Medical Devices Division

http://www.si.mahidol.ac.th/th/division/shcd/

Leakage Current Standards Simplified

IEC 60601-1 and related standards provide detailed explanations of testing strategies for leakage current. Understanding those tests and applying them effectively is a critical aspect of making electromedical equipment.

By: Leonard Eisner, Robert M. Brown, and Dan Modi

Leakage current is one of the most stringent, yet telling, parameters of possible danger to patients or caregivers. It does not take much electric current flowing through the human body to cause harm. This is especially true for patients with weakened immune systems. The potential risk is why measurement of leakage current in electrical medical products is so critical.

Leonard Eisner

Robert M. Brown

Dan Modi

The IEC 60601-1 standard, “Medical Electrical Equipment—Part 1: General Requirements for Safety and Essential Performance,” describes tests for leakage current, as do a number of related national standards.¹ This article aims to simplify these tests and the requirements of related standards and explain the rationale behind them. For an overview on other tests in the IEC 60601-1 standard, please refer to “A Primer for IEC 60601-1.”²
Leakage Current
As noted in NFPA 99: “Standard for Health Care Facilities,” 2002 Edition, just three conditions occurring simultaneously can result in a shock to patient or caregiver:
• One part of the body is in contact with a conductive surface.
• A different part of the same body is in contact with a second conductive surface.
• A voltage source drives current through the body between those two points of contact.³
Figure 1 illustrates these three conditions along with eight separate conditions that should be analyzed when evaluating the electrical safety of medical devices.
Leakage current is measured to ensure that direct contact with the medical equipment is highly unlikely to result in electrical shock. The tests are designed to simulate a human body coming in contact with different parts of the equipment. The measured leakage current values are compared with acceptable limits. These limits are based on the type of product being tested, the point of contact with the product (earth, enclosure, patient), and the operation of the product under normal and single-fault conditions.

Figure 1. Electrical shock and analysis points for medical devices (click to enlarge).

Leakage current measurements are performed with the product energized and in all conditions, such as standby and full operation. The mains supply voltage is normally delivered via an isolation transformer to the product. Per IEC 60601-1, the mains supply voltage should be at 110% of the highest-rated supply voltage and at the highest-rated supply frequency. This means that a product rated for operation at 115 V ac, 60 Hz and 230 V ac, 50 Hz would be tested at 253 V ac and a line frequency of 60 Hz.
Measuring Device

Figure 2. Human body model for IEC 60601-1 (click to enlarge).

The measuring device as defined in IEC 60601-1 is made up of two parts. One is a voltmeter with a Ž1-Mž input impedance and a frequency characteristic that is flat from dc to 1 MHz. The instrument must show the true rms value of the voltage across the measuring impedance. Indicating error must not exceed ±5%. The second part of the measuring device is a circuit, as shown in Figure 2. The circuit provides a resistance of approximately 1000 ž and frequency characteristics that take into account the human body and the risk of ventricular fibrillation.
The frequency characteristic of the circuit is based on information from a number of different studies of how electric current relates to ventricular fibrillation. Most of these studies were performed in the late 1960s through the 1970s.

Figure 3. Frequency characteristic for human body model of IEC 60601-1 (click to enlarge).

The study data showed that the risk of ventricular fibrillation is highest for frequencies from 10 to 200 Hz. The risk is slightly reduced at 1000 Hz. It rapidly decreases for frequencies above 1000 Hz. The frequency characteristic of the circuit, which is shown in Figure 3, is designed to mimic the risk of ventricular fibrillation. It has a relatively flat frequency response to 1000 Hz, then a rapid roll-off.
A number of commercially available instruments are designed to perform leakage current measurement. These instruments should have the capability to measure with the correct accuracy, input impedance, and frequency characteristic.
To illustrate the various types of leakage currents and the points from which they are measured, the measuring device in this article will be represented in the figures by a cartoon character called MD. This cartoon character will touch the various points to show where connections would be made for leakage tests.
Leakage Current Measurement Conditions

a General equipment. b  No accessible protective earth parts, no means of protective earthing of another device, mobile x-ray equipment, mobile equipment with mineral insulation (refer to notes 2 & 4, Table IV, IEC 60601-1). c  Permanently installed protective earth conductor (refer to note 3, Table IV, IEC 60601-1) (click to enlarge).

Leakage currents are measured during both normal conditions and single-fault conditions.
Normal conditions are those in which all protection against safety hazards is intact. The leakage current test is performed with the medical equipment under normal use conditions. The equipment is energized in both standby and full operation. Reversing line and neutral on the supply mains is considered a normal condition, as this occurs frequently.
There are a number of single-fault conditions. These include the opening of protective ground and the opening of each conductor on the mains supply, one at a time.
For medical devices, additional single-fault conditions may be required, depending on the classification of the medical equipment. These can include 110% of mains voltage applied on signal input/output parts (SIP/SOP) during patient leakage and enclosure leakage tests. Mains voltage on applied parts is another fault condition.
Connection
Connection for most tests is straightforward, with the measuring device connected to the conductive point under test. For example, if measuring equipment with a metal enclosure, the measuring device is connected to an unpainted portion of the enclosure. To achieve measurements on a product that has an enclosure or other measurement point made of an insulating material, a piece of conductive foil no larger than 20 ¥ 10 cm (simulating the size of the palm) is placed in direct contact with the measurement point. If the surface contacted by the patient or operator is larger than 20 ¥ 10 cm, the size of the foil is increased accordingly. The foil is normally shifted to determine the highest value of leakage current.
Acceptable Levels of Leakage Current

Figure 4. Earth leakage current (click to enlarge).

IEC 60601-1 specifies allowable limits for leakage current measurements. These limits depend on the test being performed, the classification of applied parts, and whether under normal or single-fault conditions. Leakage limits for IEC 60601-1 are shown in Table I.
Leakage Tests
In this section of this article, leakage current is simplified to illustrate typical measurements for each type of leakage test. This section is not a substitute for IEC 60601-1, related national standards, or any particular standards covering the specific medical equipment under test.

Figure 5. Enclosure leakage current (click to enlarge).

Earth Leakage Current. The earth leakage current test measures the leakage current flowing from the protective earth of the medical device through the patient (in this case, the measuring device) back to the protective earth conductor of the power cord. This is the total leakage current from all protectively earthed parts of the product. This test applies to Class I devices.
As shown in Table I, there are three different sets of limits for earth leakage current. The first set is for general equipment. The second is for equipment that has no accessible protectively earthed parts and no means for the protective earthing of other devices. These limits also apply to mobile X-ray equipment and mobile equipment with mineral insulation. The third set of limits is for devices with a permanently installed protective earth conductor.

Figure 6. (a) Patient leakage current for a Type B applied part, (b) patient leakage current for a Type BF applied part, and (c) patient leakage current for a Type CF applied part (click to enlarge).

Figure 4 illustrates the basic measurement of earth leakage current in a piece of medical equipment using a standard detachable power cord. Such measurements are taken during normal conditions as well as with single-fault conditions, which is the interruption of one power supply con-
ductor (line or neutral) at a time.
Enclosure Leakage Current. The enclosure leakage current is measured from any part of the enclosure through the measuring device to earth, and between any two parts of the enclosure. This applies only to parts of the enclosure not connected to protective earth. See Figure 5.
Enclosure leakage current is measured during normal conditions as well as during single-fault conditions, in which one supply conductor at a time is interrupted, and, if applicable, the protective earth conductor is opened.
Patient Leakage Current. This is the leakage current measured from any applied part to ground. Depending upon the type of applied part (B, BF, or CF), there are different requirements for how the leakage tests are performed and the type of fault conditions. Type CF applied parts have the most stringent test requirements.

Figure 7. Mains voltage on applied parts (click to enlarge).

The leakage current for Type B applied parts is measured between all applied parts tied together and ground, as illustrated in Figure 6a.
Type BF applied parts must be separated into applied parts having different functions. The leakage current is measured between all applied parts with similar function and ground. See Figure 6b.
Leakage current for Type CF applied parts must be measured from each applied part to ground individually. See Figure 6c.
Patient leakage is measured during normal conditions as well as during single-fault conditions consisting of the interruption of one supply conductor at a time and the opening of the protective earth conductor, if applicable.

Figure 8. Mains voltage on SIP/SOP (click to enlarge).

Mains Voltage on Applied Parts. Type F applied parts have an additional IEC 60601-1 requirement. The leakage current of each applied part is measured while applying 110% of mains voltage through a current-limiting resistor. During this test, signal input and output parts are tied to ground. The polarity of the mains voltage to the applied part is reversed, and leakage current is measured for both conditions. See Figure 7.
Mains Voltage on Signal Input and Signal Output. Type B applied parts must have the additional single-fault condition of 110% of mains applied to all signal input and signal output parts during patient leakage measurement. This is only applicable to Type B applied parts if inspection of the circuit shows that a safety hazard exists. See Figure 8.
Patient Auxiliary Leakage Current. This test measures the leakage current between any single applied part and all other applied parts tied together. Patient auxiliary leakage current is measured under normal as well as single-fault conditions. See Figure 9.
National Differences on Leakage Current

Figure 9. Patient auxiliary leakage current (click to enlarge).

United States. There are three major differences between IEC 60601-1 and UL 60601-1 for the measurement of leakage current.⁴ The UL standard incorporates the requirements of NFPA 99 and ANSI/AAMI ES1, “Safe Current Limits for Electromedical Apparatus.”⁵ NFPA 99 incorporates the requirements of the U.S. national electrical code (NFPA 70) that relate to healthcare facilities. ANSI/AAMI ES1 defines safe leakage current limits within the three parameters of frequency, equipment function, and intentional contact with patient. It is likely that ANSI/AAMI ES1 will be withdrawn when the third edition of IEC 60601-1 is adopted in the United States by ANSI/AAMI.
UL 60601-1 differentiates between patient-care equipment (6 ft around and 7.5 ft above the patient) and non-patient-care equipment for leakage current tests. The typical leakage current values for a Class I device are 300 µA in a patient-care area and 500 µA outside that area. For a Class II device, the values are 150 µA in a patient-care area and 250 µA outside that area.
UL 60601-1 allows opening of the earth conductor and one of the supply connections simultaneously for non-patient-care equipment. This would be considered a double fault under IEC 60601-1.
European Union and Australia. There are currently no differences between IEC 60601-1 and EN 60601-1 and AS/NZS 3200.1 with respect to leakage current.⁶

Figure 10. Japanese leakage current measurement circuit (click to enlarge).

Canada. There is one difference between IEC 60601-1 and CAN/CSA C22.2 No. 601.1 on leakage current.⁷ If the medical device is to carry the CSA mark, production-line leakage tests are required.
Japan. There are only a few minor differences between IEC 60601-1 and JIS T 0601-1 on leakage current.⁸
In order to differentiate between the various patient leakage measurements (normal and single-fault), JIS T 0601-1 adds the clarifying nomenclature Patient Leakage I for patient leakage in a normal condition, Patient Leakage II for patient leakage in a single-fault condition of mains voltage on SIP/SOP, and Patient Leakage III for patient leakage in a single-fault condition of mains voltage on a floating patient applied part.
JIS T 0601-1 also specifies that the risk of external voltage on SIP/SOP is very low for a device that has been evaluated to IEC 60601-1-1 with its accessories. Hence the leakage current measurements in a single-fault condition with mains applied on SIP/SOP need not be performed for such a product.
There is only one significant national deviation for leakage current measurement in Japan. For leakage currents with a frequency component greater than 1 kHz, the leakage currents must not exceed 10 µA. The IEC 60601-1 measuring device is used, but with the 10-kž resistor bypassed by a switch. See Figure 10.
Conclusion
A key step before performing leakage testing is to determine the class of the medical equipment under test and to identify the type of applied parts. Once these are determined, the appropriate tests and corresponding limits can be established. The applicable leakage tests can then be conducted under the appropriate single-fault conditions.
The leakage testing outlined here is based on the compliance testing requirements of IEC 60601-1. There are no specific requirements in that standard for leakage current measurements during production testing. Nonetheless, the manufacturer should do such testing. This can take the form of good manufacturing practice, routine production testing, or sampling.
References
1. IEC 60601-1, “Medical Electrical Equipment—Part 1: General Requirements for Safety and Essential Performance” (Geneva: International Electrotechnical Commission, 1995).
2. Leonard Eisner, Robert M Brown, and Dan Modi, “A Primer for IEC 60601-1,” MD&DI 25, no. 9 (2003): 48–58.
3. National Fire Protection Association, NFPA 99, “Standard for Health Care Facilities” (Quincy, MA: NFPA, 2002).
4. UL 60601-1, “Medical Electrical Equipment, Part 1: General Requirements for Safety” (Northbrook, IL: Underwriters Laboratories, 2003).
5. ANSI/AAMI ES1:1993, “Safe Current Limits for Electromedical Apparatus” (Arlington, VA: AAMI, 1993).
6. AS/NZS 3200.1, “Medical Electrical Systems” (Sydney: Standards Australia, 1998).
7. CAN/CSA-C22.2 NO. 601.1, “Medical Electrical Equipment—Part 1: General Requirements for Safety” (Mississauga, ON, Canada: Canadian Standards Association, 1995).
8. JIS T 0601-1, “Medical Electrical Equipment—Part 1: General Requirements for Safety” (Tokyo: Japanese Standards Association, 2000). 
Copyright ©2004 Medical Device & Diagnostic Industry

Electrical Safety

Electrical safety is very important in hospitals as patients may be undergoing a diagnostic or treatment procedure where the protective effect of dry skin is reduced. Also patients may be unattended, unconscious or anaesthetised and may not respond normally to an electric current. Further, electrically conductive solutions, such as blood and saline, are often present in patient treatment areas and may drip or spill on electrical equipment.

• Electric Current
• Leakage Current
• Extension Leads
• Double Adaptors
• Equipment Classification
  • Class I
  • Class II
  • Defibrillator-Proof
• Protective Devices
  • Residual Current Devices (RCD)
  • Line Isolation overload Monitors (LIMs)
  • Equipment Earthing
• Area Classification
  • Body Protection Area
  • Cardiac Protected Area
• Other Electrical Issues
  • Extension Leads
  • Double Adapters
  • Main Extension Devices
  • Power Boards
  • Installation of Additional Power Points

Electric Current

Injuries received from electric current are dependent on the magnitude of current, the pathway that it takes through the body and the time for which it flows.

The nature of electricity flowing through a circuit is analogous to blood flowing through the circulatory system within the human body. In this analogy the source of energy is represented by the heart, and the blood flowing through arteries and veins is analagous to current flowing through the conductors and other components of the electric circuit.

The application of an electric potential to an electric circuit generates a flow of current through conductive pathways. This is analogous to the changes in blood pressure caused by contraction of cardiac muscle that causes blood to flow into the circulatory system. For electric current to flow there must be a continuous pathway from the source of potential through electrical components and back to the source.

Leakage Current

Electrical components and systems are encased in non conducting insulation, to ensure that the electric current is contained and follows the intended pathways. If the insulation deteriorates or breaks down, current will leak through the insulation barrier and flow to earth. This may be through the protective earth conductor or through the operator.

Medical equipment and clinical areas are fitted with a number of protective devices to protect the patient and operator from harmful leakage currents.

Extension Leads

Extension leads are not permitted in clinical areas of RCH organisations. They may cause high earth resistance and excessive earth leakage current. An extension lead can allow equipment to be powered from areas other than the relevant protected treatment area. The power from the other area may not be protected to the same level as the power in the treatment area.

As the connection between the extension lead and the equipment mains cable is often on the floor there is a high danger from fluid spills, tripping and damage to the mains cable by trolleys when an extension lead is used.

Double Adaptors

Double adaptors must not be used in RCH organisations. They may not sit securely in a wall outlet, may not be able to provide adequate earth protection and may cause overloading, overheating, fire or loss of electrical supply.

EQUIPMENT CLASSIFICATIONS

There are several methods of providing protection for operators and patients from electrical faults and harmful leakage current.

Class I

Class I equipment is fitted with a three core mains cable containing a protective earth wire. Exposed metal parts on class I equipment are connected to this earth wire.

Should a fault develop inside the equipment and the exposed metal comes into contact with the mains, the earthing conductor will conduct the fault current to ground. Regular testing procedures ensure that earthing conductors are intact, as the integrity of the earth wire is of vital importance.

Class II

Class II equipment is enclosed within a double insulated case and does not require earthing conductors. Class II equipment is usually fitted with a 2-pin mains plug. An internal electrical fault is unlikely to be hazardous as the double insulation prevents any external parts from becoming alive. Class II or double insulated equipment can be identified by the class II symbol on the cabinet.

Class II Symbol:

Defibrillator-Proof

Some medical equipment within the hospital is classified as defibrillator proof. When a defibrillator is discharged through a patient connected to defibrillator proof equipment, the equipment will not be damaged by the defibrillator's energy. Defibrillator proof equipment can remain connected to the patient during defibrillation. It is identified by one of the following symbols.

Defibrillator proof symbols.

Body protected Cardiac protected

PROTECTIVE DEVICES

Most patient care areas in the hospital are fitted with protective devices. These devices are regularly tested, in accordance with the relevant guidelines published by Standards Australia. The level of protection provided is dependent upon the device and the area in which it is located.

Residual Current Devices (RCD)

RCD's (safety switches) are used in patient treatment areas to monitor and protect the mains supply. RCD's sense leakage currents flowing to earth from the equipment. If a significant leakage current flows, the RCD will detect it and shut off the power supplied to the equipment within 40 milliseconds. Hospital RCD's are more sensitive than those fitted in homes. A hospital RCD will trip at 10 milliamperes leakage current.

Power outlets supplied through an RCD have a 'Supply Available' lamp. The lamp will extinguish when the RCD trips due to excessive leakage current.

Resetting a RCD

• Lamp indicates supply is no longer available
• Disconnect all equipment from the supply
• Operate the reset button or lever on the supply panel and the 'Supply Available' lamp should illuminate. If not, contact Biomedical Engineering.
• Connect an item of equipment. If the RCD trips again, then this is the faulty item and should be labelled and sent to Biomedical Engineering.
• If the RCD does not trip, continue connecting equipment until the RCD trips. The last piece of equipment connected to the supply is most likely to be faulty as it will have caused the RCD to trip. Remove the faulty item from service, label it and send to Biomedical Engineering as mentioned above.

Line Isolation overload Monitors (LIMs)

In critical life support applications where loss of power supply cannot be tolerated, special power outlets powered by isolation transformers are installed.

Line Isolation Monitors are installed to continually monitor electrical leakage in the power supply system. If an electrical fault develops in a medical device connected to an isolated power outlet, the LIM will detect the leakage current. The LIM will alarm and indicate the level of leakage current, but will not shut off the electric supply.

The faulty equipment can be identified by un plugging one item of equipment at a time from the supply until the alarm stops sounding. Equipment that is not faulty may be reconnected. Faulty equipment should be appropriately labelled and sent to Biomedical Engineering for repair.

The LIM also monitors how much power is being used by the equpiment connected to it. If too much power is being used, the LIM will alarm and indicate that there is an overload. The power used must be reduced immediately by moving some equipment to another circuit as soon as possible until the alarm stops sounding. Failure to reduce the load on the LIM will result in the circuit breaker tripping and loss of power to the circuit.

Equipotential Earthing

Equipotential earthing is installed in rooms classified as 'Cardiac Protected' electrical areas. Equipotential earthing in treatment areas used for cardiac procedures is intended to minimise any voltage differences between earthed parts of equipment and any other exposed metal in the room.

This reduces the possibility of leakage currents that can cause microelectrocution when the patient comes into contact with multiple items of equipment, or if the patient happens to come into contact with metal items in the room whilr they are connected to a medical device.

All conductive metal in an equipotential area is connected to a common equipotential earth point with special heavy duty cable.

AREA CLASSIFICATIONS

Body Protected Area

• These areas are designed for procedures in which patients are connected to equipment that lowers the natural resistance of the skin. Applied parts such as electrode gels, conductive fluids entering the patient, metal needles and catheters provide an easy pathway for current to flow.
• The main occurrence of injury from Body-Type procedures is from high current levels causing electric shock. A direct connection to the patient's heart is not present so the risk of 'Microelectrocution' - fibrillation from minute current levels - is reduced.
• Residual Current Devices (RCD) or Isolation Transformers and Line Isolation Monitors (LIMís), are used in Body Protected areas to provide protection against electrocution from high leakage currents. Body-Protected Areas are identified with this sign.

Cardiac Protected Area

• Where the procedure involves placing an electrical conductor within or near the heart, protection against fibrillation induced from small leakage currents is required. Electrical conductors used in these procedures include cardiac pacing electrodes, intracardiac ECG electrodes and intracardiac catheters.
• Equipotential earthing in conjunction with RCD's or LIM's provides protection against microelectrocution in Cardiac-Type procedures.
• Fault currents are reduced to magnitudes that are unlikely to induce fibrillation. Used in conjunction with RCD's or LIM's, the magnitude and duration of any fault currents sourced from equipment are limited.
• Cardiac-Protected Areas are identified with this sign.

Other electrical issues

This policy aims to provide guidance to those who find that they need more electrical outlets than those available, or that the existing electrical outlets are inconveniently located.

As extension leads and multiple outlet power boards can introduce additional hazards into an area the following procedures should be observed.

Extension leads

Approved extension leads (AS 3760, 1996) may be used in some areas within the hospital but MUST NOT BE USED IN PATIENT AREAS. All electrical extension leads must be tagged with an Engineering Department maintenance tag, and require a yearly safety inspection and test, via the Engineering Department.

Double adapters

Double adapters may cause overloading or equipment earthing problems and are not to be used in WCH

Mains extension device

The only mains extension device that is to be used in "Patient care areas" is the 4-way or 8-way portable Core Balance Unit.

The Biomedical Engineering Department must approve all units prior to use. These units contain a safety switch and can detect excessive leakage current and disconnect the power in the event of a hazardous situation.

Care must be exercised in the use of a portable Core Balance Unit. It should be located off the floor and in a position that will protect it from physical abuse and possible entry of fluids. These devices are expensive and easily damaged. The device must be sent to Biomedical Engineering every 6 months for safety testing.

Power boards

Approved multiple-outlet power boards can be used across RCH but must not be used in patient care areas, except areas approved by the Biomedical Engineering Department.

The power boards must have overload protection, be fitted with internal safety shutters that protect unused outlets and be fitted with an on/off switch for each outlet.

Installation of additional power points

All of the above mentioned devices are intended to overcome a temporary inadequacy in the electrical installation. If a Department/Unit is likely to have a long term need for such mains extension devices a Project Initiation Request should be submitted to the Engineering Department for the installation of additional power points.

Cranial Electrotherapy Stimulator

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Current generated flows through clips placed on the earlobes

Output current adjustable from 80 to 600 microamperes

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Parts:

R1_____________1M5 1/4W Resistor

R2____________15K 1/4W Resistor

R3___________100K Linear Potentiometer

R4_____________2K2 1/4W Resistor

C1___________330nF 63V Polyester Capacitor

C2___________100µF 25V Electrolytic Capacitor

D1_____________3mm. Red LED

IC1___________7555 or TS555CN CMos Timer IC

IC2___________4017 Decade counter with 10 decoded outputs IC

SW1___________SPST Slider Switch

B1______________9V PP3 Battery

Clip for PP3 Battery

Two Earclips with wires (see notes)

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Device purpose:

Owing to the recent launching in Europe of Cranial Electrotherapy Stimulation (CES) portable sets, we have been "Electronically Stimulated" in designing a similar circuit for the sake of Hobbyists. CES is the most popular technique for electrically boosting brain power, and has long been prescribed by doctors, mainly in the USA, for therapeutic reasons, including the treatment of anxiety, depression, insomnia, and chemical dependency. CES units generate an adjustable current (80 to 600 microAmperes) that flows through clips placed on the earlobes. The waveform of this device is a 400 milliseconds positive pulse followed by a negative one of the same duration, then a pause of 1.2 seconds. The main frequency is 0.5 Hz, i.e. a double pulse every 2 seconds. Some people report that this kind of minute specialized electrical impulses contributes to achieve a relaxed state that leaves the mind alert.
Obviously we can't claim or prove any therapeutic effectiveness for this device, but if you are interested in trying it, the circuit is so cheap and so simple to build that an attempt can be made with quite no harm.

Circuit operation:

IC1 forms a narrow pulse, 2.5Hz oscillator feeding IC2. This chip generates the various timings for the output pulses. Output is taken at pins 2 & 3 to easily obtain negative going pulses also. Current output is limited to 600µA by R2 and can be regulated from 80 to 600µA by means of R3. The LED flashes every 2 seconds signaling proper operation and can also be used for setting purposes. It can be omitted together with R4, greatly increasing battery life.

Notes:

• In order to obtain a more precise frequency setting take R1=1M2 and add a 500K trimmer in series with it.
• In this case use a frequency meter to read 2.5Hz at pin 3 of IC1, or an oscilloscope to read 400msec pulses at pins 2, 3 or 10, adjusting the added trimmer.
• A simpler setting can be made adjusting the trimmer to count exactly a LED flash every 2 seconds.
• Earclips can be made with little plastic clips and cementing the end of the wire in a position suited to make good contact with earlobes.
• Ultra-simple earclips can be made using a thin copper foil with rounded corners 4 cm. long and 1.5 cm. wide, soldering the wire end in the center, and then folding the foil in two parts holding the earlobes.
• To ensure a better current transfer, this kind of devices usually has felt pads moistened with a conducting solution interposed between clips and skin.
• Commercial sets have frequently a built-in timer. Timing sessions last usually 20 minutes to 1 hour. For this purpose you can use the Timed Beeper the Bedside Lamp Timer or the Jogging Timer circuits available on this website, adjusting the timing components in order to suit your needs.

Electronic Stethoscope

555 Timer as an A/D converter

555 Timer as an A/D converter

I had a Basic Stamp project that needed to measure a nominal 12 volt battery, and I wanted a simple solution. This is the simplest I could come up with. The 555 timer will put out positive pulses. The pulse width is inversely proportional to the difference in voltage between the voltage at "ANALOG IN" and the voltage of the 4.7uF capacitor(let's say 2.5 volts). To calibrate this circuit, hook it up to a Basic Stamp measuring positive pulses, and give the circuit a known voltage. Let's say you get the number 2092 when you give the circuit 15 volts. Your coefficient is 2092 * (15 - 2.5) = 26150. Now you are ready to measure voltage with your Basic Stamp. Use the formula: voltage = 26150/pulse + 2.5 . You will have to modify this to work within the limits of the Basic Stamp's math. The accuracy of this circuit rivals many digital voltmeters within the range I tested it (6 volts to 18 volts), about the same as a 10 bit A/D converter. The accuracy will shift with the processor clock and the +5 supply, so it is pretty good. Conversion time is under 1/10 second. Please note it will not measure voltages below 5 volts. Also, check the accuracy of your +5 volts. If it is 5.2 volts, you will need to use 2.6 in the formula. A sample program listing follows.

'uncomment the debug lines to get pulse value while calibrating
loop:
'debug cls
pulsin 0,1,w2 'I used pin 0
'debug w2
w1=26150 'This is the coefficient you will need to calibrate.
w4=w1/w2
w3=w4*100 'I am going to get around the integer-only Stamp math.
w4=w2*w4
w1=w1-w4*10 'remember the Stamp has left-to-right math
w4=w1/w2
w3=w4*10+w3
w4=w2*w4
w1=w1-w4*10
w4=w1/w2
w3=w4+w3
w3=w3+250 '250 is really 2.5 volts
debug w3,"volts * 100" 'we get a reading in hundredths of volts
goto loop

Power Supply Circuit

If you want circuit to use a higher current regulator - up to 5A regulators are available. Please see here circuit,it good idea very much.

In this circuit, the transistor Q1 is used to share some of the current supplied. The voltage regulator maintains the output voltage, and still operates short circuit protection. The current that the transistor takes is set by the resistor values R1 and R2, and is I = R2/R1 * RegulatorCurrent. The example shown converts a 1A regulator into a 5A (4A for the transistor plus 1A for the regulator) voltage regulator circuit. See the LM340 datasheet for a full description of this circuit.