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.
Does a protective diode need to be connected across the circuit? Is there back e.m.f when V out is negative? Thanks
ReplyDeleteit could be connected.
Deleteback emf arises when the switch of the relay is closed (due to electromagnetic effects). this occurs whether the output is positive or negative.
it does not depend on the output itself, BUT rather on the when switch of the relay is closed