- Pacemakers
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Objectives:
- To be able to construct and understand circuits using logic gates
- To be able to describe the job of input and output devices
Timing: 3 lessons
Pacemakers can be fitted inside a patient whose heart does not beat regularly. They use sensors to monitor heart rate and then an electronic circuit sends a message to the wires to send electrical signals to the heart. Some pacemakers now sense things like body movement and can act accordingly. You are now going to look at how these electronic circuits work. You can read more about how pacemakers work at: http://www.nhs.uk/Conditions/ and http://www.bhf.org.uk/living_with_heart_conditions/treatment/pacemakers-1.aspx.
A light dependent resistor (LDR) and a thermistor are sensors or input devices. Their resistance changes according to light intensity and temperature. Your first job is to find out how resistance and light intensity are related for and LDR and how resistance and temperature are related for a thermistor. Use: http://www.antonine-education.co.uk/Electronics_AS/ or
http://www.antonine-education.co.uk/Physics_A2/Options/Module_9/Topic_6/TOPIC_6.HTM
to help you.
An LED is an output device which can work on the low current from the logic gates. Find out what it is using: http://www.bbc.co.uk/schools/gcsebitesize/ and http://electronics.howstuffworks.com/led.htm.
Your teacher will probably get you to test these relationships out using LDRs and thermistors.
You now need to find out what a potential divider works. How can a variable resistor, LDR or thermistor affect the output? Find out the equation for calculating the output voltage. Use: http://www.bbc.co.uk/schools/gcsebitesize/design or http://www.s-cool.co.uk/ to help you.
Your teacher will probably get you to set up some circuits, either on Croc Clips or using the Kent kits, to show how potential dividers work.
Next you need to find out about logic gates. What do NOT, AND, OR, NAND and NOR gates do when high voltage (about 5V) or low voltage (about 0V) go into them. Draw truth tables for all these gates. The following sites will help you: http://www.bbc.co.uk/scotland/education/bitesize/standard/physics, http://www.bbc.co.uk/schools/gcsebitesize/ and http://www.antonine-education.co.uk/Electronics_AS/.
Your teacher will probably get you to set up some circuits, either on Croc Clips or using the Kent kits, to show how logic gates work.
You should now be able to draw circuits to show how switches, LDRs and thermistors in series with fixed resistors can provide input signals for logic gates and also how LDRs and thermisters can be used with a variable resister to provide a signal with an adjustable threshold voltage for a logic gate. You should also be able to work out the output from a circuit with up to four inputs to the logic gates and draw a truth table to show what is happening.
The trouble with some of these circuits is that the output device switches off when it would ideally need to stay on, so a sort of memory device called a bistable latch is used to keep the device on until the reset button is pressed. There is an example of this at: http://www.ffldusoe.edu/Faculty/Denenberg/Topics/Electronics/ (just read the bits about using NOR and NAND gates). There's also a bit about it in Physics for You and the revision guide. Other than that, there's not a great deal of information on them.
You need to know that:
- a brief high at one input leads to a permanent high signal at the output
- a brief high at the other input leads to a low signal at the output
- a low signal at both inputs leaves the output unchanged.
You could do the following worksheet:
"Logic circuits using AND, OR and NOT gates"
"Logic circuits using AND, OR and NOT gates 2"
"Electronic control systems 1"
"Electronic control systems 2a"
"Electronic control systems 2b"
