Question 11
Part of a circuit
incorporating an operational amplifier (op-amp) is shown in Fig. 9.1.
Fig. 9.1
(a)
A relay is connected to the output of the op-amp circuit so that a
lamp may be switched on or off.
(i)
Complete Fig. 9.1 to show the relay connected into the circuit. [2]
(ii)
State and explain whether the output of the op-amp is positive or
negative for the lamp to be switched on. [2]
(b)
State the device in Fig. 9.1 that could be used so that the circuit
indicates a change in:
(i)
the bending of a rod [1]
(ii)
the level of daylight to switch on a street light. [1]
[Total: 6]
Reference: Past Exam Paper – June 2019 Paper 42 Q9
Solution:
(a)
(i)
relay coil shown connected between diode and
earth
switch shown connected across lamp
(ii)
For the diode to conduct, current must flow into the
output of the op-amp. So, earth must be at a higher potential than the output
of the op-amp {as current flows from higher potential to lower potential}. So,
the output of the op-amp must be negative.
(b)
(i)
strain
gauge
{The
resistance of a strain gauge changes with length of the wire/rod.}
(ii)
light-dependent
resistor
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