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IC advice to create a regolable oscillator from 0 to 500kHz
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Yaroooo:

--- Quote from: Zero999 on April 01, 2019, 08:10:08 am ---
--- Quote from: Yaroooo on April 01, 2019, 07:55:49 am ---I don't really need a 50% duty cycle, as I said it's just for test purposes, so will never feed something that require particular clock parameters.

About 555, so, if I place small Res like 150 and 1.5k, with a 0.001uF , this will be a good choice of resistors and capacitor for this IC? I'll able to generate about 400kHz.

Than I'll regulate my frequency with a POT in series to 1.5k. Theoretically, since R1 is very small (150 R), I'll able to get between 50.5% and 53% duty cycle.

I'm right with this assumptions?
--- End quote ---
Yes, but 150R is a little on the low side. I'd probably go for C = 330pF, R1 = 390R and a 4k7 potentiometer.


--- Quote ---I'd liked your version with D-FF, I've tried to simulate it but I've got some problems.
--- End quote ---
That means nothing. Simulators are notoriously bad at modelling digital logic chips. I've built it before and it worked perfectly, although I admit I used the CD4013, not 74HC74, but it should work with that too.

--- End quote ---

Thanks for your advise.

Anyway, about D-FF circuit, how can I simulate it using a valid simulator? I admit Falstad seems an entry level simulator and may fail.
Zero999:

--- Quote from: Yaroooo on April 01, 2019, 08:21:58 am ---
--- Quote from: Zero999 on April 01, 2019, 08:10:08 am ---
--- Quote from: Yaroooo on April 01, 2019, 07:55:49 am ---I don't really need a 50% duty cycle, as I said it's just for test purposes, so will never feed something that require particular clock parameters.

About 555, so, if I place small Res like 150 and 1.5k, with a 0.001uF , this will be a good choice of resistors and capacitor for this IC? I'll able to generate about 400kHz.

Than I'll regulate my frequency with a POT in series to 1.5k. Theoretically, since R1 is very small (150 R), I'll able to get between 50.5% and 53% duty cycle.

I'm right with this assumptions?
--- End quote ---
Yes, but 150R is a little on the low side. I'd probably go for C = 330pF, R1 = 390R and a 4k7 potentiometer.


--- Quote ---I'd liked your version with D-FF, I've tried to simulate it but I've got some problems.
--- End quote ---
That means nothing. Simulators are notoriously bad at modelling digital logic chips. I've built it before and it worked perfectly, although I admit I used the CD4013, not 74HC74, but it should work with that too.

--- End quote ---

Thanks for your advise.

Anyway, about D-FF circuit, how can I simulate it using a valid simulator? I admit Falstad seems an entry level simulator and may fail.

--- End quote ---
Why do you want to simulate it? There's little point in doing so. It won't actually prove whether it works or not. The simulator will only be as good or bad as the component models.

You could try LTSpice, but it's a lot of effort, because it doesn't come with the CD4013/74HC74 in the default model library.

Try building it on a piece of breadboard. Start with larger capacitors to give a lower frequency, which will be easier to work with on a breadboard, which is not ideal for high frequencies.
iMo:
You cannot get 50% duty off a 555. The math does not allow it.
You may use the wiring with two diodes in the lower resistor, that works well (tested in hw).
See below the 555 with regolable duty and frequency/
Zero999:
The 7555 will theoretically give 50% duty cycle, with the timing resistor connected to the output.


However in practise, it won't give exactly 50% duty cycle. I've built it before and it's slightly off, depending on the value of R and the power supply voltage. Generally, it gives better results with as higher power supply voltage and value of R, as possible, but it's dependant on the device, which is pot luck. See the thread linked below.
https://www.eevblog.com/forum/beginners/portable-low-frequency-square-wave-generator-circuit/msg2167870/#msg2167870

This is why simulation is not the answer. If you tried simulating the above circuit, it will no doubt give exactly 50% duty cycle, but it doesn't in reality. Indeed the default LTSpice model for the NE555 gives 50% duty, with this circuit, but a real 555 timer won't. The simulator is wrong, because it doesn't model the 555's output stage properly.
ogden:
To get 50% duty cycle, build 0..1MHz clock and divide it by two:



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