When AC is applied to the primary winding of the power transformer it can either be stepped down or up depending on the value of DC needed.
In our circuit the transformer of 230v/15-0-15v is used to perform the step down operation where a 230V AC appears as 15V AC across the secondary winding.
One alteration of input causes the top of the transformer to be positive and the bottom negative.
The next alteration will temporarily cause the reverse.
The current rating of the transformer used in our project is 2A. Apart from stepping down AC voltages, it gives isolation between the power source and power supply circuitries.
In the power supply unit, rectification is normally achieved using a solid state diode.
A commonly used circuit for supplying large amounts of DC power is the bridge rectifier.
A bridge rectifier of four diodes (4*IN4007) are used to achieve full wave rectification.
The output obtained is not a pure DC and therefore filtration has to be done.
FILTERING UNIT: Filter circuits which are usually capacitors acting as a surge arrester always follow the rectifier unit. This capacitor is also called as a decoupling capacitor or a bypassing capacitor, is used not only to ‘short’ the ripple with frequency of 120Hz to ground but also to leave the frequency of the DC to appear at the output.
A load resistor R1 is connected so that a reference to the ground is maintained. C1R1 is for bypassing ripples. C2R2 is used as a low pass filter, i.e. it passes only low frequency signals and bypasses high frequency signals. 1000f/25v : for the reduction of ripples from the pulsating. 10f/25v : for maintaining the stability of the voltage at the load side. O, 1f : for bypassing the high frequency disturbances.
VOLTAGE REGULATORS: The primary purpose of a regulator is to aid the rectifier and filter circuit in providing a constant DC voltage to the device. Power supplies without regulators have an inherent problem of changing DC voltage values due to variations in the load or due to fluctuations in the AC liner voltage. With a regulator connected to the DC output, the voltage can be maintained within a close tolerant region of the desired output. IC7812 and 7912 is used in this project for providing +12v and –12v DC supply.
Reference analog supply after being regulated is given to MCLR pin of PIC through the potential divider.
Potential divider used for setting the dynamic response range of the reference supply i.e.., reference 5v can be used as it is or it can be made into a fraction of the 5v for example 1v so that readings in this range can be read with more precision. This is because the ADC has 10-bit resolution, which can be totally used for representing the 1v rather than 5v.
The pins 2-5,7-10,35 and 36 are used as the 10 channels of the ADC. To these pins the analog inputs to be processed by the ADC are given.
Crystal oscillator is also used to provide clock input. It is of 10 MHz and gives a baud rate of 9600 bits/s.
PIC Circuit Diagram:
MICROCHIP SERIES – PIC 16F877 BASIC FEATURES: Operating speed: DC - 20 MHz clock input Up to 8K x 14 words of FLASH Program Memory Up to 368 x 8 bytes of Data Memory (RAM) Up to 256 x 8 bytes of EEPROM Data Memory Input / Output Ports – 33 Pins Interrupt capability (up to 14 sources) Watchdog Timer (WDT) with its own on-chip RC oscillator for reliable operation In-Circuit Debugging via two pins
Relay Circuit: Relays are electro mechanical devices and basically consist of an electro magnet and a number of contact sets. The prime function of a relay is the switching of large currents from small currents. In fact currents of many hundreds or even thousands of Amps can be switched by just a couple of hundred milli Amps.
Our 2 changeover relay the coil, marked CL1 and the two changeover contact sets A & B. Each set has three connections, the common being A2 & B2, normally A1 & B1 are open contacta and A3 & B3 are closed contacts. The 'normally' word in the context of relays always refers to the de-energised (no power to the coil) state. The circuit shows a battery B1, to supply power to operate the relay and a simple pushbutton switch SW1. SW1 is shown in the OPEN (not pressed) position so no Volts are connected to the relay coil. In this condition 2 separate circuits via contact sets A2 / A3 and B2 / B3 are electrically connected. When voltage is applied to the relay coil CL1, the magnet energises and closes the contact set changing the connected circuits to A1 / A2 & B1 / B2.
DC Motor: DC motor reversing can be achieved with the use of a simple changeover switch, and when wired, it changes over the polarity of the voltage when operated. As DC motors can draw lots more current under loaded conditions, it's a good idea to ensure the DC Amps rating of your switch can handle the required loaded current of the motor. Figure 1 shows a typical hook-up with the changeover switch shown as SW1, the switch has six connections and when wired the motor will change direction when the switch is operated. This simple solution may be enhanced by using a 3 position double pole changeover switch. This will then give FORWARD, REVERSE and OFF control without any additional wiring. When DC Amperage exceeds 8 to 10 Amps it is often no longer practical to use a simple switch - this is when the use of a clever device called a relay comes to the rescue !
A Light Dependent Resistor (aka LDR, photoconductor, or photocell) is a device which has a resistance which varies according to the amount of light falling on its surface.
Light dependent resistors are a vital component in any electric circuit which is to be turned on and off automatically according to the level of ambient light - for example, solar powered garden lights, and night security lighting.
An LDR can even be used in a simple remote control circuit using the backlight of a mobile phone to turn on a device - call the mobile from anywhere in the world, it lights up the LDR, and lighting (or a garden sprinkler) can be turned on remotely!