# 555 Clock Circuit - how to choose resistor value? [closed]

I need to design a 555 clock circuit to output a clock pulse of a specific frequency. The circuit I'm using is this one (from here):

I've used this circuit before and it works fine but the website only shows what resistor value to use for a few predetermined frequencies, which are all far too low for what I need. Is there an equation for working out what value of that resistor I should be using?

edit:

After looking at the datasheet, I'm still struggling to find the solution as the circuit in the sheet is different, though the one I'm using is tried and tested by myself recently. I'm also limited on what ICs I can use as I only have whats provided to me which is why I'm using an NE555.

Can anyone help with what would be a could resistor value to output the highest frequency clock possible for that chip?

## closed as too broad by Edgar Brown, Sparky256, Finbarr, Bimpelrekkie, Lior BiliaFeb 19 at 21:45

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

• Sorry, your schematic is missing. Also provide a link to "the website". Have you looked at the 555 datasheet for the equations you need? – Elliot Alderson Feb 10 at 20:04
• Any good datasheet will include such equations. – Hearth Feb 10 at 20:04
• @Elliot Sorry, added the link now, I haven't checked the data sheet because I'm very new to this and wouldn't really now what to look for – Sam Feb 10 at 20:10
• This is probably a good time for you to give the datasheet a try. Look for "astable" circuits and formulas that are used to calculate frequency. The formula will use the values of the resistors and capacitor in the 555 circuit. Come back and ask if you are still confused. – Elliot Alderson Feb 10 at 20:16
• Note that the circuit in your link will only work with CMOS 555 variants, or with a TTL 555 that's operating with a supply above 5V (and I wouldn't count on that). A really-o truly-o NE555 running on 5V won't pull the output high enough to oscillate, and if it does it won't make square waves, or go the expected frequency. – TimWescott Feb 10 at 20:56

## 3 Answers

Good data sheets for 555 timers, like this one, from ON will have a schematic and an equation or a graph (or both) showing resistor/capacitor combinations.

Note that when you get much above 100kHz you need to be careful: there's a lot of different ICs out there called a '555 timer'; some are good old bipolar circuitry (like my linked data sheet), some are CMOS, some are fast, some are low-power (and therefore slow). If you are outside of that magic 100kHz mark, reliably, you probably can't use just any old 555 -- you need to use one that's actually capable of going that fast.

• I'm using an NE555 timer, intending to go at 18MHz though I'm aware I may need to reduce this if not possible – Sam Feb 10 at 20:57
• @Sam your NE555 won't even get close to 18MHz. – Hearth Feb 10 at 21:05
• @Sam there are several methods to generate a 18 Mhz clock, even relaxation oscillators (like the 555), but your 555 will probably not do. There are, however, numerous oscillator ICs readily available. Depending on your requirements, you could also try a simple ring oscillator or just go for a quarz. I'd suggest you think about your application and rewrite your initial question to include your requirements. – michi7x7 Feb 10 at 22:06
• "and rewrite your initial question..." I think it'd be much better to ask a new question, starting with the 18MHz part, and including the nature of the output (sine, TTL, etc.) and any requirements on jitter, frequency stability, etc. – TimWescott Feb 10 at 23:28

I did a quick simulation with LTspice and the period T seems to depend linearily on the resistance R. Also the example values provided on the website you linked to suggest this. Your desired formula therefore is simply T(R) = R / (60 MOhm) s and f(R) = (60 MOhm)/R Hz)

• So the OP just needs to use a 3.3 ohm resistor and the 555 will oscillate at 18MHz? Simple as that? – Elliot Alderson Feb 10 at 23:15
• This information was not in the question and the indicated source suggests that the OP would like to operate the circuit under moderate conditions. – Sim Son Feb 11 at 0:24
• True, the information was in a comment made an hour before you answered. My point is that your answer is misleading. You don't say anything about the practical limits of oscillation frequency, and you don't include the value of the capacitor in your formula. An inexperienced user like the OP is likely to follow the wrong path if you don't provide more guidance. – Elliot Alderson Feb 11 at 0:31

Your question,

Is there an equation for working out what value of that resistor I should be using?

is based on a poorly documented circuit, including your goals, requirements, and expectations.

In a basic astable circuit, there are two resistors. This schematic from the TI CMOS version of the datasheet shows.

So, to answer your question based on this, the equation for the frequency of an astable 555 circuit as shown from the datasheet is: f = 1.44/(RA + 2*RB) * C

Now, this doesn't produce a 50% duty cycle square wave, but you didn't specify whether or not that is a requirement. If so, a different circuit is needed.

Also, the reason I chose to show the CMOS version of the 555 is that it has the highest astable frequency, which is listed in the datasheet as 3 Mhz.

Here is a link to the datasheet, which includes lots of good infromation: http://www.ti.com/lit/ds/symlink/lmc555.pdf