In the previous article I discussed about some theory of operations and how to deal with the hardware.In this article will start to discuss more in details about the hardware we want to design based on a real example.
Let start from the rotary disc.
In an antique mall I did buy an old phone with the rotary disc, a Bell System Rotary phone built probably in 1982 by Western Electric.
After removing the rotary disc and examined it, these some data I gathered.
The rotary disc of this particular phone has 2 switches.
One is normally open and is closing when the disc is turned, indicating the start of the signaling phase.
It is closing when the user start to turn the disc and is re-opening when the disc completed the return.
One is normally open and is closing when the disc is turned, indicating the start of the signaling phase.
It is closing when the user start to turn the disc and is re-opening when the disc completed the return.
The second switch is normally close and open when the disc is released for every digit.
For example if the selected number is 1, when the disc is released there will be 1 pulse.
If the selected number is 3, then 3 pulses will be generated.
If the selected number is 3, then 3 pulses will be generated.
In this drawing an example of how is supposed to work.
Let call Switch 1 the switch that indicate the signaling phase (in the picture above is on the left, white wires) and Switch 2 the switch that generate the pulses (in the picture above is on the right, wires green and blue).
The drawing represent the number 2
Let call Switch 1 the switch that indicate the signaling phase (in the picture above is on the left, white wires) and Switch 2 the switch that generate the pulses (in the picture above is on the right, wires green and blue).
The drawing represent the number 2
Testing the rotary dial
It is a good thing to test the rotary dial and see actually what kind of signal it can generate.
Electrically speaking are just two switches.
Let continue to call the SW 1 and SW 2 as the example above.
We can set up this easy circuit in order to see with an oscilloscope the signal generated.
Electrically speaking are just two switches.
Let continue to call the SW 1 and SW 2 as the example above.
We can set up this easy circuit in order to see with an oscilloscope the signal generated.
Connecting an oscilloscope to the J1 and J2 we should be able to see the generated signals by the rotary disc. Vcc can be anything, let use 5V for simplicity.
The upper trace is the signaling switch (SW 1) and the lower trace is the pulse switch (SW 2)
The upper trace is the signaling switch (SW 1) and the lower trace is the pulse switch (SW 2)
Let see some examples :
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| Number 3 |
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| Number 0 |
Note the time per unit (500 ms), it is a quite long process.
The full cycle for the number 0, 10 pulses, it takes about 2 seconds to be completed.
To give a better idea, to compose the number 0000 will takes 8 seconds !
The full cycle for the number 0, 10 pulses, it takes about 2 seconds to be completed.
To give a better idea, to compose the number 0000 will takes 8 seconds !
In the next picture is represented the number 3 to a greater resolution to see the pulse timing.
With a base of 50 ms each pulse is about 70 ms on and 30 ms off for a total of 100 ms period and is quite precise considering is a mechanical device.
In this picture is not shown, but always remember this is an electro-mechanical device and thus is important to remember to debounce the readings.
In this picture is not shown, but always remember this is an electro-mechanical device and thus is important to remember to debounce the readings.
Some measurements did show the possibility to have bouncing about 432 uS, so AT LEAST is important to use 0.5 ms as bouncing time.
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| The upper trace is coming from the pulse switch and is showing a 432 uSec bouncing signal |
1 ms would beeven better since anyway the timing involved in reading the dial is quite high.
In the next article I will discuss about designing a circuit to decode the signals.
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