Week 1 : Introduction
|555 timer astable|
|Subsystems||Timing longer intervals|
|Block diagram||What have you learned?|
|Timing the interval|
|Back to Games timer|
|Games such as Articulate and Pictionary are provided with a sand
timer to time the interval available for a player to take his turn. Timers like this give
no audible indication at the end of the timed period and have the disadvantage that the
next player has to wait for the sand to run through, even when the first player has
finished his turn.
The brief is to design an electronic replacement for the sand timer which will provide:
Additional features which can be decided upon at the outset include:
Think about the brief. Which building blocks, or subsystems, are you going to need?
1. To generate the timed intervals, a subsytem which produces pulses is needed. What is the name given to this kind of subsystem?
2. Which components do you think you will need to change to alter the duration of the timed interval?
3. What sort of subsystem will be needed to light up LEDs to show that time is passing?
4. Which frequencies of sounds are easy to hear?
5. What sort of subsystem could you use to produce pulses with a suitable frequency for an audible signal?
6. How could the pulses be directed to an audible warning device, but only at the end of the timed period?
7. Suppose the subsystem which allows the signals to pass does not provide enough current to operate the audible warning device. Which subsystem could you use to increase the available current?
8. How is the instant reset going to work?
You may not be able to come up with precise answers to these questions, but it should still be possible for you to work out a provisional block diagram, showing how some of the necessary subsystems should be joined together.
Your list of subsystems is likely to include:
ASTABLES (pulse generators)
AUDIBLE WARNING DEVICE
What functions will these subsystems carry out?
Study the block diagram given below:
Games timer block diagram
Can you explain what the various subsystems will do?
This is just one of many possible block diagrams. The idea is to have an overall picture of the games timer which will enable you to start developing and testing parts of the circuit.
As a result of testing, the block diagram will probably be modified, but it provides a sensible framework for solving the problems.
The intervals required are 30 seconds, 1 minute, 2 minutes, and 3 minutes. Suppose you decide that 10 LEDs will be used to indicate the passing of time:
If you want one pulse every 30 seconds at Y, how many pulses do you want at X?
The answer is that you want 10 times as many, that is, one pulse every 3 seconds. 3 seconds is the period of the pulses at X. What should be the period and frequency of the pulses at X for the other time intervals?
You can calculate the frequency of pulses from:
Complete the table by writing in the missing values:
|period at Y||period at X||frequency of pulses at X|
|30 seconds||3 seconds||0.33 Hz|
One possible circuit for the timed period astable is built using an integrated circuit called a 555 timer:
The frequency of the output pulses is given by:
The design formula is just what you need if you know the values of R1, R2 and C and want to calculate the frequency of the output pulses which these values will give.
In this case, you know that the frequency you want is 0.33 Hz, one pulse every 3 seconds, but you don't know what values of R1, R2 and C you should use.
You can approach this problem by rearranging the design formula:
The design formula works with fundamental units of resistance, capacitance, frequency and time. However, you can use other combinations of compatible measurement units:
Very often, it is convenient to use kW for resistance values and µF for capacitance values. You need to remember that this means that times must be in milliseconds, ms, and frequencies in kilohertz, kHz.
How do you go about choosing suitable values for R1, R2 and C ?
You are going to work through the calculation for 0.33 Hz.
R1 is usually chosen to be 1 kW. This gives an output pulse waveform with nearly equal HIGH and LOW times, provided R2 is a significantly larger resistance.
Substituting these values in the rearranged version of the design equation gives:
Note that the R1 and f values have been entered in kW and kHz. This means that the R2 value will come out in kW, while the C value will be in µF.
Use your calculator to check the value of the right hand side of the equation:
You can't get any further with the calculation without choosing a value either for R2, or for C.
Rearranging again gives:
Suppose C is chosen to be 10 µF. (If the value you choose doesn't work well, you can come back and try again!). Then:
. . . and finally:
Check through these calculations again to make sure you understand what is going on.
The way in which the 555 works places limitations on the values which can be used for R1, R2 and C. The resistor values cannot be less than about 1 kW (because this would result in excessive currents) and should not be greater than about 1 MW (currents too small). Capacitor values should be between 100 pF and about 10 µF.
Within these limits, you should choose large resistor values and small capacitor values. This is because small non-polarised capacitors have much smaller leakage currents than polarised ones and also because they are much more accurate, sometimes within 1%. Polarised capacitors have high leakage currents and tolerances as high as 25%. Whereas resistors of all values cost the same, larger value capacitors are larger-sized and often cost more.
In the example above, with C = 10 µF, close to its maximum value, the value for R2 is already quite large.
The 555 works well as an astable for higher frequencies but it doesn't work as well for slow frequencies because the R and C values needed are often outside the sensible practical range.
With a 555, the slowest pulses which can be accurately produced have a frequency of around 1 pulse every 10 seconds, 0.1 Hz. (Why?)
What do you do if you want pulses at frequencies slower than this? The answer is to combine a pulse generator, astable, circuit with a dividing circuit which reduces the frequency of the signals. A single clocked bistable, also called a toggle bistable, will half the frequency - if a 10 Hz signal is connected to the input, 5 Hz appears at the output. A chain of clocked bistables can divide by any power of two to produce extremely slow pulses. This is what happens in a digital clock.
A cmos integrated circuit, the 4060B, combines an oscillator and binary counter/divider. The chip provides an excellent source of pulses at slow frequencies.
The diagrams below show the pin connections for the 4060B and give an outline of its internal oragnisation:
4060 pin connection diagram
4060 internal organisation
The circuit around pins 9, 10, and 11 can be used to make a resistor/capacitor controlled astable, using just three external components:
4060 astable circuit
In this circuit, RT and CT determine the frequency of the pulses produced, according to the design formula:
You are now going to build a prototype circuit, using R1 = 470 kW, RT = 10 kW, and CT = 10 nF = 0.01 µF.
Follow the diagram carefully and probe the counter outputs using the oscilloscope:
If you can't remember the colour code for the resistors, use the colour code convertor program.
Using the 4060 design formula, calculate the expected astable frequency:
Use the oscilloscope to measure the actual astable frequency, as observed at pins 9 and 10. (Estimate the period of the waveform from the oscillscope display and then calculate the frequency from :
How do the expected and actual frequencies compare?
If you have done the calculation correctly and built the circuit with the correct values of R1, RT and CT , the two frequencies should be similar.
Now monitor the outputs of the 4060 at pins 7, 5, 4, 6, 14, 13, 15, 1, 2 and 3 in turn and write a sentence to explain what the 4060 does. You might be able to compare the frequencies of the counter outputs using a frequency meter.
The 4060 can divide its intial frequency by up to 214. What should be the final frequency from pin 3?
Follow the diagram below to add an LED indicator to show pulses from the slower 4060 outputs:
Up to previous stage
The 100 µF capacitor provides power supply decoupling, that is, it helps to prevent the transfer of 'spikes' or 'glitches' along the power supply connections.
You start designing an electronic project by defining a brief.
Next you consider which subsystems will be required.
You link the subsystems to make a block diagram, showing how the device might function.
You take each subsystem in turn and explore its function, finding out about and testing suitable circuits in prototype form.
For the games timer, the 4060 is an excellent choice to provide an accurate source of pulses at slow frequencies.
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