Thermistor with OPAMP control circuit

Thermistor with OPAMP control circuit

 

Thermistor with LM324 OPAMP

A thermistor is a type of resistor used to measure temperature changes, relying on the change in its resistance with changing temperature. Thermistor is a combination of the words thermal and resistor. The Thermistor was first invented by Samuel Ruben in 1930.

If we assume that the relationship between resistance and temperature is linear (i.e. we make a first-order approximation), then we can say that: 

ΔR = Kδt 

Where ΔR = change in resistance ΔT = change in temperature k = first-order temperature coefficient of resistance 

Thermistors can be classified into two types depending on the sign of k. If k is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC) thermistor, Posistor. If k is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor. Resistors that are not thermistors are designed to have the smallest possible k, so that their resistance remains almost constant over a wide temperature range. 

Circuit Description: 

In this circuit the thermistor is used to measure the temperature. Thermistor is nothing but temperature sensitive resistor. There are two type of thermistor available such as positive temperature co-efficient and negative temperature co- efficient. Here we are using negative temperature co-efficient in which the resistance value is decreased when the temperature is increased. Here the thermistor is connected with resister bridge network. The bridge terminals are connected to inverting and non-inverting input terminals of comparator. 

The comparator is constructed by LM 324 operational amplifier. The LM 324 consist of four independent, high gains, internally frequency compensated operational amplifier which were designed specifically to operate from a single power supply over a wide voltage range. The first stage is a comparator in which the variable voltage due to thermistor is given to inverting input terminal and reference voltage is given to non-inverting input terminal.

Initially the reference voltage is set to room temperature level so the output of the comparator is zero. When the temperature is increased above the room temperature level, the thermistor resistance is decreased so variable voltage is given to comparator. Now the comparator delivered the error voltage at the output. Then the error voltage is given to next stage of preamplifier.

Here the input error voltage is amplified then the amplified voltage is given to next stage of gain amplifier. In this amplifier the variable resistor is connected as feedback resistor. The feedback resistor is adjusted to get desired gain. Then the AC components in the output are filtered with the help of capacitors. Then output voltage is given to final stage of DC voltage follower through this the output voltage is given to ADC or other circuit.

Digital to Analog Converter DAC 0800

Digital to Analog Converter : DAC 0800

Digital to Analog Converter(DAC 0800) with Current to Voltage converter(LM741)

In electronics, a digital-to-analog converter (DAC or D-to-A) is a device for converting a digital (usually binary) code to an analog signal (current, voltage or electric charge). Digital-to-analog converters are interfaces between the abstract digital world and analog real life. An analog-to-digital converter (ADC) performs the reverse operation. A DAC usually only deals with pulse-code modulation (PCM)-encoded signals. The job of converting various compressed forms of signals into PCM is left to codecs. Basic Operation: The DAC fundamentally converts finite-precision numbers (usually fixed-point binary numbers) into a physical quantity, usually an electrical voltage. 

Normally the output voltage is a linear function of the input number. Usually these numbers are updated at uniform sampling intervals and can be thought of as numbers obtained from a sampling process. These numbers are written to the DAC, sometimes along with a clock signal that causes each number to be latched in sequence, at which time the DAC output voltage changes rapidly from the previous value to the value represented by the currently latched number. The effect of this is that the output voltage is held in time at the current value until the next input number is latched resulting in a piecewise constant output. This is equivalently a zero-order hold operation and has an effect on the frequency response of the reconstructed signal. 

Basic Operation: The DAC fundamentally converts finite-precision numbers (usually fixed-point binary numbers) into a physical quantity, usually an electrical voltage. Normally the output voltage is a linear function of the input number. Usually these numbers are updated at uniform sampling intervals and can be thought of as numbers obtained from a sampling process. These numbers are written to the DAC, sometimes along with a clock signal that causes each number to be latched in sequence, at which time the DAC output voltage changes rapidly from the previous value to the value represented by the currently latched number. The effect of this is that the output voltage is held in time at the current value until the next input number is latched resulting in a piecewise constant output. This is equivalently a zero-order hold operation and has an effect on the frequency response of the reconstructed signal. The fact that practical DACs do not output a sequence of dirac impulses (that, if ideally low-pass filtered, result in the original signal before sampling) but instead output a sequence of piecewise constant values or rectangular pulses, means that there is an inherent effect of the zero-order hold on the effective frequency response of the DAC resulting in a mild roll-off of gain at the higher frequencies (a 3.9224 Db loss at the Nyquist frequency). 

This zero-order hold effect is a consequence of the hold action of the DAC and is not due to the sample and hold that might precede a conventional analog to digital converter as is often misunderstood. DAC0800 The DAC0800 series are monolithic 8 bit high speed current output digital to analog faturing typical setting times of 100ns. When used as a multiplying DAC, monotonic performance over a 40 to 1 refeence current range is possible. The DAC0800 series also features high complemementary current output to allow diferential output voltages of 20 Vp-p with simple resistor loads. The reference to full scale current matching of better than l LAB elimanates the need for full scale trims in most application while the nonlinearities of better than 0.1 over temperature minimize system error accumulations. 

The noise immune inputs of the DAC0800 series wil accepet TTL levels with the logic thershold pin grounded. Channging the Vlc potential will allow direct interface to other logic families. The pefrormance and characteriststics of the device are essentially unchanged over the full 4.5v to 18v power supply range power dissipation is only 33mvw with +5v supplies and is independent of the logic input states. The output of the DAC is current signal. So it is given to current voltage converter which is constructed by the LM 741 operational amplifier. Finally the anlaog voltage is given to Triac or SCR control circuit.

PROXIMITY SENSORs

PROXIMITY SENSOR

A proximity sensor detects an object when the object approaches within the detection boundary of the sensor. Proximity sensors are used in various facets of manufacturing for detecting the approach of metal objects. 

Various types of proximity sensors are used for detecting the presence or absence of an object. The design of a proximity sensor can be based on a number of principles of operation, some examples include: variable reluctance, eddy current loss, saturated core, and Hall Effect. Depending on the principle of operation, each type of sensor will have different performance levels for sensing different types of objects. 

Common types of non-contact proximity sensors include inductive proximity sensors, capacitive proximity sensors, ultrasonic proximity sensors, and photoelectric sensors. Hall-effect sensorsdetect a change in a polarity of a magnetic field. 

Variable reluctance sensors typically include a U-type core and coils wound around the core legs. Inductive proximity sensors have a lossy resonant circuit (oscillator) at the input side whose loss resistance can be changed by the proximity of an electrically conductive medium. 

An electrical capacitance proximity sensor converts a variation in electrostatic capacitance between a detecting electrode and a ground electrode caused by approaching the nearby object into a variation in an oscillation frequency, transforms or linearizes the oscillation frequency into a direct current voltage, and compares the direct current voltage with a predetermined threshold value to detect the nearby object. 

Ultrasonic sensing systems provide a much more efficient and effective method of longer range detection. These sensors require the use of a transducer to produce ultrasonic signals. 

Eddy-current proximity sensors are well known and operate on the principle that the impedance of an ac-excited electrical coil is subject to change as the coil is brought in close proximity to a metallic object. 

Proximity sensors often are employed in manufacturing industries in which the sensors are exposed to harsh environmental conditions. Inductive proximity sensors are used in automation engineering to define operating states in automating plants, production systems and process engineering plants.

Magnetic proximity detectors are commonly used on ski lifts and tramways for detecting a derope condition of the steel cable used as a haul line or haul rope. 

Proximity sensors are widely used in the automotive industry to automate the control of power accessories. For instance, proximity sensors are often used in power window controllers to detect the presence of obstructions in the window frame when the window pane is being directed to the closed position.

IC ULN 2803 driver circuit

IC ULN 2803 driver circuit

Logic diagramm

A ULN2803 is an Integrated Circuit (IC) chip with a High Voltage/High Current Darlington Transistor Array. It allows you to interface TTL signals. A TTL signal operates from 0-5V, with everything between 0.0 and 0.8V considered "low" or off, and 2.2 to 5.0V being considered "high" or on. The maximum power available on a TTL signal depends on the type, but generally does not exceed 25mW (~5mA @ 5V), so it is not useful for providing power to something like a relay coil. Computers and other electronic devices frequently generate TTL signals. On the output side the ULN2803 is generally rated at 50V/500mA, so if can operate small loads directly. 

Alternatively, it is frequently used to power the coil of one or more relays, which in turn allow even higher voltages/currents to be controlled by the low level signal. In electrical terms, the ULN2803 uses the low level (TTL) signal to switch on/turn off the higher voltage/current signal on the output side. The ULN2803 comes in an 18-pin IC configuration and includes eight (8) transistors. Pins 1-8 receive the low level signals; pin 9 is grounded (for the low level signal reference). Pin 10 is the common on the high side and would generally be connected to the positive of the voltage you are applying to the relay coil. Pins 11-18 are the outputs (Pin 1 drives Pin 18, Pin 2 drives 17, etc.).

The ULN2803 is a small integrated circuit that contains 8 transistor driver channels. Each channel has an input to a resistor connected to the base of a transistor and a 1 amp open collector output capable of handling up to about 30volts .Each of the collectors has a reverse biased diode connected to a common Vcc pin that provides inductive spike protection. Typical uses are for micro-processor interfaces to relays, lamps, solenoids and small motors. A 2803 with a set of relays is a simple and effective way of switching mains voltages for example.

Driver Features 

• TTL, DTL, PMOS, or CMOS Compatible Inputs 

• Output Current to 500 mA 

• Output Voltage to 95 V 

• Transient-Protected Outputs 

• Dual In-Line Package or Wide-Body Small-Outline Package 

Relay Vs Switch

A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and they are double throw (changeover) switches. 

Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits; the link is magnetic and mechanical. The coil of a relay passes a relatively large current, typically 30mA for a 12V relay, but it can be as much as 100mA for relays designed to operate from lower voltages. 

Most ICs (chips) cannot provide this current and a transistor is usually used to amplify the small IC current to the larger value required for the relay coil. The maximum output current for the popular 555 timer IC is 200mA so these devices can supply relay coils directly without amplification. 

Relays are usually SPDT or DPDT but they can have many more sets of switch contacts, for example relays with 4 sets of changeover contacts are readily available. For further information about switch contacts and the terms used to describe them please see the page on switches. Most relays are designed for PCB mounting but you can solder wires directly to the pins providing you take care to avoid melting the plastic case of the relay. The supplier's catalogue should show you the relay's connections. The coil will be obvious and it may be connected either way round. Relay coils produce brief high voltage 'spikes' when they are switched off and this can destroy transistors and ICs in the circuit. To prevent damage you must connect a protection diode across the relay coil. The animated picture shows a working relay with its coil and switch contacts.

You can see a lever on the left being attracted by magnetism when the coil is switched on. This lever moves the switch contacts. There is one set of contacts (SPDT) in the foreground and another behind them, making the relay DPDT. 

The relay's switch connections are usually labeled COM, NC and NO: 

• COM = Common, always connect to this,it is the moving part of the switch. 

• NC = Normally Closed, 

COM is connected to this when the relay coil is off.

• NO = Normally Open, COM is connected to this when the relay coil is on. 

• Connect to COM and NO if you want the switched circuit to be on when the relay coil is on. 

• Connect to COM and NC if you want the switched circuit to be on when the relay coil is off. 

 Choosing a relay You need to consider several features when choosing a relay: 

1. Physical size and pin arrangement If you are choosing a relay for an existing PCB you will need to ensure that its dimensions and pin arrangement are suitable. You should find this information in the supplier's catalogue. 

2. Coil voltage The relay's coil voltage rating and resistance must suit the circuit powering the relay coil. Many relays have a coil rated for a 12V supply but 5V and 24V relays are also readily available. Some relays operate perfectly well with a supply voltage which is a little lower than their rated value. 

3. Coil resistance The circuit must be able to supply the current required by the relay coil. You can use Ohm's law to calculate the current: Relay coil current = supply voltage coil resistance 

4. For example: A 12V supply relay with a coil resistance of 400 passes a current of 30mA. This is OK for a 555 timer IC (maximum output current 200mA), but it is too much for most ICs and they will require a transistor to amplify the current. 

5. Switch ratings (voltage and current) The relay's switch contacts must be suitable for the circuit they are to control. You will need to check the voltage and current ratings. Note that the voltage rating is usually higher for AC, for example: "5A at 24V DC or 125V AC". 

6. Switch contact arrangement (SPDT, DPDT etc) Most relays are SPDT or DPDT which are often described as "single pole changeover" (SPCO) or "double pole changeover" (DPCO). For further information please see the page on switches. 

Protection diodes for relays Transistors and ICs (chips) must be protected from the brief high voltage 'spike' produced when the relay coil is switched off. The diagram shows how a signal diode (e.g. 1N4148) is connected across the relay coil to provide this protection. Note that the diode is connected 'backwards' so that it will normally not conduct. Conduction only occurs when the relay coil is switched off, at this moment current tries to continue flowing through the coil and it is harmlessly diverted through the diode. Without the diode no current could flow and the coil would produce a damaging high voltage 'spike' in its attempt to keep the current flowing. 

Advantages of relays: • Relays can switch AC and DC, transistors can only switch DC. • Relays can switch high voltages, transistors cannot. • Relays are a better choice for switching large currents (> 5A). • Relays can switch many contacts at once.

Disadvantages of relays: • Relays are bulkier than transistors for switching small currents. • Relays cannot switch rapidly (except reed relays), transistors can switch many times per second. • Relays use more power due to the current flowing through their coil. • Relays require more current than many chips can provide, so a low power transistor may be needed to switch the current for the relay's coil.

LIQUID CRYSTAL DISPLAY 

Liquid crystal cell displays (LCDs) are used in similar applications where LEDs are used. LCD displays are common in Engineering applications and play a vital role.  These applications are display of numeric and alphanumeric characters in dot matrix and segmental displays. Now a days Computer monitors and TV, Watches and advertisements  use  LCD technology for display. LCD display produces high quality images and thinner compared with CRT technologies. The basic idea behind the working of LCD is given here. 

LCDs are of two types: 

I. Dynamic scattering type 

II. Field effect type 

The construction of a dynamic scattering liquid crystal cell:

The liquid crystal material may be one of the several components, which exhibit optical properties of a crystal though they remain in liquid form. Liquid crystal is layered between glass sheets with transparent electrodes deposited on the inside faces. When a potential is applied across the cell, charge carriers flowing through the liquid disrupt the molecular alignment and produce turbulence. When the liquid is not activated, it is transparent. When the liquid is activated the molecular turbulance causes light to be scattered in all directions and the cell appeas to be bright.This phenomenon is called dyanamic scattering. 

source of image: Wikipedia

The construction of a field effect liquid crystal display 

This is similar to that of the dynamic scattering type,with the exception that two thin polarizing optical filters are placed at the inside of each glass sheet. The liquid crystal material in the field effect cell is also of different type from employed in the dynamic scattering cell. The material used is twisted nemayic type and actually twists the light passing through the cell when the latter is not energised. i. Transmittive type ii. Reflective type In the transmittive type cell, both glass sheets are transparent, so that light from a rear source is scattered in the forward direction when the cell is activated. In reflective type cell has a reflecting surface on one side of glass sheets. The incident light on the front surface of the cell is dynamically scattered by an activated cell. Both types of cells appear quite bright when activated even under ambient light conditions. The liquid crystals are light reflectors are transmitters and therefore they consume small amounts of energy (unlike light generators). 

The seven segment display, the current is about 25 micro Amps for dynamic scattering cells and 300micro amps for field effect cells. Unlike LEDs which can work on d.c. the LCDs require a.c. voltage supply. A typical voltage supply to dynamic scattering LCD is 30v peak to peak with 50 Hz

Analog to Digital Converter module

Analog to Digital Converter module

   Analog to Digital converter modules are used in Micro controller based projects where the analog signals are required to be converted into digital signal for further processing in Micro controller. The integrated chip used for this purpose is 0809 ADC. This post describe briefly the PIN diagram, block diagram and details of this specified chip here.

Pin Diagram

Circuit Diagram of ADC 0809

The ADC totally consists of 28 pins with 8 inputs and 8 outputs.The output from the filter is given to pin 26 of ADC 0809 shown in the figure above.The address channels A, B, C are grounded so that channel 1 is enabled. The digitized output from the converter is given to port 0 of micro controller. The control signals from the ADC are given to port 2 of the Microcontroller. This circuit follows the principle of successive approximation method and when the start of conversion goes high, it marks the beginning of the process and high end of conversion marks the end of it.The two capaciors 10F and 100F acts as filter. The filter capacitors in the circuit remove the low and high frequency noises. The LED is used to check the proper functioning of the circuit.

RF transmitter and Receiver modules are available in chip which can be effectively embedded with engineering projects.

These devices are simple pass-through integrated circuits. Meaning, you set up your baud-rate (as long as its within an acceptable range of whatever pair of devices you are using), and then start sending bytes to the transmitter. Quite simply, it just sends your data out the transmitter and the receiver grabs it, acting as if you had a wired serial connection between them, minus the wire!  The following PIN diagram shows the details of TLP 434,434A, 916A chips. The input/output units, antennas are integrated in this chip.

RF Transmitter

RF Receiver

MAX 232 and RS-232

MAX232 protocol:

The MAX232 is an integrated circuit that converts signals from an RS-232 serial port to signals suitable for use in TTL compatible digital logic circuits. The MAX232 is a dual driver/receiver and typically converts the RX, TX, CTS and RTS signals. The drivers provide RS-232 voltage level outputs (approx. ± 7.5 V) from a single + 5 V supply via on-chip charge pumps and external capacitors. This makes it useful for implementing RS-232 in devices that otherwise do not need any voltages outside the 0 V to + 5 V range, as power supply design does not need to be made more complicated just for driving the RS-232 in this case. The receivers reduce RS-232 inputs (which may be as high as ± 25 V), to standard 5 V TTL levels. These receivers have a typical threshold of 1.3 V, and a typical hysteresis of 0.5 V. It is helpful to understand what occurs to the voltage levels. When a MAX232 IC receives a TTL level to convert, it changes a TTL Logic 0 to between +3 and +15 V, and changes TTL Logic 1 to between -3 to -15 V, and vice versa for converting from RS232 to TTL. This can be confusing when you realize that the RS232 Data Transmission voltages at a certain logic state are opposite from the RS232 Control Line voltages at the same logic state. 

RS232 Level shifting

RS232 Protocol 

In telecommunications, RS-232 (Recommended Standard 232) is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE (Data Terminal Equipment) and a DCE (Data Circuit-terminating Equipment). It is commonly used in computer serial ports. The standard defines the electrical characteristics and timing of signals, the meaning of signals, and the physical size and pinout of connectors. The current version of the standard is TIA-232-F Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange, issued in 1997.

In RS-232, user data is sent as a time-series of bits. Both synchronous and asynchronous transmissions are supported by the standard. In addition to the data circuits, the standard defines a number of control circuits used to manage the connection between the DTE and DCE. Each data or control circuit only operates in one direction, that is, signaling from a DTE to the attached DCE or the reverse. Since transmit data and receive data are separate circuits, the interface can operate in a full duplex manner, supporting concurrent data flow in both directions. The standard does not define character framing within the data stream, or character encoding. The RS-232 standard defines the voltage levels that correspond to logical one and logical zero levels for the data transmission and the control signal lines. Valid signals are plus or minus 3 to 15 volts; the ±3 V range near zero volts is not a valid RS-232 level. The standard specifies a maximum open-circuit voltage of 25 volts: signal levels of ±5 V, ±10 V, ±12 V, and ±15 V are all commonly seen depending on the power supplies available within a device. RS-232 drivers and receivers must be able to withstand indefinite short circuit to ground or to any voltage level up to ±25 volts. 

The slew rate, or how fast the signal changes between levels, is also controlled. For data transmission lines (TxD, RxD and their secondary channel equivalents) logic one is defined as a negative voltage, the signal condition is called marking, and has the functional significance. Logic zero is positive and the signal condition is termed spacing. Control signals are logically inverted with respect to what one sees on the data transmission lines. When one of these signals is active, the voltage on the line will be between +3 to +15 volts. The inactive state for these signals is the opposite voltage condition, between −3 and −15 volts. Because both ends of the RS-232 circuit depend on the ground pin being zero volts, problems will occur when connecting machinery and computers where the voltage between the ground pin on one end, and the ground pin on the other is not zero. This may also cause a hazardous ground loop. Use of a common ground limits RS-232 to applications with relatively short cables. If the two devices are far enough apart or on separate power systems, the local ground connections at either end of the cable will have differing voltages; this difference will reduce the noise margin of the signals. Unused interface signals terminated to ground will have an undefined logic state. Where it is necessary to permanently set a control signal to a defined state, it must be connected to a voltage source that asserts the logic 1 or logic 0 level. Some devices provide test voltages on their interface connectors for this purpose.

HT12D DECODER for Engineering projects

HT12D DECODER  for Engineering projects

The 212 decoders are a series of CMOS LSIs for remote control system applications. They are paired with Holtek 212 series of encoders (refer to the encoder/decoder cross reference table). For proper operation, a pair of encoder/decoder with the same number of addresses and data format should be chosen.

The decoders receive serial addresses and data from a programmed 212 series of encoders that are transmitted by a carrier using an RF or an IR transmission medium. They compare the serial input data three times continuously with their local addresses. If no error or unmatched codes are found, the input data codes are decoded and then transferred to the output pins. 

The VT pin also goes high to indicate a valid transmission. The 212 series of decoders are capable of decoding information that consists of N bits of address and 12_N bits of data. Of this series, the HT12D is arranged to provide 8 address bits and 4 data bits. Operation The 212 series of decoders provides various combinations of addresses and data pins in different packages so as to pair with the 212 series of encoders. The decoders receive data that are transmitted by an encoder and interpret the first N bits of code period as addresses and the last 12_N bits as data, where N is the address code number.

The oscillator is disabled in the standby state and activated when a logic high signal applies to the DIN pin. That is to say, the DIN should be kept low if there is no signal input. The received data and address is checked thrice before setting the VT high.

HT12E Encoder for Engineering projects

HT12E Encoder for Engineering projects

  Most of the Engineering projects need the HT12E encoder for remote controlled systems applications.

They are capable of encoding information which consists of N address bits and 12_N data bits. Each address/ data input can be set to one of the two logic states. The programmed addresses/data are transmitted together with the header bits via an RF or an infrared transmission medium upon receipt of a trigger signal. The capability to select a TE trigger on the HT12E or a DATA trigger on the HT12A further enhances the application flexibility of the 212 series of encoders.

Operation The 212 series of encoders begin a 4-word transmission cycle upon receipt of a transmission enable (TE for the HT12E or D8~D11 for the HT12A, active low). This cycle will repeat itself as long as the transmission enable (TE or D8~D11) is held low. Once the transmission enable returns high the encoder output completes its final cycle and then stops as shown in the timing diagram shown above. 

The status of each address/data pin can be individually pre-set to logic _high_ or _low_. If a transmission- enable signal is applied, the encoder scans and transmits the status of the 12 bits of address/ data serially in the order A0 to AD11 for the HT12E encoder and A0 to D11 for the HT12A encoder. During information transmission these bits are transmitted with a preceding synchronization bit. If the trigger signal is not applied, the chip enters the standby mode and consumes a reduced current of less than 1_A for a supply voltage of 5V. Usual applications preset the address pins with individual security codes using DIP switches or PCB wiring, while the data is selected by push buttons or electronic switches. For the HT12E encoders, transmission is enabled by applying a low signal to the TE pin. 

The address pins of the encoder are joined with controller’s port 0. The data pins of the encoder are linked with controllers port P2.0 to P2.3.Once the data and address are set by the controller, the transmission enable pin of the encoder connected with the pin P2.4 of the controller is set to LOW. Thus enabling the transmission, the data is sent to the RF transmitter module , transmitting the signal continuously till the transmission enable pin to set to HIGH.