Download Electronic Circuits 4

Practical Electronic Circuits
Formal Home work 4
1.
An automatic hand dryer used in a washroom is shown in the diagram below:
Inserting hands into the dryer breaks the light beam, this is detected using a light
dependent resistor (LDR). The LDR is part of a switching circuit which activates the dryer
when the hands are inserted.
Part of the circuit for the hand dryer is shown.
(a) The variable resistor RV is set to a resistance of 60k Ω. Calculate the voltage across
the LDR when its resistance is 4 kΩ
(3)
(b) Name component X in the circuit diagram.
(1)
(c) Explain how this circuit operates to activate the motor in the dryer when the
light level falls below a certain value.
(2)
2.
A high intensity LED is used as a garden light. The light turns on automatically when it
becomes dark.
The light also contains a solar cell which charges a rechargeable battery during daylight
hours.
(a) Part of the circuit is shown below.
(i)
State the energy transformation in a solar cell.
(ii)
At a particular light level, the voltage generated by the solar cell is 1.5 V.
Calculate the voltage across the rechargeable battery at this light level.
(b) The LED is switched on using the following circuit.
(1)
(3)
2 (b) continued….
(i)
Name component X
(1)
The graph below shows the voltage across the LDR in the circuit for different light
levels.
(ii)
(iii)
For the LED to be lit, the voltage across the LDR must be at least 0.7 V.
What is the maximum light level for the LED to be lit?
(1)
Explain the purpose of the resistor R.
(1)
Total 13
2.
Total 12