B2.
The moisture sensor circuit shown in Figure B2 is used to detect rain drops. The
circuit has the following parameters:
Resistance of the moisture sensor when it is wet, $R_M(wet) = 0 \Omega$
Resistance of the moisture sensor when it is dry, $R_M(dry) = \infty \Omega$
The forward voltage, $V_{LED} = 2 V$ and $V_{CE(sat)} = 0.2V$
(a) What is the voltage across $R_2$ when the moisture sensor is dry?
(b) When the moisture sensor is wet, voltage across $R_2$ is 3.2 V determine
(i) the base current, $I_B$;
(ii) the saturated collector current, $I_C(sat)$.
(c) Name an application of the circuit.
Moisture
Sensor $R_M$
$R_1$
470 $\Omega$
$R_2$
220 $\Omega$
$R_B$
25 k$\Omega$
$I_B$
$V_{CC}$
10 V
$I_C$
$R_C$
330 $\Omega$
Figure B2
(2 marks)
(3 marks)
(3 marks)
(2 marks)
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