delabs Circuits

Saturday, August 12, 2017

Build RFID transmitter and receiver

Guide on how to build my own RFID transmitter and receiver?

Post by C1

You will find them here ...some entities misuse this technology, hence people have become less interested in this as it is invasive. Properly used this technology is an asset, example - inventory or asset tracking and management.

RFID Circuits - - Low Power RFID Transponder -  For some time railroad companies have been wrestling with the problem of tracking rail cars.

Need help in RFID circuit - Electro Tech -  need the circuit for RFID receiver and transponder and any useful information regarding RFID. I thought i would use PIC controller to find out the distance b/w the product and the reader.

Build RFID transmitter and receiver

RFID - - Radio-frequency identification (RFID) uses electromagnetic fields to automatically identify and track tags attached to objects. The tags contain electronically stored information. Passive tags collect energy from a nearby RFID reader's interrogating radio waves.

Fall 2004 RFID Honors Project - RFID can be categorized as low frequency or high frequency and as passive or active. This project looks mainly at low frequency passive RFID because it is the least expensive and the most common.

Low frequency passive RFID works by magnetic induction. A reader device generates an alternating magnetic field from a coil of wire. A tag also has a coil of wire that induces current from the magnetic field. This current provides power to the passive tag. The tag then sends back its own varying magnetic field which is read by the reader device.

This is an answer of mine to an old post - Tuesday 27 June 2006 

Friday, July 21, 2017

Continuity Tester

Question - i want a continuity tester circuit or get one.

mail by RB

Here are some pages that might help

Simple Component and Continuity Tester - This simple project may be used for testing components, as well as checking circuit board tracks, wires and connections for continuity (conduction). It tries to pass a small current through the item being tested and the LED will light brightly, dimly or not at all according to the resistance of the item.

POPTRONICS Continuity Tester - This is another invaluable piece of test equipment for digital work. It has been designed by us to cater for a particular application . . . servicing digital projects sent in for repair. 

Continuity Tester, Smart - A tester by Tony Van Roon. This uses a 741 opamp and works off a dual supply and hence rather complex but more accurate. This may need some design changes too.

Silicon Chip Online - A Programmable Continuity Tester - Let's face it, almost every analog and digital multimeter does have built-in capabilities for testing continuity. However, this function is somewhat limited.

It would be a good idea to buy one like this

Continuity Tester CT6101

Heavy-Duty Continuity Tester CT6101- The Continuity Tester is very easy to use and is used to test for breaks in non-energized wires or cords. Continuity has a bright 360 degree visual indicator located near the tip along with a flexible lead for ease of use.

Thursday, July 20, 2017

Simple Battery Characterizer

I was reading about some IC's from Maxim/ Dallas saying " Simple battery characterizer" for plotting a charging/ discharging cycles & graphs are available on computer for study I want to plot an extensive data of these batteries.

From MO

This circuit is a battery characterizer that applies a fixed load to a charged NiCad or NiMH cell and measures milliamp-hour capacity as it discharges.

PIC-based circuit example from Westford MicroSystems - Battery Characterizer that determines milliamp

Simple Characterization Circuit

Simple Characterization Circuit for Rechargeable AA Cells - Maxim.

Abstract: This characterization circuit, plus a PC and some software, accurately measures the full discharge cycle for a rechargeable AA cell. Cell capacity and output resistance can easily be determined from the curve resulting from these measurements. 


Friday, July 14, 2017

Desoldering Methods

Dear Solderman, i hope you know how to dessolder a wave soldered component from a board. There is problem because the pins are two face soldered. - thanks

mail from JS

Sending a few sites, read them all and then tell me.

Desoldering - Wiki

A component with one or two connections to the PCB can usually be removed by heating one joint, pulling out an end of the component while the solder is molten (bending the other lead to do so), and repeating for the second joint. Solder filling the hole can be removed with a pump or with a pointed object made of a material which solder does not wet, such as stainless steel or wood.

Desoldering Methods

How to Desolder

One of the most frustrating parts of building electronic circuits is getting something wrong. For a board that should take ten minutes to put together, you could spend an hour just trying to fix a component that you got backwards. Sadly, desoldering it’s not as easy as soldering in reverse.

Desoldering requires two main things: a soldering iron and a device to remove solder. Soldering irons are the heat source used to melt solder. Irons of the 15W to 30W range are good for most electronics/printed circuit board work.

Desoldering THM and SMT

Desoldering is a topic that no one seems to get to excited about until the time comes when people discover that a lot of damage is being done during this operation.


Wednesday, June 14, 2017

Active Camera trigger shutter

well listen i read about an idea but i didn't get it so well and i want to develop it if i understood it with ur help active camera trigger shutter something like that i want to know input output solutions and i was wondering if i can send the picture to a mail and how

mail from MK

Is this what you are looking for....

The HSP DIY system must therefore be a control system. Suitable control systems range from amazingly simple to extraordinarily complex. The tips on the following sections are designed to help you select the type of control system you wish to build and us

Active Camera trigger shutter

Photoflash circuit from pocket camera - This schematic was traced from an electronic flash unit removed from an inexpensive pocket camera, a Keystone model XR308.  Errors in transcription  are possible.

Flash slaves are used when you need to supplement one flash unit with one or several more. This slave trigger simply triggers those other units.

Slave Flash Trigger - A Programmable Optical Slave Flash Trigger  for Digital Cameras with Processor PIC 12F675


Monday, June 12, 2017

RF Microwave and Wireless Projects

hi im a communications engineering student can u help me to find an idea for my secondary project. i want a hardware project that is related to for example wireless communications , microwave or fiber optics something like that can u help.

mail from MK

Think of the project you want, enter multiple keywords, then see the results. you may need download pdf files by right click save target ,

Cambridge Wireless Resources
"development and application of wireless and mobile technologies to solve business problems."

RF Microwave Projects

Look at these links too....

 Application Notes - RF Cafe -  Useful Learning Application notes on RF and Parts vendor sites.

New Wave Instruments - RF snd Microwave resources

A comprehensive collection of RF & microwave resources for wireless communication engineers. Includes tutorials, theory, design aids, circuit diagrams, products, and more.

First try out above links and google scholar, Then give me more specific ideas, so i can help you better.


Wednesday, June 07, 2017

Application for 8051 mcu project

Need an unique Application for 8051 mcu project ? - mail by J9

Look at this place - The Online 8052 Resource - This web site is dedicated exclusively to the 8052 microcontroller, related products (both hardware and software), and 8052 derivative chips. This includes the traditional 8052, 8051, 8032, and 8031....

The best place is PJRC: 8051 Projects - This 8051 development board provides an easy and low-cost way to develop projects based on the 8051 microcontroller, without the need to purchase any other equipment, such as EPROM programmers or emulators.


8051SBC V1.0 - The old day chips, say 8031, 8751, 8032 or the new chips,89C51, 89C52, 89C51RD2 can be used without any problem. Some may think that your old chips were useless, now you can use it for many projects.

This is an answer of mine to an old post - 2006

Friday, April 21, 2017

How does RTD Work

How Resistance temperature detector works?

Post by JG

The most popular RTD is PT-100, Platinum 100 Ohms, As all Metals have a Temperature Coefficient of Resistance. The Resistance of RTD increases with rise in temperature.

How does RTD Work

Even Copper wire can be used, but you need to have a very thin and long wire to make usable measurements.

Resistance: Temperature Coefficient - Since the electrical resistance of a conductor such as a copper wire is dependent upon collisional proccesses within the wire, the resistance could be expected to increase with temperature since there will be more collisions

Resistance thermometer - Wikipedia - . The RTD wire is a pure material, typically platinum, nickel, or copper. The material has an accurate resistance/temperature relationship which is used to provide an indication of temperature. As RTD elements are fragile, they are often housed in protective probes.

What are RTDs? - in the case of Platinum known variously as PRTs and PRT100s, are the most popular RTD type, nearly linear over a wide range of temperatures

What is a Resistance Temperature Detector (RTD)? - Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core.
The material has a predictable change in resistance as the temperature changes; it is this predictable change that is used to determine temperature.

This is an answer of mine to an old post - 2006

Thursday, April 20, 2017

Spice Simulation for CD4060

I found your web site today. I am looking for the CD4060 spice model for Orcad Capture. My Orcad version is the 9.2 (complete but without CD4060 !!)

I saw on your web pages a design using this component. How can I have the CD4060 model (to use it in simulation) ?

Mail from EG France

Try using 74HCT4060 or maybe 74HC4060

74HCT4060 or  74HC4060

Have a look at these links.

SPICE Simulation Fundamentals - Basic SPICE Simulation Model Parameters, SPICE Simulation Control Statements. SPICE Simulation Source Type and Parameters.

PSPICE Resources -  Where can I get my own copy of PSPICE using Schematics? (the student version 9.1 is free)

SPICE Resources - SPICE is available in many different "flavors." Several versions are presented below for your downloading pleasure.

SPICE - A Brief Overview - The description is far from complete, as SPICE is a powerful circuit simulator with many capabilitie

The SPICE circuit simulator and models - SPICE is an acronym for Simulation Program with Integrated Circuit Emphasis and was inspired by the need to accurately model devices used in integrated circuit design. It has now become the standard computer program for electrical and electronic simulation.

"The Orcad demonstration package includes several libraries for circuit layout, but only the EVAL.LIB has SPICE models as well. This library includes a range ...."

Please study all links above and tell me if it solves your problem, Look and study each link above. If you have a problem still, then i will search again or find a solution


Tuesday, April 04, 2017

Bioamp for muscle signals - EMG

message: I'm working an a bioamp for muscle signals. This is the base design, but it does not work. Basically what I want to do is a circuit that amplifies my muscle signals and is able to run a small dc motor when I flex a muscle. Any help? Thanks

Mail from JX US

Electromyography (EMG)  - An electromyograph detects the electric potential generated by muscle cells. when these cells are electrically or neurologically activated.

Electromyography EMG

Study these links and send me a feedback ....

Positive Feedback Circuit for Isometric Muscle Experiments - he part of which I am most proud is the innovative Automatic Gain Control (AGC) circuit, without which the project would not have succeeded. This AGC circuit could well be used in many other applications too

EMG design parameters - The device must be able to detect the small signals arising from muscular contraction and amplify them without also amplifying ambient noise which may exist in the recording environment and which is likely to be of approximately the same magnitude as the signals of interest.

Important Factors in Surface EMG Measurement - Small electrical currents are generated by muscle fibres prior to the production of muscle force. These currents are generated by the exchange of ions across muscle fibre membranes, a part of the signaling process for the muscle fibres to contract. The signal called the electromyogram (EMG) can be measured by applying conductive elements or
electrodes to the skin surface, or invasively within the muscle


Sunday, March 19, 2017

Coffee maker PID Temperature Control

My name is RP  i'm working on a project to regulated the temperature of a coffee maker. I don''t know were to start. what i want is to regulate the temperature of the coffee. Could you please help me.

Mail from RP

This is what you may have to do. study below pages and try.

Murph's Silvia PID Page - This page describes my effort to improve temperature control in a home espresso machine by installing a PID temperature controller. If you are reading this page, then you probably already know what Silvia is, what PID means and why anyone would want to add one to the other.

Coffee maker PID Temperature Control

Tuning a PID (Three-Mode) Controller - . If the controller does not include an autotune algorithm or the autotune algorithm does not provide adequate control for the particular application, the unit must then be tuned using a trial and error method.

PID controller - Wikipedia - A PID controller continuously calculates an error value {\displaystyle e(t)} e(t) as the difference between a desired setpoint and a measured process variable and applies a correction based on proportional, integral, and derivative terms.

Temperature Control - PID and LabVIEW - First, let's take a look at how the PID controller works in a closed-loop system using the schematic shown above. The variable (e) represents the tracking error, the difference between the desired input value (R) and the actual output (T). This error signal (e) is sent to the PID controller.


Saturday, March 18, 2017

Halogen bulb lamp dimmer circuit

Can someone provide me a (simple) schematic for a dimmer that would control 12 VAC about 80 watts. This would be used to control a lamp with halogen bulbs.

A comment in Hobby Hound

Here is a listing study them and choose, try out, learn and then build the one right for you.

1200 Watt Lamp Dimmer Circuit The following circuit is a lamp dimmer circuit that is capable of controlling up to 1200 Watts. The circuit uses the Q4015LT combination Diac and Triac that is available from Digikey Electronics.

PWM Motor Speed Controller / DC Light Dimmer - Here is a listing study them and choose, try out, learn and then build the one right for you.

DC Controlled Dimmer Velleman kit - K8064

Halogen bulb lamp dimmer circuit

Simple brightness adjustment by means of a DC voltage, optically isolated adjustment input. Pre-set adjustment for full brightness, suitable for incandescent lamps, mains voltage halogen lighting and low voltage halogen lighting in combination with a conventional transformer. "Soft start" feature to ensure lamp longevity, transformer protection in case of defective light sources

Electronic Transformer for 12V Halogen Lamp - STMicro
The topology of the circuit is the classic half-bridge. The control circuit could be realised using an IC (so fixing the operating frequency), but there is a more economical solution which consists of a selfoscillating circuit where the two transistors are driven in opposing phase by feedback from the output circuit.


Friday, March 17, 2017

How to Connect RTD to transmitter

Can you explain me were i have to conect the three cables of the PT100 in the Circuit RTD-Pt-100-Transmitter Circuit?

RTD PT100 Transmitter and Multiplexer

Comment from CV of Spain

You have a three wire RTD, there is also a 4 wire RTD type, This circuit can be used for both.

In the 3W RTD you have two wires coming from the same point (zero ohms between them) the third wire will be 100 ohms from these two. From the two wires that are at same point take one wire and give 1mA from Q3 + OUT. The other wire of the pair connect to input +..... input - can be connected to the 100E end.

How to Connect RTD to transmitter

So imagine that I have three wires one white and two blacks.

One black to +OUT 1ma
Other black to input-

And waht about the withe one that is the one that give me arround 100ohms??

Reply from CV

Black 1 - +Out 1mA
Black 2 - +Input
White goes to -Input

See NI Developer Zone - RTD Connections


Saturday, February 11, 2017

Battery Management and Supply Design

As you have have pointed out, my circuit is not driving any motor or such loads...but it is driving a Wireless Module which draws about 165mA to 240mA from my circuit during transmission [for 4 sec] and normally it takes upto 114mA. After a time-out of 1min, I put my uC P89V51RD2 to powerdown mode [200uA in this state].

Read articles in the link you provided, thanks for that! - Continued from uC Board is Misbehaving on Brown Out.

My other problem is that my batteries get supercharged upto 5.8V, and as per spec's of most IC' they are bound to operate faithfully upto 5.25[max 5.5v], so how do I solve this issue? I want that my boards get only 5.25v even if my battery is having voltages 5.25+, without increasing my existing current consumption to noticeable value.

Moreover I'm in need of a cutt-off circuit which will take away the load from my batteries when my battery has reached 4.00V, & the charger charges my battery independently. Do you have any thing which will suite my purpose?

Mail from MO

Have two supplies, the one for uC can be a small Ni-Cad battery charged by a diode by the bigger battery which supplies the 200mA for RF drivers.

So when the big battery is loaded and the supply dips the shock is not felt by uC. or isolate it by SMPS and isolate the grounds. The ground return current of RF module 200mA must be lifting the ground of uC or some Logic.

Start by trying to locate the problem, first power RF and uC boards with isolated lines from two lab-test 10A power supplies. If both are in same board, cut tracks and power them separately. The grounds should meet at point the two sections interface.

If that works without problem, you know it is related to supply, now step-by-step go back to the battery and charger you have built keeping isolation intact. Look for spikes-glitches on power lines of uF when RF module active. Check if that is close to or leaking RF to uC circuits.

The battery charger circuit should limit voltage and current to battery specs. If you are not using a charger chip then while testing make sure the charger has a regulator or use LM317 to clamp voltage to 5.6 or 5.5 as per your design.

There are charger chips from maxim, national for battery charge management. If you have a need for extra voltage the you have to use SMPS boost switcher. which may take the low battery 4 to 8 which can be regulated to 5 using 7805.

This may be getting bit complex, first try to find what is the problem, it may be just the board design.

Battery Management and Supply Design

You can also use a Single Maxim Chip for all these functions. ...

Supervisor ICs Monitor Battery-Powered Equipment

A feature-laden µP supervisor (IC1), with the help of the µP itself, performs a variety of functions in this typical application circuit. You can get these functions all together or in various combinations, by selecting one of the many available microprocessor (µP) supervisor IC....


Wednesday, February 08, 2017

RS Latch Erroneously setting by Itself

I am having trouble preventing my rs latch from erroneously setting. The circuit is a voltage threshold detection circuit with a nand gate whose output is tied to a SR latch.

I have capacitors across the VDC and ground for the chips and I verified the power supply's input is fairly stable. As further troubleshooting I probed with a oscilloscope at the SR latch's input and looked for anything above 1 V. The voltage never moved above 1V, but the latch still set.

I was able to place a .3 uF capacitor from the SR latch's input to ground and this seemed to fix the problem. However, it does not explain what is causing the latch to set. Any ideas or suggestions would be much appreciated. Thanks.

Some info:
Nand gate: SN74LS30
SR Latch: CD4043B
Supply to ic's is 5V from a LM7805 (with .1uF cap's across input/output)
Voltage threshold detection coming from LM339 comparator

Basic idea:
Vin ----------> |LM339 pin 4| -> |nand gate| -> |SR latch| -> |(relay driver circuit)|
Vthreshold -> |LM339 pin 5|

Mail from SM

If the power supply is stable and you do not have ground loops then that is not the problem.

If a relay or LED or some output is being driven, the current pulse may be triggering the circuit. Isolate Grounds of Loads even LEDs and Relays, Ensure EMI-RFI immunity.

RS Latch Erroneously setting by Itself

Flip-flop (electronics)

Use a 400MHz Scope and set timebase to uS-nS and see if a Spike is causing the trigger. Set the brightness level a bit high and look as close as you can.

Even a IC consuming a little extra power for a mS can set up a spike in the rail. Keep power and signal circuits on different PCBs when designing.

Check the environment and mains wiring, It could be a Laser Printer sharing the mains. If a Loose contact is present in mains box then your test circuit links with printer. Finally a good earth or ground is a must, if the unit is to operate on mains.

In the LM339 comparator use a Hysteresis Resistor feedback of 10M or 1M, Also a small cap can be placed across 10M, like 10nF if the system is a slow response one. Ensure you do not put a cap in a way to cause oscillations. A RC at input of LM339 may help.

Finally a LM339 output may have to go thru a schmitt nand buffer like 4093. to Clean the signal. Interface from a LM339 may be good with CMOS and not TTL, so a Scmitt can be tried. Nand gate: SN74LS30 and SR Latch: CD4043B seem to be different families. Use the 74HCT30 and 74HCT4043, use any one single family in Logic.


One thing i left out, The power on Start of the circuits must be defined. So a power on Set or Reset Cap with a pullup has to be provided if it is triggered by a Low. If you use a Latch which triggers by a Positive Edge then A pull down R and a Pull up Cap is required. - delabs

Sunday, February 05, 2017

Isolated RS232 interface to Microcontroller

I've seen on the net the schematics for RS232 with Opto-Isolation. The supply on the PC side is from the PC. The supply from the line side is external? I've seen the 3 connectors (RX, TX & GND) together with 2 connectors 5V. One of them is connected to the device. How about the other? Needs additional power source?

Could you send me the PCB and component layout, as well as the component list, I'm afraid to get mistaken

mail from CS

The the circuit you are referring to is... RS232 with Opto-Isolation
The Blue line indicates the Optical Isolation, no wires or cooper should cross this line. Yes the PC Powers the left side, The supply to the right side is the Microcontroller supply. The Microcontroller will have a +5 and GND use this for the right side of opto. I have no PCB for this. You can make your own PCB using eagle cad. I made this circuit as i could not risk my PC and had no other components. I had to make a high voltage measurement with the PC, so this caution.
Also use the Max232 if you do no need 1KV Optical Isolation.

Here is a interface from RE Smith Inc, the one below is not isolated. They have Isolated versions too.

The RS422S is a fast Async bi-directional RS232 to RS422 interface converter that will operate at data rates up to 115.2Kbps. The unit has jumpers termination, RS232 DTE/DCE selection and, TTL/CMOS operation. The unit has two RJ11/12 connectors an a pin header for power, common TTL/CMOS TX data and RX data.

Discover Circuits - Opto-Isolator Coupler Circuits
RS232 to RS232 Port Powered Optical Isolator: Brochure with schematic in PDF format / (added 7/02). RS232C Circuit has Galvanic Isolation:


Wednesday, January 18, 2017

Room Temperature Monitor and Alarm

I am required to design an electronic system that monitors the temperature between(22-24 degrees Celsius). Given are power supply, Analogue subsystems and digital subsystems. the accuracy of the digital control circuit is limited by the accuracy of the temperature sensors. A green LED is lit to indicate correct temp range.

Room Temperature Monitor and Alarm

A red LED indicates out of range temperature and in sustained events( where this indicates out of range temp which is counted, displayed and timed) a large LED is lit. In a event where the sustained event exceeds 6 days an alarm is sounded to indicate a possibility of a system failure.

Now I need help on how to come up with the correct circuit diagrams which can work , how to simulate and to get the project done, please help, even direct me to books on the project or other sites please...

Mail from AM South Africa

First acquire the room temperature as an analog value. This requires a sensor for the range you indicated a Semiconductor sensor like LM335 will do fine.

Use the ideas on this page - Circuit Design Service and Projects 

Now monitor and generate Logic outputs for temperatures out of range using a Window Comparator. Then for the timed events you need a Timer circuit. And a Logic to set off the Alarm or Annunciator.

All this can also be done by a PIC circuit if you can write some code. I have given some pages to see, study them, that will be sufficient

Tuesday, January 17, 2017

RTD and Connections - Mini 3W 4-20mA transmiter

I´ve been looking at the mini-three wire 4-20mA transmitter and it is everything I need to try a new way of measuring the temperature. It is great but there are a few questions about it I hope you can answer them:

- What is the meaning of RYIN and RXIN?

- You say that it is a three wire circuit but I can only seen two points of connection for the PT100. Is it because the signal conditioner of the three wire PT100 isn´t drawing?

- The OUTPUT connector is where I can find the 4-20 mA?

I hope there is no problem to answer these quetions to me, I would be really pleased if you can do it and I´d appreciate it.

Many thanks and hope to hear soon from you.

Mail from AD - Spain

All the transmitter circuits can be seen here. Industrial Process Control Circuits

RTD and Connections - Mini 3W 4-20mA transmiter

As this circuit was popular i have made a separate page for it. Mini RTD Pt-100 Three Wire Transmitter

A ex-customer of mine, (ex because i don't manufacture now.) wanted a low cost transmitter, so i made this.

What is the meaning of RYIN and RXIN? - Connect it to any two wire RTD

For RTD read more RTD Omega - RTD Intro

You say that it is a three wire circuit ....

+V .... -V .....OUTPUT ...... on the left are the three wires.
+V and -V is the Supply. OUTPUT is a Current Sink Directly Proportional to Temperature on RTD.

In the Costlier 2 Wire systems, Current Drawn by Transmitter is Directly Proportional to Temperature on RTD. Two wire systems can be just looped, 3 Wire like mine has to be wired, more wire.

The OUTPUT actually drinks 4-20mA prop to Temp. Put a Ammeter (fused) in 0.2A range in Series with a 50 Ohm Resistor. Black Lead of ammeter to this OUTPUT. Red Lead to one end of 50 Ohm. The other End of 50 E to +V.


Thank you for your quick answer. I think I´ve got it. Although there is another question that has come up to me while reading your email. +V and -V is the supply I apply to the PT100? I am applying 24DCV so I suppose the +V is the 24V and -V is the ground, isn´t it?

Reply from AD

YES - +V is the 24V and -V is the ground, anything above 12V will work.

The 24 V should be applied to +V and -V only. Not to RTD Directly. RTD Directly Connected to 24V will BURN the RTD.

RTD is a Thin Platinum Film or Wire of 100 Ohms worth. The Resistance varies with temperature.

A 1mA ONLY Constant Current is sent thru the RTD to measure the Voltage across it. From the measured voltage 100mV for the 100 Ohms is subtracted using opamps.

The balance mV is Directly Proportional to Temperature. This mV is converted to 4-20 mA. 4-20 mA is a Strong Signal for Current Loop Transmission.

4mA is 0% 12mA is 50% and 20mA is 100% of the range you calibrated.

Related Reading

Monday, January 09, 2017

Comparison of Thermocouple and RTD

message: what is the pros and cons of rtd,over thermocouple

Mail from MU

Read the pages in links below, that ought to clarify your doubts. Last edited February 16, 2007

Comparison of Thermocouple and RTD

Comparison of Thermistors, Thermocouples and RTD's ... Unlike RTD's and thermistors thermocouples have plus and minus legs so polarity must be observed.
Comparision of Thermistors, Thermocouples and RTD's

Thermocouple - No resistance lead wire problems, Fastest response;  RTD - Most stable, accurate, Contamination resistant,; Thermistor - High output, fast, Point temperature sensing.
Watlow - Temperature Sensor Comparison Guide

Since thermocouples measure wide temperature ranges and are relatively rugged, they are very often used industrial and process applications where accuracy may be a less important factor.
RTD vs. Thermocouple comparison chart