# Ways of making an astable without a 555?

Similar, but is using a 555 differently, as opposed to not using one at all.

So, for a circuit I am in the process of building, I need an astable, and am attempting to avoid using a 555, partly because of the amount of noise it introduces and partly because I am trying to learn of other ways of doing it.

A quick google returned different uses of a 555 (and the link above is using a 555 to make a sine wave). On the assumption there are other ways of making a 555, what are they, and what are the major pros/cons?

First, you need to realize that the 555 is not a particularly noisy chip. ANY signal source which generates square waves will generate noise. If you are having problems, the likely source is your ground construction (or lack of it) and you need to address this before you go any farther.

As for how to make a square wave generator, the possibilities are endless. How about a ring oscillator? Note that you can use comparators in place of the gates, but you must include some hysteresis to get clean transitions. If you're willing to provide 2 - 5 volts as power, the 74HC14 will do this automatically. simulate this circuit – Schematic created using CircuitLab

Or you can use a unijunction transistor (such as a 2N2646) set up as a relaxation oscillator, followed by a flip-flop to convert the pulse output to a square wave.

A.J. 's answer will work just fine, and will have the virtue of being intrinsically less noisy due to the slow rise and fall times of an op amp vs a logic chip. Although if you are trying to drive logic (you don't say) you need to use a different op amp - one with rail-to-rail outputs.

• For the HC line, 3 V should be fine as well, or are there any effects I'm not aware of? – Arsenal Nov 23 '17 at 15:48
• Perhaps a 3-gate ring will start oscillating more reliably than a 2-gate (or even-#-of-gates) ring. – glen_geek Nov 23 '17 at 16:21
• @glen_geek - Oops. No perhaps about it. As in, an even number of gates won't work at all. My bad. – WhatRoughBeast Nov 24 '17 at 19:34

To generate a square wave, a free running multivibrator such as this will work: simulate this circuit – Schematic created using CircuitLab

The frequency of oscillation is

$$f_o=\frac{1}{2R_3C\ln{(1+2\frac{R1}{R2}})}$$

With the values on the diagram that would mean $f_o=4.56$ KHZ wit 50% duty cycle.

The opamp displayed is a TL081, which might not be the best to achieve a sharp square wave, using faster opamps will achieve faster rise/decay times, even the cheap LM301 will do the job.

The link to the question you provided includes an output filter, do you need a sine wave? If so, you might want to consider using something like a Wien Bridge Oscillator instead.

• i do not need a sine wave. however, we don't have that chip where I am, would a 741 do the job (although with a different equation for frequency probably)? – Alex Robinson Nov 27 '17 at 10:59
• A 741 will work but the rise/fall times will not be as fast as with other chips so the square wave wont look as sharp, the equation is the same for a 741. – S.s. Nov 27 '17 at 19:18