2. Current in Parallel circuits

You are going to use 'Crocodile Clips' to investigate how current behaves in parallel circuits.

3. Controlling Current

You are going to use 'Crocodile Clips' to investigate how a resistor can be used to control the current flowing through an LED (Light Emitting Diode). You will be measuring the current which flows through the LED and the voltage across the LED for various values of resistor.

Current Limiting Resistors

The circuit above shows a resistor in series with an LED. The ammeter measures how much current goes through the LED.

Red LED's are rated at about 2V when a current of more than a few milliamps flows through them. (This means that a potential difference of about 2V is present across the anode and cathode when current is flowing through the LED)

4. Controlling Voltage 1

You are going to use 'Crocodile Clips' to investigate how voltage can be controlled using circuits called potential dividers.

A diagram showing the first circuit you are going to build is shown above. It shows two resistors R1 (1K) and R2 in series. A voltmeter is connected in parallel to measure the voltage (potential difference) between the junction of the two resistors and the 0V rail.

The voltage at the output (V out) will depend on the value of R2 the pull-down resistor and R1 the pull-up resistor.

  5. Controlling Voltage 2

You are going to use 'Crocodile Clips' to investigate a potential divider or voltage divider which uses a light-dependent resistor or LDR. The system will act as a simple light sensor.

A diagram showing the circuit you are going to build is shown above. It shows two resistors R1 (LDR) and R2 in series. A voltmeter is connected in parallel to measure the voltage (potential difference) between the junction of the LDR and resistor and the 0V rail.

The voltage at the output (Vout) will depend on the value of R2 the pull-down resistor and the LDR which acts as the pull-up resistor. The resistance of the LDR depends on the amount of light hitting its surface. The output of this system Vout will indicate how much light is falling on the LDR.

a. Press the S.P.S.T. switch. If the circuit is connected correctly the output voltage should change as you illuminate the LDR. Watch the value of p.d. measured by the voltmeter as light is increased.

b. Redesign this circuit so that Vout is high when the LDR is in the dark and low when it is illuminated.

c In your notes, write today's date and the title Light/ Dark Sensors. Beneath this heading either print a copy of thecircuit and stick it neatly in your notes or draw it neatly by hand. Describe what the circuit does. Print and paste or draw your redesigned circuit and explain how this works.

A diagram showing the circuit you are going to build (Circuit 13) is shown on the right. It shows two resistors R1 (a Thermistor) and R2 in series. A voltmeter is connected in parallel to measure the voltage (potential difference) between the junction of the Thermistor and resistor and the 0V rail.

The voltage at the output (Vout) will depend on the value of R2 the pull-down resistor and the Thermistor which acts as the pull-up resistor. The resistance of the Thermistor depends on the temperature at its surface. The output of this system Vout will indicate the temperature of its surface.

a. Press the S.P.S.T. switch. If the circuit is connected correctly the output voltage should change as you change the temperature. Watch the value of p.d. measured by the voltmeter as the temperature is increased.

b. Redesign this circuit so that Vout is high when the temperature of the thermistor is decreased and low when it is increased

c. In your notes, write today's date and the title Hot/Cold Sensors. Beneath this heading either print a copy of the circuit and stick it neatly in your notebook or draw it neatly by hand.

Describe what the circuit does.

Print and paste or draw your redesigned circuit and explain how this works.

 6. Current and Voltage in a potential divider

You are going to use 'Crocodile Clips' to investigate the effect of current on the output voltage of a potential divider or voltage divider.

The circuit is shown above. It consists of two resistors R1 and R2 which form the voltage divider and R3 which acts as the load resistor (something to draw current from the potential divider). As the value of the load resistor is decreased the current drawn from the output of the potential divider Vout will change (it will increase).

 7. Output Buffering

You are going to use 'Crocodile Clips' to see how an integrated circuit (a 555 Timer IC) can be used to boost (buffer) the current available at the output of a voltage divider.

The first circuit you need to build is shown above. It consists of two resistors R1 and R2 (an LDR) which form the voltage divider and a bulb which acts as the load. In theory the bulb should glow each time the light falling on the LDR is reduced. In practice it will not work.

Try to model a pressure sensor and indicator using the buffered circuit as the basis for your design. The pressure pad can be simulated using a push to make switch. The indicator can be a light or sound.

Write today's date and a suitable title. say what your system is designed to do.

Draw a block diagram of your system and explain how it works.

 8. Charging and Discharging a Capacitor

You are going to use 'Crocodile Clips' to explore the properties of a capacitor. A capacitor is very like a small rechargeable cell. It can be charged up by connecting it to a power supply and can then be used to drive current through other components for a short period of time.

The circuit you need to build is shown above. It consists of two resistors and two LED's which are used to charge the capacitor or are driven by the capacitor once it has been charged. The S.P.D.T. switch connects the positive plate of the capacitor first to the supply rail via R1 and LED1 and then to LED2 and its current limiting resistor R2. The push to make switch allows you to completely discharge the capacitor.

a. Assemble the circuit. Set the S.P.D.T. switch in the upper position so that it charges the capacitor through R1 and LED1 (LED1 will glow while this happens). Then operate the switch so that the capacitor can discharge itself through the resistor R2 and LED2. After a short time the capacitor will be discharged and LED2 will stop glowing.

b. Use this circuit to find the answer to the following questions.

* allow the capacitor to charge up for 10s. For roughly how long does LED2 glow ?

* Allow the capacitor to charge for just 1s. How long does LED2 glow now ?

* What difference(s) do you notice if the 470 ohm resistors are replaced with 1K resistors ?

* What difference(s) do you notice if you replace the 1000 uF with a 10mF(10 000 uF) capacitor ?

c. In your notebooks, write today's date and the title Charging and Discharging Capacitors. Beneath this heading either print a copy of the circuit and stick it neatly in your book or draw it neatly by hand.

Describe what you did to charge and discharge the capacitor.

Copy and answer the questions (*) above.

Working Voltage

Capacitors have a maximum working voltage. If this figure is exceeded the capacitor is destroyed.

Build the circuit shown here and increase the voltage. What is the maximum working voltage of the capacitor used in crocodile clips ?

  9. Voltage/Time Graphs

You are going to use the 'Crocodile Clips' oscilloscope simulation to find out how the voltage across a capacitor changes with time and use this to plot a voltage / time graph for a 1000 uF capacitor which is charged and discharged through identical LED's and their limiting resistors.

The circuit you need to build is shown above. It consists of two resistors and two LED's which are used to charge the capacitor or are driven by the capacitor once it has been charged. The S.P.D.T. switch connects the positive plate of the capacitor first to the supply rail via R1 and LED1 and then to LED2 and its current limiting resistor R2. The push to make switch allows you to completely discharge the capacitor. The Oscilloscope probe is placed as shown.

a. Assemble the circuit. Click on the oscilloscope this will open the scope window. 'Click' on 'OK'. The scope window can the be dragged to a suitable size by moving the sizing bar up or down.

b. Operate the S.P.D.T. switch so that it charges the capacitor wait 2s then switch it to discharge wait 2s. repeat this procedure six times to obtain a voltage/time graph similar to the one shown on the below.

.c. In your notebooks, write today's date and the title Charging and Discharging Capacitors (voltage/time graphs). Beneath this heading either print a copy of the circuit and the voltage time graph and stick it neatly in your book or draw it neatly by hand.

Answer the following questions.

1. At which points on the graph is the voltage rising or falling most rapidly ? Mark these with a red asterisk *

2. At which points on the graph is the voltage rising or falling least rapidly ? mark these with a blue asterisk *

  10. Diodes

You are going to use 'Crocodile Clips' to investigate the way diodes are used in a circuit.

A diagram showing the circuit you are going to build is shown above. It shows a switch connecting two diodes to a bulb. One diode is forward biased (current can flow through it), the other is reverse biased (current cannot flow through it).

The two leads of a diode are known as the anode (+) and the Cathode (-).

If the anode is connected to the more positive part of the circuit and the cathode to the more negative the diode is said to be forward biased. The symbol for a diode is similar to an arrow pointing in the direction of current flow.

a. Build the circuit shown. does the bulb light ?.

b. Operate the switch does the bulb light ?

c. In your notebooks, write today's date and the title Diodes. Beneath this heading either print a copy of the circuit and stick it neatly in your notebooks or draw it neatly by hand. Then complete the paragraph below.

When the switch is not pressed the current is .................... and the bulb is .............. this is because the diode is ................. biased.

When the switch is pressed the current is ................. and the bulb .............. because the diode is now ................. biased.

The positive lead of a diode is called the ..................

The negative lead is called the ..............

  11. Investigating switches

You are going to use 'Crocodile Clips' to investigate how switches work.

The simplest form of control consists of a simple manually operated switch. The electrical supply provides a current in the control circuit.

Switches are not electronic components, but mechanical devices which can make (connect) or break (disconnect) a circuit.

If the switch is closed (switched ON), the voltage from the electrical supply reaches the object under control and makes an electric current flow through it.

Switches give us control over the circuits we build by

i) allowing a current to flow through them;

ii) preventing a current from flowing through them or;

iii) by changing its path, so that current can be diverted (rather like points on a railway track.)

SPST Single pole single throw (Single pole ON/OFF)

SPDT Single pole double throw (Single pole changeover)

DPST Double pole single throw (Double pole ON/OFF)

DPDT Double pole double throw (Double pole changeover)

The contacts on switches can be NO or NC.

NO means Normally Open. This means that the switch is normally not making contact; the circuit is not complete. The switch has to be operated to complete the circuit.

NC means Normally Closed. This means that the switch is normally making contact; the circuit is complete. When the switch is operated, the circuit is broken.

a. Build the circuits shown.

b. Operate the switch in each case.

c. In your notebooks, write today's date and the title Switches. Beneath this heading either print a copy of the circuit and stick it neatly in your notebooks or draw it neatly by hand. Then explain how each circuit works.

  12. Creating a Time Delay

You are going to use 'Crocodile Clips' to build a simple time delay circuit using the IC Buffer looked at in an earlier circuit.

The circuit you need to build is shown above. It consists of a push to make switch used to charge the capacitor (C).and a current limiting resistor (R). The output of this capacitor /resistor circuit is fed into a 555 timer acting as an IC buffer.

When the push to make switch is pressed point (A) on the circuit goes high causing the output of the buffer (Q) to go low, making the LED glow.

When the switch is released there is a time delay before point (A) goes low again, because the capacitor has to discharge through resistor (R).

Eventually point (A) does become low and Q then goes high and the LED stops glowing.

You will be investigating how the time delay in seconds (S) depends on the size of the capacitor (C) and the resistor (R).

 13. Capacitors in Series and Parallel

You are going to use 'Crocodile Clips' to find out what happens to the time constant if you connect capacitors in series and parallel.

The circuit you need to build is shown above. It is identical to the circuit used for our time delay investigation.

When the push to make switch is pressed point (A) on the circuit goes high causing the output of the buffer (Q) to go low, making the LED glow.

When the switch is released there is a time delay before point (A) goes low again, because the capacitor has to discharge through resistor (R).

Eventually point (A) does become low and Q then goes high and the LED stops glowing.

You will be investigating how the time delay in seconds (S) is affected by putting capacitors in series and parallel.

  14. Building a 555 Monostable

  15. A 555 Oscillator

You are going to use 'Crocodile Clips' to find out what makes a 555 timer oscillate. You will initially explore the transfer characteristics (upper and lower switching points) of the NOT gate inside the 555 timer and then add a resistor and capacitor to make it oscillate.

The circuit you need to build is shown on the left above. A variable voltage V IN is measured by the voltmeter and fed into the 555 IC through pins 6 and 2. The output voltage V OUT (pin 3) is monitored by an LED and its current limiting resistor.

Does the period roughly equal 2RC if so why ?

Copy and complete this sentence: As V IN is raised from OV to +9V, V OUT goes from....... to ....... when V IN passes .........V. This is the ............ switching point. As V IN goes from +9V to 0V, V OUT goes from ......... to ......... when V IN passes ..........V. This is the ............ switching point.

  16. Logic Gates

You are going to use 'crocodile clips' to work out the truth tables for the five different logic gates shown below. A truth table is a useful way of summarising the way a logic gate works. If the inputs are labelled A and B the resulting output Q can be found for each combination of inputs. The numbers 1 or 0 are used to signify logic 1 (on or high) and logic 0 (off or low).

The circuits you need to build are shown below. Firstly check the way the switches work. Make sure that in the 'view' menu you select 'logic signals'.

a. Assemble the circuits shown above. Set the input switches to match the inputs in the truth tables note the output in each case.

b. In your notebooks, write today's date and the title Logic Gates and Truth Tables. Beneath this heading either print a copy of the circuits and stick them neatly in your book or draw them neatly by hand. Copy and complete a truth table for each logic gate.

 

  17. Transistors- part 1

You are going to use 'crocodile clips' to investigate the currents which flow through a transistor. The type of transistor used in this investigation is known as a bipolar npn junction transistor. It has three terminals called the base (b), collector (c) and emitter (e). The currents that enter or leave these three terminals are called the base current (Ib), the collector current (Ic) and the emitter current (Ie)

a. Assemble the circuit shown. Begin with the switch open, so that no current can flow through L2 into the base. Is current flowing through L1 ?

b. Close the switch S1. Does current flow through LP1? Does current flow through LP2?

c. Which currents flow into the transistor? how big are they? Which current flows out? How big is it?

d. In your notebooks, write today's date and the title Currents in transistors. Beneath this heading either print a copy of the circuits and stick them neatly in your notebook or draw them neatly by hand.

Copy and complete the following questions:

1. Which currents flow into the transistor?

2. Which current, must therefore flow out of the transistor?

3. Which current is bigger Ib or Ic?

4. If all the current that enters a transistor must also leave it what can you say about Ie?

  18. Transistors - part 2

You are going to use 'crocodile clips' to investigate how the currents which flow through a transistor can be controlled by adjusting the base/emitter voltage (Vbe).

a. Assemble the circuit shown. Begin with Vbe set to OV, so that no current can flow through the 10K resistor into the base.

b. Slowly increase the value of Vbe until current begins to flow into the base. What is the value of Vbe at this point?

c. Continue to increase the value of Vbe Note the currents Ib and Ic in each case.

d. In your notebooks, write today's date and the title Controlling currents in transistors. Beneath this heading either print a copy of the circuit and stick it neatly in your book or draw it neatly by hand.

copy and complete the table below.

e. calculate the gain of the transistor for each setting of Vbe What do you notice about the gain ?

                    Ic
Gain (hFE) =  __
                    Ib

Copy and complete the following questions:

1. What is the value of Vbe when the transistor begins to conduct ?

2. Does Vbe change as the value of Ic and Ib increase?

3. What do you notice about the gain at each stage?

e. Move the ammeter measuring Ic and add a lamp complete the experiment again with a 500ohm resistor. What do you notice about the gain now ? Can you explain why there is a difference at higher values of Vbe?

  19. Transistors - Part 3

You are going to use 'crocodile clips' to investigate how to increase the gain of a transistor circuit using a Darlington Pair arrangement. This is usually required when the base current is not large enough to turn the transistor on fully, so that the collector current is not large enough to drive the load.

a. Assemble the circuit shown. Begin with Vbe set to OV, so that no current can flow through the 10K resistor into the base.

b. Slowly increase the value of Vbe until current begins to flow into the base. What is the value of Vbe at this point?

c. Continue to increase the value of Vbe Note the currents Ib and Ic in each case.

d. In your notebooks, write today's date and the title Controlling currents in transistors. Beneath this heading either print a copy of the circuit and stick it neatly in your book or draw it neatly by hand.

copy and complete the table below.

e. calculate the gain of the transistor for each setting of Vbe What do you notice about the gain ?

                    Ic
Gain (hFE) =  __
                    Ib

Copy and complete the following questions:

1. What is the value of Vbe when the transistor begins to conduct for a darlington pair?

2. Does Vbe change as the value of Icand Ib increase?

3. Compare the gain of a single transistor with a darlington pair. What do you notice about the gain, can you explain why it is so large?

4. For a darlington pair the total gain is found by multiplying ...

  20. Transistors - part 4

You are going to use 'crocodile clips' to investigate how to use the transistor as an electronic switch. This is used in a variety of circuits, usually with a sensor.

We are going to build a number of circuits which use different methods of 'biasing' the transistor.

a. Assemble circuit No1. as shown. Begin with VR1 set to OK (make VR1 a 47K variable resistor), so that the current can flow through the light sensor straight into the base.

b. Slowly increase the amount of light on the light sensor what happens? can we adjust the point at which the transistor switches on?

c. Try using VR1. Increase its value to 11.8K, 14.1K, 16.5K, 18.8K and finally 21.2K. note what happens in each case. Their is some adjustment possible but it is limited.

d. Assemble circuit No2. as shown. Begin with VR1 set to O (make VR1 a 1K variable resistor), Adjust the light sensor can you switch the transistor on?.

e. Adjust VR1 to give readings of .05K, .5K and 1K. Adjust the light sensor for each of these settings and note how much the sensor is illuminated.

f. Swap the positions of VR1 and the light sensor so that VR1 is at the top and repeat (e.) above. Note what happens

g. Swap the light sensor for a temperature sensor and repeat (a-f) above.

h. In your notebooks, write today's date and the title The transistor as a switch. Beneath this heading either print a copy of the circuit(s) and stick them neatly in your book or draw them neatly by hand.

Explain how the circuit(s) work.

  21. Operational Amplifiers - 1

You are going to use 'crocodile clips' to investigate how the output voltage of an op-amp depends on the voltage of its two input terminals

The circuit you are going to use is shown on the right. The two potentiometers generate two signals V+ and V- which are fed into the corresponding inputs of the op-amp. The two LED's indicate whether Vout is positive or negative and V1, V2 and V3 can be used to measure all three voltages.

An op-amp used in this way is called a comparator. It is used to compare the two inputs V+ and V-

a. Assemble the circuit as shown. Begin with V- set to 1.8V and V+ to -5.4V. Vout should be negative, because V- is set to the highest voltage.

b. Slowly increase the value of V+, note the value of V+ at which Vout changes. What are the two values of Vout ?

c. In your notebooks, write today's date and the title Operational Amplifiers. Beneath this heading either print a copy of the circuit(s) and stick them neatly in your book or draw them neatly by hand.

Explain how the circuit(s) work.

Copy and complete this passage. Vout will be either ......V or .......V. When it is positive LED .........glows. When it is ............. LED 1 glows. If V+ is higher than V- then Vout will be .......V If V+ is lower than V- then Vout will be .......V

  22. Operational Amplifiers - 2

You are going to use 'crocodile clips' to investigate how an op-amp can be used to amplify signals which are normally too small to affect the next part of a system.

The circuit you are going to use is shown on the right. The output voltage follows the same pattern as the input voltage, but its size can be controlled by the ratio of Rf and Ri. Normally Rf and Ri are chosen to amplify the signal (make the output voltage larger than the input) but they can also make the output smaller (called attenuation).

You will notice that the output voltage is inverted (or 1800 out of phase). Notice also that the output voltage is limited by the supply voltage.

An op-amp used in this way is called an inverting voltage amplifier.

a. Assemble the circuit as shown. Double click the 0.25Hz figure and set the amplitude of the sine wave to 500mV.

b. Double click the Osciloscope ans set the Oscilloscope voltage to read from +7V to -7V

c. Operate the oscilloscope you should see both the input trace and the output trace. As one goes positive the other will go negative. The output should be 10 times larger than the input voltage (gain = Rf/Ri)

d. Change the 10K resistor to 15K the gain should then be 15. Change the oscilloscoipe to read +9v and -9V operate the oscilloscope again. Does the output signal amplify the input correctly? You should see a flat appear at the top of the output trace, this is known as cut-off and is due to the supply voltage limiting the amount of amplification possible. At the cut-off point the amplifier is said to be saturated.

e. In your books, write todays date and the title Inverting Amplifier. Beneath this heading either print a copy of the circuit(s) and stick them neatly in your book or draw them neatly by hand.

Explain how the circuit(s) work.