) time/voltage grid and ID details, on 4 sheets of plain paper. This software is available at nominal non-profit cost, on floppy disk from the author, or by EMAIL If you request this by EMAIL, I will be grateful for a brief account of where you are, and for what purpose you intend to use it.
It should perhaps be noted that Dataq, the makers of the sampler are not able to guarantee continuing availability of the kit containing this sampler, but I am told that they will probably continue to supply something similar. It was originally introduced as a "Starter Kit" to introduce their range of digital equipment to potential users; I find it ideal for the limited needs of my system. This unit does not provide an opto-coupled output - See below. Alternative methods of sampling would include various software oscilloscope packages for the PC, which are available as Shareware or Freeware from various sources. If these are used some means of connecting the analog amplifier direct to the PC will be needed; (See note *). Other, and probably better alternatives would include hardware oscilloscopes or digital sampling devices. Software oscilloscope simulators depend for their accuracy on the particular sound card fitted to the computer, whch may have inadequate low frequency response for this purpose, which requires a very low figure.Any digital sampling device should if possible contain facilities for opto-coupling to the computer. I am at present (May 2005) working on developing such a device.
* NB. Safety
concerns :
It should be remembered that patient electrodes
connected to electronic apparatus are capable of transmitting harmful,
and possibly lethal currents in the reverse direction, from equipment to
patient, and that electronic equipment can BREAK DOWN, possibly allowing
such currents to leak. In commercial professional equipment, stringent
regulations include compulsory provision for opto-isolation between
any high voltage equipment and patient, and other additional safety measures.
In the case of this device, the hardware device is powered by low-voltage
battery, and capacitors are placed in the input leads, but it should be
remembered that a direct pathway from the computer
via the serial cable to sampler, and sampler to amplifier, exists.ANY
USER OF THE EQUIPMENT SHOULD SATISFY THEMSELF AS TO THE SAFETY TO PATIENTS
OF ITS USE, AND IF IN ANY DOUBT OBTAIN PROFESSIONAL ADVICE ABOUT THIS ASPECT.
It might be pointed out that capacitors such as those employed here
would block DC voltages, but if an AC supply were applied, would act like
resistors; reactance calculations indicate that small currents in the region
of a few milliamps could in fact be passed between the active electrodes
and the (earthed) right leg. It is possible to obtain a device for opto-coupling
the serial connection, but I have not tried it, and the only device I have
found so far, would cost the buyer more than the price of this whole equipment;
I have no doubt that it should be capable of ensuring complete safety.THE
USER MUST MAKE AN ASSESSMENT OF POSSIBLE RISKS INVOLVED.
Parts List:
(Optional ) Ramsey Electronics Model No. ECG 1 (Supplied as kitset
of parts, or as fully built-up unit)
Ramsey Electronics Inc, 590 Fishers Station Drive, Victor, New York
14564, USA.
Dataq DI-194RS Serial Port Data Acquisition Module and software
as supplied in DI-194 Starter Kit.
Dataq Instruments, 241 Springside Dr., Suite #200, Akron, Ohio 44333,
USA.
I have used plain plastic-covered wire for the leads, and in my experience this is quite adequate, but some users might prefer to make up a set from shielded cable. Small alligator clips will provide suitable connection to the patient extremity electrodes, and some form of electrode for the chest leads will be needed - ideally one of the miniature suction-type supplied with commercial machines - however, I have made my own suction electrode from 'bits and pieces'. Also I make my own re-usable electrodes from sheet stainless steel, and velcro-fastened straps, for the extremities.I find stainless steel sufficiently conductive, and much more resistant to corrosion than the conventional silver-plated brass electrodes.
Patient lead selector switch-box. - see enclosed design, including a second, small PCB..
Parts for assembly of an independent analog amplifier:
PCB - see enclosed design.
Capacitors:
C1, C2 0.1uF (600 VW or higher)
C6, C7, C8, C9 0.1uF.
C5 470pF ceramic disk.
C4 1uF electrolytic
C3, C10, C12, C13, C16 10uF electrolytic.
C11 100uF electrolytic.
Resistors: (all 1/4 watt)
R1, R2, R6 10Meg ohms
R3, R4, R13 10K ohms
R7, R8, R9, R10, R12 100K ohms
R11 4.7K ohms
R15 47K ohms
R1A 1K ohms [R18 on check plot]
R2A 25K ohms [R19 on check plot ]
R3A 20 Ohms [R20 on check plot ]
Potentiometers:
R5 100K ohms linear PC mount
RV3 100 Ohms horizontal trimpot
Semiconductors:
D1 1N4148 diode
Q1 2N3904 transistor
Q2 LM336-2.5 Reference diode.
D2 Red LED (5 mm.)
U1 LM324 Quad Op-Amp
1 14 pin DIP IC holder (optional - use if in doubt
about soldering in ICs)
Misc.:
1 DPDT PC-mount push-button switch (or any suitable SPST
power switch)
1 PC-mount 3.5mm. stereo jack (Optional -could be replaced
by 3 PC pins).
1 PC-mount 3.5mm. mono jack (Optional - could be
replaced by 3 PC pins)
1 3.5mm stereo plug (see above)
1 3.5 mm mono plug ( see above )
1 2.1mm. DC socket; panel mount, switched.
1 9 Volt battery clip connector with leads.
1 Holder for 9 volt battery (Can be fabricated from sheet metal)
1 Holder for 2 AAA batteries.
1 9 Volt NiMH re-chargeable battery
2 AAA dry batteries.
1 DPDT Momentary 'On' push button.
Tinned copper wire for links (from spare cut from resistors)
Small quantity of 3 core shielded cable.
Serial cable with 9 pin male plug.
1 Box to suit. (Dim. approx.150 x 175 x 35 mm).
Switchbox parts for patient lead selection:
8 x miniature slider switches (SPST) - OR - 1 8-way
DIL-switch
1 small piece of Veroboard - OR
- small PCB to author's design, suitable for above DIL switch
1 5 pin DIN socket - panel mount..
1 5 pin DIN line plug
1 Panel-mount 3.5 mm stereo socket.
4 Miniature alligator clips.
Insulated 'hook-up' wire (Q.S.).(Sufficient !)
Suitable box - e.g. clear plastic 90 x 35 x 35 mm
Software: The program ECGDATAQ.EXE is available by EMAIL, or on floppy disk, by request. The disk will contain also EASYTRAX files for my PCB patterns, with overlays, and the full PROTEL EASYTRAX programs; these will make it possible to display the PCBs in their original form, suitable for sending to just about any professional PCB manufacturer.The Easytrax file for my main board may also be downloaded from this site (see link on PCB page).