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Fig 1.1 circuit Diagram

The LASER guided door open system contains a LASER light as transmitter and a photodiode acts as a receiver. When a person crosses the laser light, the photodiode and its control circuit will excite the relay circuit, and then the motor starts rotates. The circuit diagram is shown in figure 1.1
The entire circuit diagram is divided into four sections
  • ·         LASER and Photo diode interface.
  • ·         LM358 OPAMP circuit for digital output values.
  • ·         Relay and Buzzer interface circuit.
  • ·         12V DC converter circuit.

LASER and Photo diode interface circuit.

Any LASER diode can act as a transmitter (any normal LASER diode). Here one end (positive terminal) of the diode is connected to 5V power supply as shown in figure 1.1 and other end is connected to ground via R1 to protect the LASER diode by not allowing more current from power supply.
R1 can be calculated as R1 = (Vs-Vdiode)/Idiode = (5-3)/20mA = 100 Ohms.
Current ‘Idiode’ is chosen in such a way that the LASER diode produce more intensity.

The photodiode act as LASER receiver detects the light falling on it and varies its resistance according to the LASER light intensity. Note that ambient light conditions (sunlight, etc.,) may affect the circuit performance. Therefore the photo diode should be enclosed in a closed surface and provide a small hole to allow the LASER light fall on it. Whenever LASER light fall on the photodiode its resistance will decrease with respect to the light intensity LASER light, otherwise it offers huge amount resistance in order of Mega ohms.

A 1k pot (potentiometer) is connected to adjust the sensitivity of the photodiode circuit that means the circuit will respond for small changes in light. The photodiode and the trim pot (1k) combination act a voltage divider network and the output is connected to simple transistor based amplifier. When there is no light the photo diode offers high resistance. Therefore, no current will flow through trim pot. Hence the voltage drop across the trim pot is zero. Hence the transistor is off. On other hand when light falls on photodiode, its resistance decreases and the current starts flowing through 1k trim pot then some voltage is appear across the 1k trim pot. If this voltage is greater than Vbe of the transistor then the transistor conducts
            R2 = (5v – Vbe - Vled)/I

LM358 OPAMP section:

Fig 1.2 Pin diagram of LM358

LM358 contains dual differential input amplifiers and its operating voltage ranges from 3.3V to 30V. The main advantage of LM358 is single power supply and it requires low power when compare to LM741. Pin diagram is shown in figure 1.2. The use of this opamp in open loop configuration in this circuit is used to produce to two discrete levels of voltage (+Vsat and -Vsat). The output of the photo diode circuit which is taken from the emitter of transistor (T1 BC 547) and it is connected to pin 2 of LM358. A 1k trim pot (VR1) is connected to pin 3, if the voltage at the pin 2 is more at pin 3, the ouput is –Vsat. Otherwise the output is + Vsat. Comparator circuit is shown in figure in 1.3.

Fig 1.3 LM358 as comparator

In this circuit, we make use of both opamps provides in the LM358. The output taken from the pin 7 is connected to buzzer driving circuit and pin1 is connected to relay driving circuit.

Buzzer and relay circuit:

The base of the transistor T2 is connected to the output of the opamp at pin7. A resistance 4.7kohms is used to limit the base current of the transistor which in turn limits the collector current flowing through the buzzer.

A base resistor can be calculated as follows.
                                    Rb = (Vopamp - Vbe)/Ib
The resistance R6 is used to limit the collector current. The same base resistance is used for the relay driver transistor (T3).

The 12V relay coil require 70mA to excite, hence transistor circuit should be designed to allow more than 70mA current. Here BC547 is best suited and allow up to 100mA current.
The diode D4 is called fly wheel diode. When the transistor T2 is off, the current flowing through the relay is also off. But inductor does not allow sudden changes in the current in it. Hence a huge voltage is produce across the transistor which may damage the transistor. Hence D4 is connected across the coil to protect the transistor when relay is suddenly off.


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