Switching Circuits Learning Intention To combine knowledge of

Switching Circuits

Learning Intention: To combine knowledge of potential dividers, transistors and input/output devices to explain how automatic switching circuits work. Success Criteria: Being able to successfully describe how automatic switching circuits work to switch on output devices.

SWITCHING CIRCUITS Switching circuits combine potential dividers, transistors and input/output devices in circuits which automatically switch an output on or off. The circuits that we look at will change dependent on temperature (hot or cold) or light (light or dark).

SWITCHING CIRCUITS +5 V A variable resistor is normally used as this allows the left temperature/light level The side of the circuit theresistance output comes Changing the of a isata which potential divider. The on to be adjusted. component in the two resistors will potential share the divider will cause supply voltage, in the ratio The component atits thevoltage to morepotential resistance of change; their resistances. bottom of the One of the components in means aisbigger share of the divider thedivider important the potential will be voltage supply. This will also component. If or thethermistor. voltage either an LDR mean the variable resistor across this goesofabove The resistance thesea takes smallerthe share. certaina value, components change transistor due to will switch on. You must light/temperature. reference this in your 0 V answer. This is the output device that will be switched on or off. It could be a lamp, LED, motor, relay switch etc. The transistor is positioned here. It will either be an npn transistor or MOSFET.

SWITCHING CIRCUITS Example 1 – Explain how the following circuit works to switch on the LED when it is dark. +5 V Answer Light level down, Resistance of LDR up. VLDR up. When V LDR = transistor switch on voltage: transistor on, LED on. You must then state Firstly, reference that the important light/temperature component’s voltage Next, state what will component. reach a level which happens toat the voltage theoftransistor will the important State clearly what will switch on. component. happen to that Remember; if the component given resistance of theinon LDR This will described also switch scenario it will take a theincreases, output device. the question. bigger share of the Light level supply. down, voltage Resistance of LDR up. 0 V

SWITCHING CIRCUITS Example 1 – Explain how the following circuit works to switch on the LED when it is dark. +5 V Answer – Note in workbook Light level down, Resistance of LDR up. VLDR up. When VLDR = transistor switch on voltage: transistor on, LED on. 0 V The answers to all of these questions will take a similar format to this.

SWITCHING CIRCUITS Example 2 – Explain how the following circuit works to switch on the motor when it is light. +5 V Answer Light level up, Resistance of LDR down. VLDR down VVAR up. When V VAR = transistor switch on voltage: transistor on, motor on. You must then state that the important Next, statevoltage what (in component’s happens this case, to thethe voltage As before, reference of the important VARIABLE resistor) will the light/temperature component. reach a level at which component. the transistorifwill Remember; the Light level switch on. up, of the LDR resistance increases, ofit. LDR will down. take a Resistance bigger of theon This will share also switch voltage supply. the output device. 0 V M

SWITCHING CIRCUITS Example 2 – Explain how the following circuit works to switch on the motor when it is light. Answer – Note in workbook +5 V Light level up, Resistance of LDR down. M VLDR down VVAR up. When VVAR = transistor switch on voltage: transistor on, motor on. 0 V

SWITCHING CIRCUITS Example 3 – Explain how the following circuit works to switch on the bulb when it is hot. +5 V t 0 V Now, solve the examples from your workbook in your classwork jotter showing full working for each one.

Summary

TRANSISTORS 1 A transistor is an electrical switch. There are two different types: npn transistor collector MOSFET drain base gate emitter source The main question asked, is “How does the circuit work to switch on the output? ”. Two possible examples are on the next study card. Note; A variable resistor is normally used in these circuits as this allows the temperature/light level at which the output comes on to be adjusted.

Example 1: The “bottom” component is the important one. We need its voltage to go up to switch on the transistor. In this case, that is VLDR. Example 2: The “bottom” component is the important one. We need its voltage to go up to switch on the transistor. In this case, that is VR. +5 V TRANSISTORS 2 R Light up, RLDR down. VLDR down. Light down, RLDR up. VLDR up. When VLDR is at switch on voltage, transistor on, LED on. 0 V +5 V Light up, RLDR down. VLDR down. Therefore, VVAR up. R When VR is at switch on voltage, transistor on, LED on. 0 V Note; these circuits could feature a thermistor instead of a LDR.

Success Criteria: Being able to successfully describe how automatic switching circuits work to switch on output devices. Tick off the box in your workbook when you have met the success criteria.

Now, use the space in your workbook to produce a summary of switching circuits. You may wish to produce concise bullet points, or draw a mind-map, or use any other useful revision technique. PUPIL CHECKPOINT : You should then swap your workbook with another pupil, and both of you should give feedback.