Schematics Electronic Thermometer Circuit

There are plenty of digital thermometers with ·1ºC displays but the accuracy is about + – 1ºC and you can not calibrate them. I made this thermometer circuit from components that were available at the local electronics hobby shop and it was an educational experience. If you want a simple modern circuit that requires no calibrating I recommend you look at a circuit that uses the LM34 or LM35.

Electronic Thermometer Circuit Description

In the circuit above T2 is a fixed current device that sinks approximately 70 Micro amps. P1 is adjusted so that this current does not vary with temperature by balancing the negative temperature coefficient of T1 with the positive coefficient of T2. R3 reduces the effect that adjusting P1 has on the set current.

T4 provides a current proportional to the absolute temperature and is adjusted to equal the fixed current through T2 at 0ºC. Thus at any temperature other than 0Cº a current must flow into or out of the voltage divider formed by R7 and R6 and provide a voltage for the meter as it flows through P3 and R5 .

P3 is adjusted so that at some standard temperature the 200 milli Volt meter reads correctly. Simply put T4 is a Kelvin thermometer and T2 subtracts 273.15 to convert it to a Centigrade one.

The LM334z (T4 - T2) is a readily available device that provides a current source that is directly proportional to the absolute temperature ( ºK ) over the range 0 to 70°C for larger ranges there are other devices in the same family that can be substituted.

Great care was taken to ensure that temperature changes to the body of the instrument do not cause changes to the readings. T2 has changes nulled by P1 but the resistors themselves change value with temperature and this is also canceled out as follows. Any change to the resistance of R1,P2 and P1,R2,R4, R3 with the temperature of the instrument body will cause changes to the current of T4 and T2 but the same change will occur to P3 and R5 and keep the voltage across them constant.

For example if all the resistors went up 10% there would be 10% less current through T4 and 10% less through T2 and the difference in the currents would be 10% less through P3 and R5 but as they would have 10% more resistance the output voltage would stay the same. Any change in the output of T3 with temperature has no effect.

T3 regulates the voltage to 5 Volt and C1 and C2 provide filtering.


Electronic Thermometer Construction

The circuit was laid out paying attention to keeping T2 and T1 close together and the resistors that could cause temperature drift in close proximity. An Acrobat reader .pdf file of this is provided and can be printed full size and used to hand make a PCB using a etch resist pen or to photo etch a board if required

The sensors T4 is connected by 3 wires. Thin twin shielded wire was used with the shield connected to the + rail (V+). The connection to the multimeter was made with flying leads fitted with pin sockets insulated by heat-shrink. Left shows a bottom view of T4.

Power consumption is a couple of milli-amps so it can be powered by a 9 Volt battery. A panel meter if used can be run from the 5 Volt supply but it must be a meter that can be run with a common power supply and floating inputs.

T1 and T2 should be in close thermal contact. Winding copper wire around them and applying heat-sink thermal grease will aid stability. The red wire with the knot just passes through the board as the switch is on the opposite side of the board to the battery connector. The 4 unused holes (top right) are are for the power leads to an optional internal panel meter.

Source: Electronic Thermometer Circuit

read more "Schematics Electronic Thermometer Circuit"

Schematics Measuring Milliohms with a Multimeter

Measuring low values of resistance is not easy. Low cost multimeters do not include a milliohm measurement range and specialist equipment is expensive. The simple circuit described here allows milliohm measurements to be made safely on a standard multimeter.

The circuit consists of little more than a 6 V voltage regulator and a mains adapter capable of supplying around 300 mA at 9 to 12 V.

The circuit supplies a fixed current output of 100 mA or 10 mA selected by switch S1. This connects either the 60 Ω or 600 Ω resistor into the constant current generator circuit. The resistor values are produced by paralleling two identical resistors; 120 Ω and 1.2 kΩ from the E12 standard resistor range. Two test leadswith probes are used to deliver current to the test resistance. The resultant voltage drop is measured by the multimeter (M1). With the test current set to100 mA a measurement of 1 mV indicates a resistance of 10 mΩ. At 10 mA (with S1 in the position shown in the diagram) a measurement of 1 mV indicates a resistance of 100 mΩ while 0.1 mV is equal to 1 mΩ. Diode D1 protects the meter from too high an input voltage.

With the voltmeter connected as shown in the diagram it measures not only the voltage drop across RX but also that produced by the resistance of the test leads, and probes. To make a true measurement, first touch the probes close together on the same lead of the test resistance and note the reading, now place the probes across the test resistance and note the reading again. The first reading measures just the test leads and probes while the second includes the resistance RX. Subtract the first measurement from the second to get the value of RX.

The accuracy of the measurements are influenced by the contact resistance of switch S1, the precision of resistors R1 to R4, the 6 V supply level and of course the accuracy of the measuring voltmeter.

For optimum decoupling C1 should be fitted as close as possible to pin1 of IC1. An additional electrolytic capacitor of around 500 µF can be used at the input to the circuit if the input voltage from the AC power adapter exhibits excessive ripple. (Author: Klaus Bertholdt, published in Elektor Magazine (Germany)

read more "Schematics Measuring Milliohms with a Multimeter"

Schematics Transistor Tester Circuit

Transistor tester is a very simple circuit that can b e used to check the hfe of transistors. Both PNP and NPN transistors can be checked using this circuit. Hfe as high as 1000 can be measured by using this circuit. The circuit is based on two constant current sources build around transistors Q1 and Q2.The Q1 is a PNP transistor and the constant current flows in the emitter lead. The value of constant current can be given by the equation; (V D1 -0.6)/ (R2+R4).The POT R4 can be adjusted to get a constant current of 10uA.

The Q2 is an NPN transistor and the constant current flows into the collector lead. The value of this constant current can be given by the equation; (VD2-0.6)/(R3+R5).The POT R5 can be adjusted to get a constant current of 10uA.This constant current provided by the Q1 circuit if the transistor under test is an NPN transistor and by Q2 circuit if the transistor under test is a PNP transistor is fed to the base of transistor under test. This current multiplied by the hfe flows in the collector of the transistor and it will be indicated by the meter. The meter can be directly calibrated to read the hfe of the transistor.

Transistor Tester Circuit and Parts List

Notes

• Assemble the circuit on a general purpose PCB.
• The circuit can be powered from a general purpose PCB.
• J1 and J2 are transistor sockets.
• The Zener diodes must be rated at least 400mW.

Source: Transistor Tester Circuit

read more "Schematics Transistor Tester Circuit"

Schematics RF Power Meter 0-500MHz

RF Measurement has been an expensive work so far the cost of measuring instruments are concerned. RF Meter is based on PIC16F876 microcontroller, AD8307 and 2x20 LCD display. Frequency range measurement is 0-500 MHz and Full documentation is included.

This power meter kit has two push buttons to control the functions. Very cost effective and simple to use. 4 Push buttons used in this kit are right angle type and are sufficiently long enough to come out of the box, if you decide to house this meter in a case.

 

Download :
Project Documentation,
Schematic,
Hex Code.

Source: RF Power Meter 0-500MHz With PIC16F876

read more "Schematics RF Power Meter 0-500MHz"

Schematics Basic Multimeter Circuit

A number of shunts and multipliers selected by a switch can be used in association with a single basic meter to form a multirange instrument, known as a multimeter. this is capable of measuring volts, current and resistance.

A multirange meter can be constructed in two units, the first containing the 0-1mA meter movement with switches to select various shunt and series resistors to give six d.c. current ranges up to 1 amp and eight d.c. voltage ranges up to 1000 volts. An internal battery provides an ohms range readable up to 200,000 ohms which corresponds with the first division of the meter (0.02mA)

An Add-on unit contains a meter rectifier with associated switched series resistors to give four a.c. voltage ranges up to 100 volts while additional shunt and series resistors (Ra and Rb) extend the range to 10A and 5Kv. When using the add-on unit the main instument is set to measure 1mA FSD and the add-on unit is connected to its terminals by its lugs.

The series resistors are 1% tolerance high stability types while the shunts are made of lengths of Eureka resistance wire. The wire used on electric fire elements is ideal for the lower value shunts. The values have been calculated for a meter resistance of 60 ohms internal resistance but would need modifying for other values. In any case, the precise value of each shunt should be adjusted experimentally to give the correct reading against a meter of known accuracy.

To calculate Rb: Rb= 1000V/I

Where I is the FSD of the meter and V is the desired voltage range

To calculate Ra: Rs= Rm/n-1

Where Rm is the meter resistance and n is the scale multiplication factor.

For example: If a milliammeter of 10 ohms resistance and a FSD of 1mA is to be used to measure 100mA, a shunt must be provided to carry the excess current, that is 100-1 milliamps,i.e 99mA. Thus the required shunt resistance is:

Rs= 10/100-1 = 10/99 = 0.101 ohms

Source : Basic Multimeter

read more "Schematics Basic Multimeter Circuit"

Schematics Time Domain Reflectometer (TDR)

This Time Domain Reflectometer (TDR) based on circuit idea published in Electronics Design October 1, 1998 magazine. Time Domain Reflectometer modified version by Tomi Engdahl which you see in this document. The circuit has been tested with wide variety of cables.

This reflectometer circuit is best powered with 4.5V battery or three 1.5V AA batteries connected in series. The + from battery goes to IC1 pin 14. The pin 7 of IC1 is connected to circuit ground which is connected to circuit ground. Remember to put a 100 nF (ceramic or polypropylene) capacitor between IC1 pins 7 and 14 to guarantee stable operating voltage for the circuit.

Circuit use

TDRs are used in all phases of a cabling system's life, from construction to maintenance and to fault finding. Historically, the TDR has been reserved for only large companies and high level engineers. This was due to the complexity of operation and high cost of the instruments.

If a cable is metal and it has at least two conductors, it can be tested by a TDR. TDRs will troubleshoot and measure all types of twisted pair and coaxial cables. TDRs can locate major or minor cabling problems including; sheath faults, broken conductors, water damage, loose connectors, crimps, cuts, smashed cables, shorted conductors, system components, and a variety of other fault conditions. TDR can be used to locate the problem type and in which place along the calbe the fault is.

The TDR works on the same principle as radar. When that pulse reaches the end of the cable, or a fault along the cable, part or all of the pulse energy is reflected back to the instrument. Any impedance change in cable will cause some energy to reflect back toward the TDR and will be displayed. How much the impedance changes determines the amplitude of the reflection. The TDR measures the time it takes for the signal to travel down the cable, see the problem, and reflect back. The TDR then displays the reflected signal as information on waveform display.

This circuit in this article is made to be used with a normal oscilloscope. The circuit which you build is used as the signal source and the oscilloscope is used as a waveform monitor. Connection diagram:

See Time Domain Reflectometer (TDR) in detail.

read more "Schematics Time Domain Reflectometer (TDR)"

Schematics Cable Reflection Tester-Reflectometer

A Time Domain Reflectometer (TDR) is a test set used to characterize and locate faults in metallic cables. It transmits a short rise time pulse along the cable pair. If the cable pair is of a uniform admittance and properly terminated, the entire transmitted pulse will be absorbed in the far-end termination and no signal will be reflected toward the TDR. Any impedance discontinuities will cause some of the incident signal to be sent back towards the source. This is similar in principle to radar.

An increase in the impedance (an open cable pair) creates a reflection that reinforces the original pulse. A decrease in the impedance (a solid short) creates a reflection that opposes the original pulse.

The resulting reflected pulse that is measured at the output input to the TDR is displayed or plotted as a function of time, and because the speed of signal propagation is relatively constant on a cable pair, the total travel time of the pulse down and back can be read as a function of cable length.

Here is a schematic for a homebrew cable reflection tester from the December 1996 issue of Electronics Now. It's very useful for checking coax cable runs for shorts or even impedance mismatches.

This reflectometer circuit works by sending a pulse down the cable, then checking the return signal on an oscilloscope. You can then determine the distance to a short or impedance mismatch using simple distance = time x speed equations. Be sure to divide your time by 2, and take in account your cable's velocity factor.

Source:  Cable Reflection Tester

read more "Schematics Cable Reflection Tester-Reflectometer"

Schematics LED Power Meter Using Digital Multimeter

LED power Meter circuit is a simple RF detector using diodes to charge a capacitor. The voltage developed across the capacitor is indicated by a multimeter set to a low voltage range. The circuit is soldered together without the need for a PC board, as can be seen in the diagram below and paper clips are used for the positive and negative terminals of the multimeter.

The level power output of an FM transmitter is indicated by the illumination of a LED and the voltage reading on the multimeter gives a further indication of the output.

A digital multimeter may be used but the presence of RF may produce a false reading. Likewise, the radiated energy may upset some analogue meters and you may get full scale deflection on the 15v range as well as the 250v range! But the LED won't lie. It will accurately indicate the RF and you can see the change in brightness as you adjust the coils in the output stage. Some of the cheapest and simplest multimeters will give the best results as they have a low sensitivity and the radiated RF energy will not induce a reading. Even a damaged multimeter can be used, provided the 10v or 15v DC scale is operating.

The reading is not calibrated and does not represent milliwatts output. It is only a visual indication.
We have designed over 10 FM transmitters for inclusion in the pages of this e-magazine and each one has different features and characteristics. Some are designed for 3v operation, some are for 9v operation, some are stable for hand-held situations and others are designed for high output. The illumination of the LED will range from barely visible to very bright.

LED Power Meter Parts
1 - 470R
1 - 100p ceramic
1 - 100n ceramic
2 - 1N 4148 diodes
1 - 5mm Red LED
1 - 2in (5cm) hook-up wire
2 - paper clips
No PC board required

Read more: LED Power Meter

read more "Schematics LED Power Meter Using Digital Multimeter"

Schematics Analog Milliamp Meter Used as Volt Meter

By adding a series resistance, a milliamp meter can be used as a volt meter. The resistance needed is the full scale voltage reading divided by the full scale current of the meter movement. So, if you have a 1 milliamp meter and you want to read 0-10 volts you will need a total resistance of 10/.001 = 10K ohms.

The meter movement itself will have a small resistance which will be part of the total 10K resistance, but it is usually low enough to ignore. The meter in the example below has a resistance of 86 ohms so the true resistor value needed would be 10K-86 or 9914 ohms. But using a 10K standard value will be within 1% so we can ignore the 86 ohms. For a full scale reading of 1 volt, the meter resistance would be more significant since it would be about 8% of the total 1K needed, so you would probably want to use a 914 ohm resistor, or 910 standard value.

By adding a parallel resistance, the milliamp meter can also be used to measure higher currents . The meter resistance now becomes very significant since to increase the range by a factor of ten, we need to bypass 9/10 of the total current with the parallel resistor. So, to convert the 1 milliamp meter to a 10 milliamp meter, we will need a parallel resistor of 86/9 = 9.56 ohms.

Source: Analog Milliamp Meter Used as Voltmeter

read more "Schematics Analog Milliamp Meter Used as Volt Meter"

Schematics Digital Blood Pressure Meter

This Digital Circuit describes a Digital Blood Pressure Meter concept which uses an integrated pressure sensor, analog signal-conditioning circuitry, microcontroller hardware/software and a liquid crystal display. The sensing system reads the cuff pressure (CP) and extracts the pulses for analysis and determination of systolic and diastolic pressure. This design uses a 50 kPa integrated pressure sensors (Freescale Semiconductor, Inc.P/N: MPXV5050GP) yielding a pressure range of 0 mm Hg to 300 mm Hg.

CONCEPT OF OSCILLOMETRIC METHOD
This method is employed by the majority of automated non-invasive devices. A limb and its vasculature are compressed by an encircling, inflatable compression cuff. The blood pressure reading for systolic and diastolic blood pressure values are read at the parameter identification point.

The simplified measurement principle of the oscillometric method is a measurement of the amplitude of pressure change in the cuff as the cuff is inflated from above the systolic pressure. The amplitude suddenly grows larger as the pulse breaks through the occlusion. This is very close to systolic pressure. As the cuff ressure is further reduced, the pulsation increase in amplitude, reaches a maximum and then diminishes rapidly.

The index of diastolic pressure is taken where this rapid transition begins. Therefore, the systolic blood pressure (SBP) and diastolic blood pressure (DBP) are obtained by identifying the region where there is a rapid increase then decrease in the amplitude of the pulses respectively. Mean arterial pressure (MAP) is located at the point of maximum oscillation.


HARDWARE DESCRIPTION AND OPERATION
The cuff pressure is sensed by Freescale's integrated pressure X-ducer‰. The output of the sensors is split into two paths for two different purposes. One is used as the cuff pressure while the other is further processed by a circuit.

Since MPXV5050GP is signal-conditioned by its internal op-amp, the cuff pressure can be directly interfaced with an analog-to-digital (A/D) converter for digitization. The other path will filter and amplify the raw CP signal to extract an amplified version of the CP oscillations, which are caused by the expansion of the subject's arm each time pressure in the arm increases during cardiac systole.The output of the sensors consists of two signals; the oscillation signal ( ≈ 1 Hz) riding on the CP signal ( ≤ 0.04 Hz).

Hence, a 2-pole high pass filter is designed to block the CP signal before the amplification of the oscillation signal. If the CP signal is not properly attenuated, the baseline of the oscillation will not be constant and the amplitude of each oscillation will not have the same reference for comparison.

Oscillation signal amplifier together with the filter.

Digital Blood Pressure Meter Schematic

Author: C.S. Chua and Siew Mun Hin, Sensor Application Engineering Singapore, A/P
More about Digital Blood Pressure Meter

read more "Schematics Digital Blood Pressure Meter"