A-LevelPhysicsD.C. circuits9702/41 · May/June 2017 · Q69 Marks

A comparator circuit is designed to switch on a mains lamp when the ambient light level reaches a set value. An incomplete diagram of the circuit is shown in Fig. 6.1. [Figure 6.1]

(a)(i)A relay is required as part of the output device. This is not shown in Fig. 6.1. Explain why a relay is required.[2]
(a)(ii)On Fig. 6.1, draw the symbol for a relay connected in the circuit as part of the output device.[2]
(b)(i)the variable resistor R_V,[1]
(b)(ii)the diode D.[1]
(c)State whether the lamp will switch on as the light level increases or as it decreases. Explain your answer.[3]

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Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

[B1] lamp needs 'high' power/'large' current/'large' voltage
[B1] op-amp can deliver only a small current/small voltage
[B1] correct symbol for relay coil connected between output and earth
[B1] switch between mains supply and lamp
[B1] vary light intensity at which lamp is switched on/off
[B1] so that relay operates for only one current/voltage direction
[B1] or so that relay/lamp operates for either dark or light conditions
[B1] when light level increases, LDR resistance decreases
[M1] (R_LDR low,) so V⁻ > V⁺, so V_OUT negative/–5 V (must be consistent with B1 mark)
[(B1)] or when light level decreases, LDR resistance increases
[(M1)] (R_LDR high,) so V⁻ < V⁺, so V_OUT is positive/+5 V (must be consistent with B1 mark)
[A1] lamp comes on as light level decreases
[A1] or lamp goes off as light level increases

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