Wednesday, August 18, 2010

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LADDER LOGIC INPUT

PLC inputs are easily represented in ladder logic In Figure 2.11 there are three types of inputs shown. The first two are normally open and normally closed inputs, discussed previously. The IIT (Immediate InpuT) function allows inputs to be read after the input scan, while the ladder logic is being scanned. This allows ladder logic to examine input values more often than once every cycle.

Monday, August 9, 2010

Ladder Logic

ladder logic  is the main programming method used for PLCs. As mentioned before, ladder logic has been developed to mimic relay logic. The decision to use the relay logic diagrams was a strategic one. By selecting ladder logic as the main programming method, the amount of retraining needed for engineers and tradespeople was greatly reduced.


Modern control systems still include relays, but these are rarely used for logic. A relay is a simple device that uses a magnetic field to control a switch, as pictured in Figure 2.1. When a voltage is applied to the input coil, the resulting current creates a magnetic field. The magnetic field pulls a metal switch (or reed) towards it and the contacts touch, closing the switch. The contact that closes when the coil is energized is called normally open. The normally closed contacts touch when the input coil is not energized. Relays are normally drawn in schematic form using a circle to represent the input coil. The output contacts are shown with two parallel lines. Normally open contacts are shown as two lines, and will be open (non-conducting) when the input is not energized. Normally closed contacts are shown with two lines with a diagonal line through them. When the input coil is not energized the normally closed contacts will be closed (conducting).



Relays are used to let one power source close a switch for another (often high current) power source, while keeping them isolated. An example of a relay in a simple control application is shown in Figure 2.2. In this system the first relay on the left is used as normally closed, and will allow current to flow until a voltage is applied to the input A. The second relay is normally open and will not allow current to flow until a voltage is applied to the input B. If current is flowing through the first two relays then current will flow through the coil in the third relay, and close the switch for output C. This circuit would normally be drawn in the ladder logic form. This can be read logically as C will be on if A is off and B is on. 


The example in Figure 2.2 does not show the entire control system, but only the logic. When we consider a PLC there are inputs, outputs, and the logic. Figure 2.3 shows a more complete representation of the PLC. Here there are two inputs from push buttons. We can imagine the inputs as activating 24V DC relay coils in the PLC. This in turn drives an output relay that switches 115V AC, that will turn on a light. Note, in actual PLCs inputs are never relays, but outputs are often relays. The ladder logic in the PLC is actually a computer program that the user can enter and change. Notice that both of the input push buttons are normally open, but the ladder logic inside the PLC has one normally open contact, and one normally closed contact. Do not think that the ladder logic in the PLC needs to match the inputs or outputs. Many beginners will get caught trying to make the ladder logic match the input types. 



Many relays also have multiple outputs (throws) and this allows an output relay to also be an input simultaneously. The circuit shown in Figure 2.4 is an example of this, it is called a seal in circuit. In this circuit the current can flow through either branch of the circuit, through the contacts labelled A or B. The input B will only be on when the output B is on. If B is off, and A is energized, then B will turn on. If B turns on then the input B will turn on, and keep output B on even if input A goes off. After B is turned on the output B will not turn off. 



  







Sunday, August 8, 2010

Infrared Flame Sensor Switch

the infrared detector switch is a very sensitive circuit that can be use to detect the presence of a flame, match, or a heat source, such as an iron or soldering iron. up to 3 feet away an then activate a relay. the heart of the infrared flame detector circuit is two tiny thermistor, as shown in figure 1.
a thermistor is a temperature sensitive resistor which changes its resistance as the temperature varies. glass bead or bulb thermistor are recommended for tis project.

schematic diagram for Infrared Flame sensor Switch

part list :
1. T1, T2 25K to 50K ohm thermistor
2. R1 33K ohm
3. R2 50K potensio
4. R3 1K ohm
5. R4 47 ohm
6. D1 1N4002
7. Q1 2n2222 
8. RY-1 SPDT 6 volt relay
9. U1 LM741
10. B1 9V battery
11. S1 SPDT toggle switch 

HEAT DETECTION

heat is transferred from one place to another in one of three ways; conduction, convection, and radiation. conduction is the proses of transferring heat from molecule to molecule in a substance.
convection is the process of transmitting heat by means of the movement of heated meter from one place to another. convention thus take a place in liquid and glasses. a room is heated by means of convection by circulating warm air through the the room. this bring us to radiation.
heat waves and light waves are of the same nature; they are both electromagnetic radiations thats differ only in wave length. heat waves near the radio portion of the spectrum are called the infrared.
in here we will construct an infrared flame detector, which can sense a match or flame up to 3 feet away. and learn how to construct a freeze alarm, an over temperature monitor, and an analog data logger for sending temperature data remotely.
more advance project include an LCD data thermometer, a night vision viewer, and an infrared motion detector.

ULTRASONIC Listener

whit this ultrasonic listener receiver you will be able to hear sound that are too high frequency to be heard by human ears, such as glass breaking and electric arcing. this project will enable you to listen to a world of sound that few people even know exist.

The addition of a parabolic reflector further enhances the performance of this project. because ultrasonic frequency spectrum is beyond our hearing range, they can listened to only by indirect means, such as frequency heterodyning. frequency heterodyning is a method widely used in modern radio receiver.

in an ultrasonic receiver with frequency heterodyning, a local oscillator (LO) is used to generated square wave. the output of the LO is from 20 to 100KHz. the incoming signal (FI) is first pick up by an ultrasonic transducer and then amplified by a three stage amplifier. the input signal is then mix by a the mixer section to produce a sum (LO+FI) of the frequencies and a different of frequencies(LO-FI) at the mixer output. because the sum of frequency (LO+FI) is too high to be heard, it is filtered out. the difference of frequency (LO-FI) is just within the audio range of frequencies. therefore after amplification, it can be heard from a loudspeaker or headphones. a system block diagram is shown in figur 1.

Figure 1. Ultrasonic Listening Block Diagram



a special piezoelectric ultrasonic transducer acts as a microphone and it used to detect the high frequency ultrasound waves. whenever sound pressure is applied to the transducer, it will produce a small voltage at its output terminal. the frequency response of this transducer peaks around 40KHz but will work from 20 to 100KHZ. this weak signal from the transducer is passed to input pin 9 of the CD14069 integrated circuit (IC) at U1: a, a Hex inverter IC. this digital IC operates in the linear mode by connection feedback resistor R2,R3, and R5 from the inverters' output back to their inputs. the weak signal from the transducer goes through three stages of amplification at U1:d U2:e and U1:f the signal is then rectified and coupled to the mixer by C7. the schematic diagram of ultrasonic listener can be see at figure 2.




ultrasonic listener part list :
1. R1, R9 10K
2. R2,R3,R5 1 M
3. R4 100K
4. R6 470
5. R7,R8 470K
6. R10 10 ohm
7. VR1 10K
8. VR2 200K
9. D1,D2 1N4148
10. C1,C2,C7 0.01 uF 25V
11. C4,C11 20pF 25V
12. C5 200 uF
13. C6,C13 0.04uF
14. C3,C8, C9 0.022 uF
15. C10 100pF
16. C12 100uF
17. C14 10uF
18. C15 47uF
19. Cx 100pF

20. U1 CD4069 IC
21. U2 LM386 audio amplifier IC
22. Speaker 8ohm
23. BT 9V transistor radio battery
24. Y1 ultrasonic transducer 

Saturday, August 7, 2010

Underwater Hydrophone

the hydrophone is an underwater listening device, microphone, or electroacoustic receiving tranducer, designed specially for continued use in salt or fresh water. it operated on water in much the same manner that an ordinary microphone operates in air. it convert audio sound waves in water into analog electrical signals, which which are then amplified by your audio amplifier to level where you can hear them. you can use this devices to listen to amplified sound, or you can tape record underwater sound of all types.