A student designs an electronic sensor that is to be used to switch on a lamp when the light intensity is low. Part of the circuit is shown in Fig. 10.1.

Question 6

A student designs an electronic sensor that is to be used to switch on a lamp when the light intensity is low. Part of the circuit is shown in Fig. 10.1.

Fig. 10.1

(a) State the name of the component labelled X on Fig. 10.1. [1]

(b) On Fig. 10.1, draw the symbols for

(i) two resistors to complete the circuit for the sensing device, [2]

(ii) a relay to complete the circuit for the processing unit. [2]

(c) (i) State the purpose of the relay. [1]

(ii) Suggest why the diode is connected to the output of the operational amplifier

(op-amp) in the direction shown. [2]

Reference: Past Exam Paper – June 2012 Paper 41 & 43 Q10

Solution:

(a) light-dependent resistor

(b)

(i)

two resistors in series between +5 V line and earth

midpoint connected to inverting input of op-amp

(ii)

relay coil between diode and earth

switch between lamp and earth         

(c)

(i)

The relay allows the mains supply to be switched on and off using a low voltage.

(ii)

So that the relay will switch on for one polarity of the output voltage – which is when the output voltage is negative.