Download Electronics and Photonics Revision Sheet

Unit 3 Electronics Revision
Save the electronics revision files on the Physics VLA to your “My Documents” folder.
Start the Crocodile Clips program and open the following files in order.
Voltage Divider
Red flags = +ve Voltage
Blue flags = -ve Voltage
Size of flag = relative size
of Voltage
Voltmeter: small
black dot connected
to +ve terminal.
Small arrow shows
direction of
conventional current
through device.



What is the function of the two voltmeters?
Label R1 and R2 in the circuit diagram above.
Circle the voltmeter that reads Vout.

Calculate the current flowing through each of the resistors and the battery.
Check your answer by placing the mouse cursor over each component.
What current flows through the voltmeter?



Predict the effect on Vout if you doubled the value of R2. Check your answer by
changing the value of R2 to 20k  .
What effect did this increased resistance have on the current through each
resistor and the battery?

Predict the voltage drop across each resistor if the Voltage supply were to be
increased to 330V DC.

The resistors can dissipate a maximum of 2W of power each before breaking.
Use calculations to decide if a 330V supply is safe. Test your prediction by
increasing the battery’s voltage.
Potentiometer Voltage Divider
A potentiometer is another name for a variable resistor.


Circle Vout on the circuit diagram.
Move the slider of the pot up and down. What happens to Vout? Why does the
value of the 10k  pot not change?

Place the slider exactly in the middle. Predict wall will happen to V out if the
supply voltage is increased to 12V. Adjust the circuit to check your answer.

Reset the mf to 9V and ensure the slider is exactly half way. Predict what will
happen if the resistance of the pot is increased to 20k  . Make the change to
check. Explain the observed behaviour.
LDR Voltage Divider


Identify Vout and the LDR.
Move the light source up and down and observe the changes in LDR’s resistance.
Does this LDR have a positive or negative gradient characteristic?

What happens to Vout as the illumination is increased?

Use the voltage divider formula to calculate V out if the LDR has a resistance of
4.5k  .
LED in Parallel

Identify the resistor and LED which are in parallel.


Start the simulation by clicking the PAUSE button.
Slowly increase the resistance of the variable resistor by moving the slider to
the right. What happens to the Voltage drop across the parallel part of the
circuit?

What is happening to the brightness of the diode? How does this relate to the
current through the diode?

VERY SLOWLY move the slider of the variable resistor to the LEFT. Observe
what happens to the current through the diode. Stop the simulation and draw
the current graph of the diode in the space above.

Why did the diode blow? Move the mouse cursor over the blown diode and try to
fix the problem.
Amplifier


You can start and stop the simulation using the pause button. The top graph plots
Vin and the bottom graph plots Vout.
On the circuit diagram label the component that supplies V in , the component
that measure Vout and the amplifier.

What is the Vin?

What is the Vout?

What is the voltage gain of this amplifier?

Is it inverting or non-inverting?

Stop and reset the simulation.

Click on the signal generator symbol in the circuit diagram. Choose the
PROPERTIES option at the bottom left of your screen and set the Voltage to
0.001V. Start the simulation.

Describe the output signal.
Phototransistor in Voltage divider
Phototransistors are similar to photodiodes.
 What happens to the current and Vout when
you slide the torch up and down?

What is the effective resistance of the
transistor when there is no light? How do you
know?

Compare the current that flows when 100%
of light shines with the current for 50% of
light. Is the current directly proportional to
the light intensity?
Electro-Optical System

This system consists of a circuit containing a light source and a second circuit
containing a light detector. Use two different coloured highlighters to mark the
two circuits.

What type of light source is used?

What type of light detector is used?

Adjust the variable resistor in the circuit containing the light source. What
happens to the current in the left circuit as the resistance decreases? What
happens to the brightness of the LED? What happens to the current of the
detector circuit?

Now increase the resistance and observe.

Set the resistance to 1.5kΩ. Record the currents in both circuits. Adjust the
resistor to double the current in the left circuit. What is the current in the
detector circuit? Has the current also doubled? Are the signals that are sent by
the transmission circuit replicated by the detector circuit?