Question

Given an automatic street light control circuit using transistors, a relay and an LDR shown in Figure 10:

1. Explain why the components are used in the circuit;
2. Explain the modes of operation of BJT working for the circuit;
3. Concisely explain the operation of the circuit.
4. Design an automatic street light control circuit using op-amp, relay and LDR
   
   Vcc +12 V 1 LDR R2 15k_2 Relay 220V 50Hz RB1 R1 4.7622 Q2 C1815 Lamp Q1 C1815 VR 50k 470uF Figure 10
   

Answer 1

components are used in the circuit: o LDR: • LDR is Light Dependent Resistor. • LDR made from semiconductor materials to enable them to have their light sensitive properties. • cadmium sulphide (Cds) is used in LDR. • LDR works on the principle of “Photo Conductivity". • Principle: whenever light falls on the surface of the LDR, the conductance of the element increases or the resistance of the LDR falls when the light falls on the surface of the LDR. • This property of the decrease in resistance for the LDR is achieved because it is a property of semiconductor material used on the surface. o Relay: • It provides isolation between the controller and the device. • Because devices work on AC as well as on DC but they receive signals from microcontroller which works on DC hence we require a relay to bridge the gap. The relay is extremely useful when you need to control a large amount of current or voltage with the small electrical signal. o Capacitor: It is a passive component with two terminals and used to store charges

o Resistor: It is a passive component having two terminals that are used to manage the current flow in the circuit. • A current that flows via a resistor is directly proportional to the voltage that appeared into the resistor. O AC bulb used as street light o 50K pot is also used for setting up the sensitivity of the circuit. • Explain the modes of operation of BJT working for the circuit: o Whenever light falls over LDR, its resistance get decreased and transistor Q1 turns ON and collector of this transistor goes LOW. o This makes Q2 turns OFF due to getting a LOW signal at its base, so relay also remain turned OFF. LDR senses Darkness(no light), then transistor Q1 turned ON due to increase in the resistance of LDR which is responsible for voltage drop at the base of Q1. o Due to a LOW signal at the Q1 base, Q2 transistor gets a HIGH signal from the collector of Q1 and turns on the relay. o Relay turned on the AC load that is connected to relay.

• the operation of the circuit: o LDR responsible for detecting light and darkness. o The resistance of LDR increases in darkness and reduces in presence of light. This circuit is same as a Dark Detector or Light Detector Circuit, only here we have replaced simple LED with a AC load, using a Relay. o Two NPN transistors are used to drive the relay. o Whenever light falls over LDR, relay also remain turned OFF o LDR senses Darkness, Relay turned on the AC load that is connected to relay Because of this, automatic Street Lights turns on in the night and turn off in the day. Design an automatic street light control circuit using op-amp, relay and LDR: 5 pin Relay Ri 10K NO LDR DL TCOM INC 230V AC IC VR1 R3 IK 3R2 Bulb 10K 330R BCSU 8 C1 104F DI : IN4004 IC CA3140

Working: o IC CA3140 which is basically an operational amplifier. o Pins 2 and 3 of these IC are used to compare the voltage and give us the output in return i.e. it works as a potential divider in its inverting and non-inverting inputs (pins 2 and 3) o LDR and VR1 form one potential divider which is used to provide a variable voltage at the inverting input (that is 2) and the second potential divider is built around non-inverting input (pin 3) with the help of R1 and R2 which will grant half of the supply voltage to non inverting pin. o property of LDR that during the time of day resistance is low therefore voltage at the inverting input (IC pin 2) is higher than the voltage at the non-inverting input (pin3) hence the output at the pin6 is low so the transistor goes into the cutoff state which means LED or bulb will not glow o in dimness or in night we know that resistance of LDR is high hence voltage at inverting input pin 2 of the IC CA3140 decreases than the non inverting input pin 3 as a result output pin 6 moves to high state which make transistor to conduct and the LED or bulb associated to it start simmering