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555 Timer Help needed

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nwo4life

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switching an npn transistor with a 555 timer

Hi,

I need to make an FSK (frequency shift keying) generator using the 555 timer. My marks are as follows,
'0' = 1000 Hz '1' = 1200 Hz

I have calualeted all the resistor and capacitor values for both frequencies, but I don't know how to hook everything up with one 555 timer. One idea is to use an active logic switch (NPN BJT) but I don't know how?

If anyone has any ideas or suggestions please post a reply.
 

1000hz 555

.. switching a cap using NPN transistor .. see picture below ..

Regards,
IanP
 

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    nwo4life

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An idea is to make two oscillators with two separated 555 chips, then using external MUX, select outputs according to its selection lines (1's and 0's)..

Or, you can use only one 555 chip, if you could put an intermediate adjustable component.. e.g. a resistance that's been shorten or something like that, when an NPN switch is ON, apply your modulating signal to its base, and let it parallel to the desigred resistance.. You may use combination of more than one NPN.. Calculate your components to get the required mark and space frequencies...

Regards
Ahmad
 

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    nwo4life

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To ahmad_abdulghany

Would you be able to give more detail on your second idea.

THANKS
 

Hi nwo4life,

I thought this was easy, I expected a lot more replies.

18_1172430522.GIF


Adjust 1000 Hz first.

78_1172430584.JPG


The first period is 1000 Hz, logical "0", 0 volts applied.
The second period is 1200 Hz, logical "1", 5 volts applied.

Vertical 2V / div. Horizontal 200µS / div.

on1aag.
 

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    nwo4life

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nwo4life said:
Hi,

I need to make an FSK (frequency shift keying) generator using the 555 timer. My marks are as follows,
'0' = 1000 Hz '1' = 1200 Hz

I have calualeted all the resistor and capacitor values for both frequencies, but I don't know how to hook everything up with one 555 timer. One idea is to use an active logic switch (NPN BJT) but I don't know how?

If anyone has any ideas or suggestions please post a reply.
Click to expand...

Try this link "555 Timer/Oscillator Tutorial"
**broken link removed**
 

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