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I'm trying to get a 250kHz signal with 58% duty cycle from an NE555, i'm using values calculated by an online calculator but the circuit clocks at 168kHz when viewed in an oscilloscope. I'm feeding the circuit with a 9v battery. Am i doing something wrong?

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ If you modify the capacitor and resistor values by the usual 10% tolerance You can get to 177khz. That does not include the tolerance of the 555 itself. Question: What kind of capacitor have you used? Do you know the material? Can you post a picture of it? \$\endgroup\$ – Nils Pipenbrinck Apr 13 '16 at 18:58
  • \$\begingroup\$ @NilsPipenbrinck It's a totally new generic ceramic capacitor, I measuerd it and it has 1.019nF \$\endgroup\$ – DELTA12 Apr 13 '16 at 19:12
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No you're not doing something wrong, you're just pushing the 555 near it's limits so the non-idealities are getting more pronounced. If you would use 10x higher resistor values I bet that the frequency you get would better match the calculated value.

By using 1 kohm and 2.38 kohm the internal resistance of the discharge transistor probably also starts playing a role. This internal resistance adds to the resistor you used so you get a slightly higher value and a lower frequency.

I suggest that you try increasing the values of the resistors by a factor 2, 5 or even 10 and at the same time decrease the value of the capacitor by the same factor. So for example 5 kohm (instead of 1 kohm), 12 kohm (instead of 2.38 kohm) and 200 pF (instead of 1 nF).

If that still does not give you the right frequency, just change the values of the components slighly. The 555 timer concept is not so precise and certainly not at the frequency you want. In practice, you get something and then tweak it to the value you want.

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  • \$\begingroup\$ I'm going to try that when i get the components. By the way is there a better way if generating such signal? Perhaps a crystal? \$\endgroup\$ – DELTA12 Apr 13 '16 at 19:53
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    \$\begingroup\$ For generating a 250 kHz square wave ? If you need the accuracy then a crystal oscillator is a better option. A 250 kHz crystal is not so easy to find I think but a 2.5 MHz or 10 MHz is. Then just use a frequency divider circuit to divide that down to the frequency you want. Start by looking at the 74HC4060 datasheet, this chip has a crystal oscillator circuit (you need to add the crystal) and frequency divider stages. \$\endgroup\$ – Bimpelrekkie Apr 13 '16 at 20:05
  • \$\begingroup\$ 250 kHz is quite low so you could also generate it by using a microcontroller, the Atmega as used in the Arduino boards should be able to do it. But everyone uses micros these days, if you do it with a 4060 and/ or 4017, that's way more cool :-) \$\endgroup\$ – Bimpelrekkie Apr 13 '16 at 20:07
  • \$\begingroup\$ In fact i actually need the timer to drive a buck converter as a power supply for an ATtiny26 \$\endgroup\$ – DELTA12 Apr 13 '16 at 21:15
  • \$\begingroup\$ If you do not need to be too precise, than 555 will do. Just put trimmer resistor and tweak the frequency. I personally would probably put some attiny13a, from start, because they are so cheap and I would find him some other purpose also, but for this, it is overkill. \$\endgroup\$ – Darko Apr 13 '16 at 22:03
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250kHz is VERY high for an old-style bipolar 555- close to the maximum possible.

Try simulating it (you can use Circuitlab in the SE interface), and you'll get close (1-2%) to your measured result. Presumably the 'calculator' you found simply plugs the values into the datasheet formula- which is fairly accurate at low frequencies. There is a reason why the simple-Simon straight line graphs on the datasheet end at 100kHz. Perhaps there should be something like the captions allegedly placed in unknown areas of old maps- "Here be Dragons".

I suggest you substitute a CMOS version capable of 2MHz and you'll get closer to the theoretical values. Bypassing the control input makes a bit of difference too (but not in a helpful way!).

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Maybe your capacitor is not 1 nF but 1.5 nF. They are normally not very precise and 50% more is a bit on the high side but it seems possible.

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  • \$\begingroup\$ No the capacitor is at 1.019nF i just measured it with a multimeter \$\endgroup\$ – DELTA12 Apr 13 '16 at 19:13

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