Question

C3. An automatic streetlight pole with a sunlight detecting circuit is installed as shown in Fig.C3(a). The characteristic of the sunlight sensing device is illustrated in Fig. C3(b). The control circuit in Fig. C3(e) was well tested. A LED lamp Fig. C3(a) A light sensor for detecting the sunlight -10V +10V Fig. C3(0) The characteristic of the Bght sensor w 20KO K 2010 300k Fig. C3(b) Resistance ew 741 Op-amo 1000 2010 Night-bme Day-time Light Intensity (Lux) Control Circuit for LED lamp on (a) Explain the operations of the control circuit with aid of Fig. C3(b) (4 marks) and Fig. C3(e). (b) (6 marks) When the LED Lamp and control circuit are installed at the lamp pole. The LED lamp is switched off at day-time. At night-time, there is a problem, the LED lamp is continuously switched on and ofl. Explain the situation and method to resolve the problem.

Answer 1

(a) Working of control circuit-

• The 741 Op Amp used here works as a comparator.
• Non inverting terminak is connected to 10V supply via voltage divider resistances of 2k and resistance of light sensor.
• Inverting terminal is connected to supply via two equal resistance of 20K, hence Voktage of inverting terminal (V-) will be-

$V^-=10*\frac{20k}{20k+20k}$

V- = 5V

• According to characteristics of Light sensor, during daytime its, resistance is 100 ohms, so

$V^+=10*\frac{100}{100+2k}$

V+ = 0.435 V

• Since V+< V- during daytime, output of comparator will be negative, so MOSFET switch will be off, so during daytime tge LED remain off.
• During night time, we have resistance of sensor 300k ohm,

$V^+=10*\frac{300k}{300k+2k}$

V+ = 9.93 V

• So during night time V+>V- so output of Op Amp will be positive, so MOSFET switch will be ON, allowing current flow in oath of LED, so street light LED will glow.

(b)

As soon as it gets darker, the resistance of sensor starts increasing from 100 ohm to 300 kohm.

There will be a situation when for a particular combination of resistance, V+ and V- values will be very near to each other, in that case LED will glow, and since it is not completely dark outside, the sensor might trigger from the brightness of LED itself, and send signal to comparator to turn off as discussed in part a, so the cycle of on off may start in case of not completely dark atmosphere.

The solution to overcome this problem can be by providing isolation between switch and output of comparator in the form of resistance, so that when the signal voltages at input of Op Amp are very near to each other, the trigger signal to switch drops in the resistance,and only when there is a sufficient difference between two inputs is there, the switch will be turned ON.