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Showing posts with label Electrical. Show all posts

Drunken ROBOT Project


INTRODUCTION:

Parts and Tools

Here's the parts you'll need to make one pocket-sized drunken robot. But make more than one since it's no fun to drink alone.

Parts
1 vibrator motor from a pager or cell phone. (I used these. You can find the same motor here and here, though it seems distributors keep selling out. Just about any tube-shaped vibration motor with two metal tabs on the end will work. Search for more.)
1 AG13 button cell battery. A common watch battery that also goes by the aliases 357A, L1154, LR44, GPA76 or PX76A.
1 square inch of sheet tin, copper or other easy to work with metal sheeting. You can probably use a tin can but it might be hard to work with. I'll be using 0.008" sheet tin from the local hobby store.
The PDF template linked below.
Tools
Pliers
Tin snips. (or old crafty scissors you don't mind messing up to cut some tin.)
Now that you have everything, lets get started!

CUT THE TIN:
Print out the template from step 1 (also linked below) Be sure to print it at 100% and transfer the design to your piece of tin. (Cut it out and trace it or just glue it on with some temporary adhesive.)

Cut and snip your tin on the solid lines. Please be careful when cutting and handling sheet metal since it can get really sharp. Gloves and safety goggles are recommended.

NOTE THAT THE NEGATIVE SIDE OF THE BATTERY IS ON TOP


Make The Holder For The Battery:

We want to make a solid connection to the side and bottom of the battery. To do that first bend the piece of tin up at right angles where it's indicated on the diagram. Then place the battery in the middle and fold the arms around so it holds it securely.

Prepare The Motor:

Our pager motor has two leads. One needs to be connected to each side of the battery for the motor to work. 

The flat side of the battery (+) is already making contact with our tin support. To get a contact with the top of the battery (-) we bend one of the pins back underneath the motor. When we put the motor in place this lead will spring in contact with the top of the watch battery. (Picture is worth a few hundred words here.)

If you're using a different motor, say one with wires then you might need to get busy with a soldering iron to replicate what we have here. Do not solder directly to the battery. Applying that much heat to a battery is dangerous and can cause it to burst.


BEND ONE OF THE LEADS OF THE BATTERY AS SHOWN

Mount The Motor:

Now we're going to complete the circuit by crimping the motor (lightly!) into the top pair of tabs. This will press the bent tab in contact with the top of the battery and the lead into contact with the tin which will complete our circuit. 

First bend the top tabs of our tin into a U shape so we can rough everything into place. Then place the motor so that the bottom lead is in firm contact with the battery while the other lead is pressed against the metal of our tabs.

(You might want to put a piece of paper or tape over the top of the battery to keep our robot quiet while we're working on him.)

Very carefully crimp this closed with pliers. You want the motor to stay in place, but you don't want to damage the casing of the motor.



Troubleshooting:
It doesn't go at all.
First be sure that the motor leads are touching the things they need to touch, and only those things. One should be touching the top of the battery and the other should be firmly pressed against our tin framework.
Check for shorts. Make sure that the only bit of metal touching the top of the battery is
Make sure that there is nothing keeping the weight at the top from spinning.

It falls over more than I'd like.
There are several ways you can counteract this.
Bend the motor back towards the center of the battery so its center of gravity is more in the middle.
Bend down the corners of the "front" of bottom platform.
It might be too vigorous, you can try letting the battery run down a bit so it doesn't jump as much.
Try a different surface. I found that a pad of paper was the most reliable. On a harder surface they'll bounce easier.
Get out a file and remove some of the weight from the top of the motor.

It falls over less than I'd like or doesn't act very drunkenly.
Give it a double whiskey neat and wait 10 minutes.
You might be using an under powered battery., especially if you're using a bigger motor. Try a fresh battery or a more powerful cell. (If you use a different battery you'll need to rework the tin holder.
Make sure the motor isn't shaking lose. If it is, a dab of glue or tape can take care of your troubles.

DOWNLOADS: 





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10 LED Volume Unit Meter Project by LM3915 and LM324

INTRODUCTION:

VU (Volume Unit) meters are often included in analog audio equipments to display a signal level in Visual Units. We are building a mono input VU meter in this project. You can build one pair to use them in stereo mode.






Input sensitivity is about 1 Volt rms, up to +6dB. You can adjust the input amplitude by using the multiturn 50k linear potentiometer (POT1). Circuit needs symmetric ±12V power supply because of the LM324 opamp. We used metal film resistors (1% tolerance) so accuracy is sufficiently high. Visual duration can be adjusted by using linear 10k potentiometer (POT2). LEDs should be 5mm x 5mm in size for best fit. But we used standard LEDs and this caused a little shift as you can see from the image. Dot/Bar switch determines the operation mode of LM3915.



The PCB file is provided in pdf format. You can apply it to the board by using the ironing method.

DOWNLOADS:


CLICK HERE FOR THE VOLUME UNIT PROJECT PCB AND LAYOUT DOWNLOAD




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Remote control for home appliances Project

INTRODUCTION:

This project was found while surfing the web. It was just brought into our knowledge by one our team member and found to be a valuable suggestion. This basic electronic project is purely application based project. This can be constructed to improved the normal everyday living experience of a normal person. This project is about a circuit that is to be attached to any home appliance in order to control it's working. The circuit given below can be easily constructed with the guidance given and this circuit must be attached to any home appliance. The total controlling can be done via our TV, VCD or DVD remote controller. The circuit will function from a 10 meter distance.


The amplified signal is fed to clock pin 14 of decade counter IC CD4017 (IC1). Pin 8 of IC1 is grounded, pin 16 is connected to Vcc and pin 3 is connected to LED1 (red), which glows to indicate that the appliance is ‘off.’

The output of IC1 is taken from its pin 2. LED2 (green) connected to pin 2  is used to indicate the ‘on’ state of the appliance. Transistor T2 (BC548) connected to pin 2 of IC1 drives relay RL1. Diode 1N4007 (D1) acts as a freewheeling diode. The appliance to be controlled is connected between the pole of the relay and neutral terminal of mains. It gets connected to live terminal of AC mains via normally opened (N/O) contact when the relay energizes.


DOWNLOADS:


CLICK HERE TO DOWNLOAD THE PROJECT



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Maze Solving Robot using a PIC18F2525 Microcontroller Project

INTRODUCTION:

The goal of the project is to create a robot that will follow a black line on a white sheet of paper and solve a maze created out of those materials. The project also included a list of specifications that were to be followed. These specifications are: • The maze will have black lines, 1/4 to ¾ of an inch in width on white paper • The maze will be no larger than 10x10 feet. • All paths meet at 90 degree angles • Dead ends and loops possible • Robot must fit in a 6x6 inch square • Must be able to operate without a power cord • Designed to finish a maze in the fastest possible time.

PROJECT DESCRIPTION:

Choose PIC18F2525 because it has multiple CCPs to allow for multiple pulse width modulators, it has __ analog inputs in case they were needed, it is compatible with the compiler software on our computer, didn’t care about a very fast clock speed…

Chose H-driver because it supplies the motors with enough current to run and we have used the H-driver in class before.

Chose regulator because it has a heat sink, so won’t burn up easily, outputs 5 volts with 1A max current.

Motor package, PCB board, and motor chassis were all from the same company and work together.

Used analog sensors because they can be used as digital sensors and require less code to implement. The sensors used include the emitter and the receiver as one part (didn’t have to worry about the emitter and receiver working together).


THEORY OF OPERATION OF DESIGN:

A switch is used to turn the robot on or off. When it is on it is connected to a power supply of 4 AA batteries with 1.5 volts each for a total of 6 volts, this is considered the unregulated power. Unregulated power goes to a 5 volt regulator. Regulated power runs to the PIC, the H driver, the RJ11, and the 4 sensors. Unregulated power runs to the H driver as well.

The robot decides its direction based off of the outputs of the four sensors. The robot has 4 different states, and they are: Forward, Left, Right, and Turn Around. The state priority is in this order: Left, Forward, Right, and, lastly, Turn Around. The four sensors are placed close together at the front of the robot. Each sensor has a corresponding LED that lights up when the sensor is high. The left and right sensors are slightly farther back then the front two sensors and the front sensors are centered and side by side. The robot enters the Left state whenever the left sensor goes high until the front two sensors go high. When the two center sensors are high and the left sensor is low, the robot enters the Forward state. If neither of the previous conditions are true and the right sensor is high, then the robot enters the Right state until the front two sensors go high. If none of the sensors are high, then the robot enters the Turn Around state where it does a 180o right turn in place.

With the 4 previously mentioned states our robot is able to make turns, turn around from dead ends, correct itself on straight lines, and create random turns that ignore the left turn priority. The robot is able to do the second 2 mentioned abilities because of the positioning of the sensors. By having the left and right sensors extremely close to the front sensors, the robot is able to make very small left and right turns to keep itself on a straight line. The robot is able to randomly ignore state priority because of while loops used in the code. For example, the motor is coming to a 3 way intersection with left and straight directions in front of it. Under normal priority the robot should turn left at the intersection. When the robot approaches the intersection it may be caught in the Right state in order to correct itself on the straight line. The robot exits the Right state once the front 2 sensors have gone high, so there is a possibility that the robot is in the Right state as it enters the intersection. If this is true, the left sensor will be ignored until the front sensors go high and the robot will go through the intersection straight because the left option was ignored.

By being able to stay on the lines of the maze, follow turns, turnaround, and provide occasional random turn priority, the robot should be able to find its way through any maze eventually even if there are loops within the maze.


CONFUSIONS AND LESSONS LEARNED:

Overall, the final design worked as intended. However, several different versions of our wiring diagram were created before we had a working robot. This created some setbacks during the construction phase, however any problems that arose during this time were quickly found due to our methodical checking of the circuit being built at the time for any shorts or wrong connections.

Throughout the duration of this project we gained experience in building circuits that worked with each other to create a final outcome, and along the way we learned a few valuable lessons. Checking solder connections meticulously pays off and will save a lot of time in future work, and checking that everything works as intended on a system-by-system basis will further help with the overall construction of any soldering project. Once a system is checked for physical connections, the programming interface must be tested as well. We wrote a lot of very short programs to test each module for individual functionality before interfacing it with another section of the design, as its easier to debug one small section rather than one very large section consisting of several subsections.

In addition to checking for proper connections, ohmmeters are the quintessential debugging tool for a circuit physically as well as logically for code, and one should be kept nearby at all times. These two lessons saved a lot of time when creating the final version of our design, without running constant checks on our circuit with an ohmmeter we would not have been able to assemble our robot as quickly as we did.

The other important lesson learned came from the design of our circuit: DC power is a valuable asset when debugging a circuit, especially if the final design is battery powered. DC power allows the user to constantly run new tests without worrying about draining batteries and having to replace them constantly. DC power may draw more current, but in the end, if the circuit can handle the DC power it will be able to run on batteries.

FUTURE WORK:

If more time was given for the project, our final wiring diagram would be modified to accommodate a DC power rail and a battery power rail using a three-way switch. This would cut down on the total amount of current drawn when running the robot off of batteries. Another addition to the final design would be to create a more aesthetic looking enclosure for our robot and possibly creating a surface mount PCB to minimize noise from wires and increase the overall cleanliness of our circuit.

SCHEMATICS:


BLOCK DIAGRAM

DOWNLOADS:

CLICK HERE TO DOWNLOAD THE MAZE SOLVING ROBOT SOURCE CODE DOWNLOAD

CLICK HERE TO DOWNLOAD THE MAZE SOLVING ROBOT REPORT DOWNLOAD



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PWM DC Motor Controller Using MOSFETs and IR2110 H-Bridge Driver Project

INTRODUCTION:

H Bridge configuration is commonly used in electrical applications where the load needs to be driven in either direction. A typical H-Bridge structure is shown below








The current flows through the load M – Motor in one direction when S1 and S4 switches are closed and current flows in the other direction when S2 and S3 switches are closed.

The components that realize the switching action are commonly transistors. Two types of transistors, NPN and PNP for BJTs, N-Channel and P-Channel for MOSFETs are needed for the proper biasing where the high side is P-type and the low side is N-type.

In this project, we use MOSFETs because of their high switching speed and low RDS resistance for low heat dissipation. H-Bridge configuration requires both P and N type MOSFETs but since N-type MOSFETs have improved electrical characteristics, using only N-type for four of the transistors will be ideal. IR2110 half bridge MOSFET and IGBT driver IC allows us to do this. By using a boost-up capacitor, it can bias the high side N-type MOSFETs so we get rid of the P-type.

C1, C5, C6 : 100uF/16V Electrolytic Capacitor
C3, C4, C7, C8: 10uF/16V Electrolytic Capacitor
C2 : 100nF Polyester Capacitor
Con1, Con2, Con3, Con4 : 2×4 Terminal
D1 : 1N4001 Diode
D2, D3, D4, D5, D6 : 1N5818 Schottky Diode
Q1, Q2, Q3, Q4 : IRF3205 Power MOSFET
R1, R2, R3, R4 : 1/4W Resistor
U1, LM7805CV Linear +5V Voltage Regulator
U2, U3 : IR2110 High and Low Side Driver

When one current way is off, namely its control signal is low, the boost up capacitor is charged up. When this way turns on, the boost up capacitor starts to bias the high side MOSFET until it fully discharges.So it is not possible to drive the motor in one way continuously without a PWM control signal. By using PWM control signals you can easily adjust the speed of motor and continuously run the load in one way. Same is also valid for the other way of current.

The MOSFETs used in this project are International Rectifier’s IRF3205 which can handle up to 115A drain current and 55V Drain to Source voltage. It has 0.008 Ohm RDS resistance. For lower currents (~0-5A) heat dissipation will be too low. But if you will use this board for high current applications you should connect a heatsink. On the other hand you can choose a different MOSFET that suits your needs.

As it is shown in the schematic, we input +12V DC supply voltage to the board. +12V is used for gate driving of MOSFETs. A LM7805 linear voltage regulator converts +12V to +5V DC for the logical supply of IR2110 which is suitable for microcontroller applications.

The board has 8 terminal connections. From left to right;

1 and 2 : Load connection
3 : Ground
4 : +12V DC
5 : Load Supply ground
6 : Load Supply positive voltage
7 – 8 : PWM signals


The construction of the circuit board is easy. The PCB file is provided in pdf format. You can apply it to the board by using the ironing method.


DOWNLOADS:

CLICK HERE TO DOWNLOAD THE PCB OF THE PROJECT






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Web Data on an LED DisplaY Project

INTRODUCTIION:

It is an alphanumeric LED display that connects via USB to my PC and can display RSS feeds, the weather (like wind chill, above), the time, or just about anything.
When I built my Homemade Digital Clock, I shamelessly over designed it. It had an eight character alphanumeric display (plus two discrete LEDs for a colon), four more discrete LEDs for indicators, two printed circuit boards, and two microcontrollers (three, if you count the GPS it uses as its time source), one of which was running at 20MHz. That’s more processing power than the Apollo spacecraft had on board! All for a stupid clock!
I had a few reasons for this. First was that I had the idea that I might re-use the display for other things in the future. Second was that when all you have is hammer, everything looks like a nail. I’m a software engineer by trade, so my first thought is always to use a programmable microcontroller to solve a problem. I’m so bad that if I needed a square wave, I’d more likely use an 8 pin microcontroller than a 555 chip.

Anyway, when I had my clock’s printed circuit boards made, I had them make several. Now, I’ve added a USB port to one of them, connected it to my PC, and written a program in C# to control it. It can fetch data from local files or the web, extract specific pieces of data, format them and display them on the LEDs. The user can enter C# expressions that evaluate to true or false and cause the discrete LEDs to light up (and even flash). The user can also configure alarm sounds and actions to take when the front panel buttons are pressed.
I’m making my schematic, board layout, and software available for free here. The UI is definitely for techies only, but it could probably be adapted to other PC-controlled displays like those from Crystalfontz and Matrix Orbital.
Here’s a short introduction to the display’s capabilities and UI (if you’re wondering what the computer is in the background, it’s a modern PC in a small form-factor Altair 8800 replica case.

And if you’re considering downloading and playing with the UI, here’s a much more detailed description of it
Finally, here’s the schematic and layout (readable by the open source program Kicad), the PIC 18F4550 firmware (readable by Microchip’s MPLAB and using their free C18 compiler), and the PC UI (readable by Microsoft’s free Visual C# Express):

SCHEMATICS:

DOWNLOADS:

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Frequency counter or Frequency meter using PIC16F628 Project

INTRODUCTION:

In this project we are building a basic and low cost frequency counter circuit . It can measure from 16Hz to 100Hz signals with a maximum amplitude of 15V. The sensitivity is high, the resolution is 0.01Hz. The input signal can be a sine, a square or a triangle waveform.





PROJECT DESCRIPTION:


The counter can be used in many applications. For instance, to observe an oscillator’s accuracy, to measure the mains frequency or to find out the rpm of a motor that is connected to an encoder.

The PCB file is provided in pdf format. You can apply it to the board by using the ironing method.

The components are listed below.

1 x PIC16F628 – 04/P Microcontroller
4 x Common Cathode 7 Segment Display
1 × 4N25 General Purpose Phototransistor Optocoupler
5 x BC547 NPN Transistor
1 × 7805 Voltage Regulator
7 × 330 Ohm 1/4 W Resistor
7 × 1K 1/4 W Resistor
1 × 470 Ohm 1/4 W Resistor
1 × 10K 1/4 Resistor
1 × 4.7K 1/4 W Resistor
1 × 1N4148 Diode
2 × 220nF Polyester Capacitor
2 × 22pF Ceramic Capacitor
2 × 100uF 16V Electrolytic Capacitor
1 × 4Mhz Crystal Oscillator


The CCP (Capture/Compare/PWM) module of the PIC microcontroller counts the input signal. Only the capture function is used. To learn more about the CCP module of the PIC please visit http://www.microchip.com.

The displays are 14.2 mm common cathode seven segment LEDs with red light.


Before measuring the frequency of the input signal, the signal must be converted to the square waveform. So an optical isolator circuitry with 4N25 optocoupler is used for this purpose. So the input signal is safely isolated from the microcontroller circuit and converted to square wave. The signal amplitude must not exceed 15V. If this happens, 1k resistor may burn. If you want to measure the mains frequency, you should use a 220V/9V transformer first.

The supply voltage should be between 8-12V. Since the circuit may be defected, you should be careful about the polarity while connecting the supply.

The counter circuit schematic is given in the project file. There are 4 displays that are driven by the multiplexing method. To make the measurement, the RB3 pin is connected to the output of the optic isolator. The second display’s 5 numbered pin is connected to the supply via 1K resistor so the dot after the second display brights. This connection isn’t shown in the schematic.

The C code that is writen with Hi-tech PIC C compiler is available in the downloadable project file. The hex code is also included.

We used extra two sockets. One (18 pin, 2 way) is for the PIC16F628 microcontroller, and the other one is (40 pin, 2 way) for the seven segment displays.

DOWNLOADS:

CLICK HERE TO DOWNLOAD THE COMPLETE PROJECT WITH CODE AND FILES









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IR remote extender Project

INTRODUCTION:


This project describes how to build an IR remote control extender / repeater to control your electronic appliances from a remote location.

An IR detector module receives IR signal from remote control and two IR leds are re-emitting the signal to the appliance. You can place the IR emitting leds close to the device you would like to control using some wire and keep main unit close to remote control location. In the image at the left LEDs are soldered on the board. The circuit consists of three main parts, the IR receiver module, a 555 timer configured as an oscillator and the output / emitter stage. We will describe circuit operation below.

Circuit is designed by Andy Collinson and can be found here:   http://www.zen22142.zen.co.uk

IR Signal:

The IR signal emitted from a remote control caries the information needed to control the appliance. This signal consists of pulses that code 0 and 1 bits, instructing the appliance to do a certain operation. One of the most common protocols used to code the IR signal is Philips - RC5 protocol. The signal consists of two parts, the control pulses and the carrier wave as seen in the image below.

A common frequency used for the carrier is 38KHz and control pulses frequency is in the range of 1-3KHz. The carrier signal is modulated by the control pulses and the resulting signal is emitted by remote in IR band of electromagnetic spectrum. IR band is invisible to human eye. You can see if an IR led is emitting light or not using a camera. Point the camera to the led and you will see that light comes off.


Circuit description:

IR signal is received by TSOP1738. TSOP1738 is an infrared receiver at 38KHz. At the output of infrared receiver we get a demodulated signal that means we get the low frequency control pulses. Infrared receiver is powered from C1, R1 and Z1 that forms a 5V power supply. With no signal received, infrared detector output is high and Q1 is on, so pin 4 of IC is LOW and 555 timer is in reset state. Q1 also acts as a level shifter that converts 5V signal of TSOP1738 to 9V signal for IC1.
When HIGH control pulses are appearing on TSOP1738 output then timer 555 (which is configured as an oscillator) starts to oscillate at a preset frequency, for the duration of each data pulse. That means that at pin 3 we get a signal that is similar to modulated source signal. It has a carrier component and a control pulses component. Oscillating frequency of 555 timer is set by R4 and C2 and pulse period is given by:

T = 1,4 R4 C2

Trimmer R5 is used to fine tune oscillating frequency at 38KHz. That's equal to carrier frequency.   

          The output stage is formed from R6, Q2, one red LED, two IR LEDs and two current limiting resistors R7 and R8. Q2 is connected as voltage follower, that means when base of Q2 is HIGH transistor is ON allowing current to flow through LEDs. LED current is set by R7 and R8 according the following formula:
So IR LEDs are emitting a signal that is similar to the signal received by TSOP1738, that means it repeats the signal received at higher infrared radiation intensity. The red LED is used as an optical indicator of output signal. Circuit can be powered from a 9V battery.  





Parts List:

R1 = 1k 
R2 = 3k3 
R3 = 10k 
R4 = 15k 
R5 = 4k7 trimmer 
R6 = 2k2 
R7 = 470R 
R8 = 47R - 1/2W 
C1 = 47uF - 16V 
C2 = 1n - polyester 
C3 = 100uF - 16V 
C4 = 47uF - 16V 
Z1 = 5V1 zener 
Q1 = BC549C 
Q2 = BC337 
IC1 = NE555 
LED1 = red LED 
LED2,3 = IR LED 
IR receiver = TSOP138 or IR38DM



PCB:

Download PCB files in EAGLE format or PDF format

Testing:

Before powering the circuit, remove IR LEDs. With no input red LED should be off. Now press a button on a remote control, red led should flicker. If that's the case then your circuit should be working ok. Install IR LEDs. We found during testing that IR signal emitted from remote and IR signal emitted from circuit are interfering each other and that's make receiving device not to react on receiving the signal, this happens when IR from remote and IR from circuit's LEDs are on the same room. To solve that we must isolate the IR beam of remote control. To do that we used a thin pipe in front of infrared sensor as seen in photo below, so that the beam emitted from remote hits the sensor directly. Another solution to this would be to put the emitting LEDs on a different room.

Installation:

We installed the circuit on the wall the way you see on the photo below. You can see that remote control led is optically isolated from surround. You can also notice that one LED is remotely placed near the device we would like to control.

DOWNLOADS:

CLICK HERE TO DOWNLOAD THE IR REMOTE EXTENDER PROJECT CIRCUIT IN .DOC FORMAT


CLICK HERE TO DOWNLOAD THE PCB OF IR REMOTE EXTENDER PROJECT IN EAGLE FORMAT







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DIGITAL THERMOMETER AND CLOCK PROJECT

INTRODUCTION:

This device uses two digital sensors (DS1620 or DS1820), measures the ambient temperature with 0,1 °C (0,2 °F) resolution and displays it on LCD 2x16 (LM016 etc.) screen. It have a clock, which is based on DS1302 timekeeping chip. This chip stores current date and time. The main CPU used in this project is PIC16F877. The additional 8-digit 7 segment LED display can be used. It is based on PIC16F870 microcontroller. RS232C interface is applied to transmit the information from the main CPU to remote LED display. No calibration required!

MAIN CHARACTERISTICS:


Displays Inside and Outside Temperature.
Celsius and Fahrenheit scales selectable.
Temperature range: -55 °C to 125 °C (-67 °F to 257 °F).
Temperature resolution: ± 0,1 °C (± 0,2 °F).
Clock (24-hour format).
Local LCD display.
Additional LED display (optional).
Digital sensors: DS1620 or DS1820 (optional).
Power supply: DC 9 V from adaptor.

PHOTO IMAGES:



MAIN CPU

REMOTE DISPLAY UNIT







SCHEMATIC DIAGRAMS:


Original version with DS1620 and DS1820 sensors


Version with DS1620 sensors




 FIRMWARE DOWNLOAD:





REPORT DOWNLOAD:


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AUTOMATIC FAN CONTROL

INTRODUCTION:

Just download the project PDF and copy the schematics  and the project should work ok with you, here is the complete guidance for you. 
Th1, the 50K thermistor, is a standard type. Mine was a bar or rectangular looking thingy. Available from Tandy/Radio-Shack. Almost any type will do. I experimented with different models from 22K to 100K
and all worked fine after replacing the trimmer pot. The one used in the above circuit diagram was a 50K model. This 50K was measured at exactly 25 °C and with 10% tolerance. The resistance increases as the surrounding temperature decreases. Tolerance for my application (cooling a large powersupply coolrib) is 10%. Another name for this thing is 'NTC'. NTC stands for "Negative Temperature Coefficient" which means when the surrounding temperature decreases the resistance of this thermistor will increase. I replaced my thermistor for a 60K hermetically 
sealed glass type since the environment for my application may contain corrosive particles which may affect performance on a future date. P1 is a regular Bourns trimmer and adjusts a wide range of temperatures for this circuit. 

I used the 10-turn type for a bit finer adjustment but the regular type will work for your application.
R1 is a 'security' resistor just in case the trimmer pot P1 is adjusted all the way to '0' ohms. At which time the thermistor would get the full 12 volt and it will get so hot that it puts blisters on your fingers... :-)R3 feeds a bit of hysteresis back into the op-amp to eliminate relay 'chatter' when the temperature of the thermistor reaches its threshold point. Depending on your application and the type you use for Q1 and Re1, start with 330K or so and adjust its value downwards until your satisfied. The value of 150K shown in the diagram worked for me. Decreasing the value of R2 means more hysteresis, just don't use more then necessary. Or temporarily use a trimmer pot and read off the value. 120K worked for me.

Transistor Q1 can be a 2N2222(A), 2N3904, NTE123A, ECG123A, etc. Not critical at all. It acts only as a switch for the relay so almost any type will work, as long as it can provide the current needed to activate the relay's coil. D1, the 1N4148, acts as a spark arrestor when the contacts of the relay open and eliminates false triggering. For my application the 1N4148 was good enough since the tiny relay I used was only 1 amp. 

However, you can use a large variety of diodes here, my next choice would be a regular purpose 1N4001 or something and should be used if your relay type can handle more then 1 amp. If you like to make your own pcb, try the one above. The pcb is fitted with holes for the relay but may not fit your particular relay. It was designed for a Aromat HB1-DC12V type. 

The variety and model of relays is just to great. How to mount it then? Well, I left ample space on the pcb to mount your relay. You can even mount it up-side-down and connect the wires individually. Use Silicon glue, cyanoacrylate ester (crazy glue), or double-sided tape to hold the relay in place. Works well. Note that the pcb and layout is not according to the circuit diagram in regards to the hookup of the fans. The PCB measures approximately 1.5 x 3 inches (4.8 x 7.6mm) If you print the pcb to an inkjet printer it is probably not to scale. Try to fit a 8-pin ic socket on the printed copy to make sure it fits before making the pcb...

DOWNLOADS:

CLICK HERE TO DOWNLOAD THE COMPLETE PROJECT PDF




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