BASIC 
ELECTRONICS COURSE 
Page 48 INDEX

REVISION
The previous 3 pages covered INTERFACING between a pick-up device (also known as a transducer) and a digital line such as the input to a microcontroller. 
Sometimes you can connect a pick-up directly to the input of a microcontroller. Devices such as switches and some Hall Effect device are examples. The only requirement is the pick-up must have a voltage swing of at least 80% of rail voltage. The program in the microcontroller can be designed to remove noise, switch-bounce, excessive switch-pushing and all types of interference. 
Devices that produce a voltage swing less than 80% of voltage rail must have an INTERFACE CIRCUIT connected between the device and the microcontroller to bring the voltage up to the required level. 
These are the features we have covered in the previous 3 pages. 
In this revision test we will go over the design of a number of interface circuits and make sure you know how they work so you can design these types of circuits when you go to our microcontroller section.

Question 1: Draw an interface circuit between an electret microphone and a microcontroller, consisting of two common-emitter stages. The following animation will help you with each stage of the design:


Click and wait for file to download

Question 2: A coil is used to detect the passing of a magnet. The coil is connected to a common-emitter amplifier and the output of the transistor is connected to the input line of a micro-controller. 
Draw the circuit and sketch the input to the microcontroller when the magnet passes the coil. 

     
Click and SLIDE pointer to keep window open

We have not shown the output of the micro-controller as it may only change after 15 passes of the magnet or once every 24 hours! It will depend on the program.  See Question 3 for more details.

Question 3: From Question 2, suppose the PIC microcontroller produces a HIGH when the input is HIGH. Draw diagrams to show the position of the magnet for the output:  
(a) HIGH, 
(b) LOW. 
Compare your work with the animation below:

 
Note: the output of the MICRO can go HIGH for any duration, at ANY time. 
It will depend on the program you put in it.

Question 4: Name the pick-up device you would select for the following: (A number of different pick-up devices can be used for each situation - it will depend on the exact requirements)
(a) Detecting water level    Answer
(b) Detecting nails in a piece of wood    Answer          See Metal detector Page 24
(c) Customers walking though an open doorway    Answer
(d) Detecting a burglar opening a drawer    Answer
(e) Glass Breakage Detector    Answer
(f) Detecting noises and voices    Answer
(g) Counting cardboard boxes on a conveyor belt    Answer
(h) Detecting the opening of a door   Answer
(i) Detecting walking down a corridor   Answer
(j) Detecting smoke   Answer
(k) Early-Warning Smoke Detector   Answer

Question 5: A coil connected to the input of a microcontroller (as shown in the accompanying diagram) does not produce sufficient voltage for the micro to detect a HIGH when a magnet passes over it. Describe 4 ways to improve the circuit.

 

 Question 6: Did you construct the circuit below on Page 47? If not, Click HERE.
Question 6(a): Name the 3 stages in the circuit below:


Question 7: How much of the Metal Detector circuit from Question 4b above, do you remember? It's important to know what size resistor and/or capacitor goes in each location. Refresh your memory by constructing the circuit. Click HERE.

Question 8:  What is the main fault with each of the following circuits:


Question 9: Can you put more than one "pick-up" device on an input line?

Question 10: This is a memory test. Study the circuit then build it by clicking below. You can see how all the features we have covered have been combined to produce a 4-input dialing alarm. This is one of the projects we will be covering in the PIC Programming section. 


                    Click HERE to build the circuit.


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