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Why astable is not working?

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Can you post a .jpg schematic instead...
 

Many people don't install the bloatware that is Winzip, and may not have winrar, or may be viewing this on non-Windows platforms [mobile phones or Linux machines]. Also, not everyone knows what to do with a .dsn file (I don't), and I'm guessing that is Windows-only too. Could you paste a jpg of the circuit? (Easy to do - just hit the printscreen button on your PC).
 

Check this. .dsn files open in proteus isis.
 

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What is the error reported?
It's been a while since I used a 555 but it looks like it should work. Personally I would reduce the 500k resistor (to say 47k) and increase the capacitance. I'm not sure such a high resistance is a good idea.
 

Nothing happens when I connect the oscilloscope. No clock is generated. I need a 500 KHz to 640 KHz clock for feeding ADC0808.

If R1 = R2 = 751 Ohms and C1 = 1nF gives 640 KHz. http://www.ohmslawcalculator.com/555_astable.php


-----------update--------------

OK. The astable is working.
 

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Last edited:

The original design is working in Proteus and real life, but of course not with a frequency of 500 kHz. Review a 555 datasheet about frequency calculation. 555 isn't very good suited for the intended frequency range, but will hopefully work. I would prefer a simple thing like 74HC14 oscillator or similar.
 

@FvM

Can you show me how to implement the oscillator using 74HC14?

----------Update--------------

f = 1/T = 1/CR if C = 1uF and R = 1.5 Ohms, f = 640 KHz. Am I right?
 

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@FvM

What should be the value of R and C if f = 640 KHz?
 

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