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ne555 sine wave ?

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LAN_403

Duality

Mon Feb 02 2009, 05:12PM

Registered Member #1951 Joined: Sun Feb 01 2009, 01:59PM
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Posts: 105

is it possible to make a sinewave generator with a ne555,
can't seem to find it? (on the net)
(i have a stupid comp anny way)

anny way i just want to know if it's possible ;)

Dave Marshall

Mon Feb 02 2009, 05:22PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

Yes and no. It is possible to make something pretty close to a sine wave.

Since you can't achieve a perfect 50% duty cycle with a 555, you can't achieve a perfect sine wave. However, by getting as close as possible to 50%, and filtering the output with a couple of chebychev or similar low-pass op amp filters, you get something pretty darn close to sine wave.

This works because a square wave is basically a number of sine waves (the fundamental frequency plus its harmonics) all wrapped together. By filtering out the harmonics, you're left with the single sine wave at the fundamental frequency.

Dave

Dr. Dark Current

Mon Feb 02 2009, 06:22PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Dave Marshall wrote ...

Since you can't achieve a perfect 50% duty cycle with a 555, you can't achieve a perfect sine wave. However, by getting as close as possible to 50%, and filtering the output with a couple of chebychev or similar low-pass op amp filters, you get something pretty darn close to sine wave.

Well if you were going to use a "couple of" op-amps, you can as well make a sine wave oscillator with just a single op amp. And it will be a nearly perfect sine wave.

KLH

Mon Feb 02 2009, 06:55PM

Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
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Posts: 137

Dave Marshall wrote ...
Since you can't achieve a perfect 50% duty cycle with a 555, you can't achieve a perfect sine wave. However, by getting as close as possible to 50%, and filtering the output with a couple of chebychev or similar low-pass op amp filters, you get something pretty darn close to sine wave.
Dave

It is possible to achieve an exactly 50% duty cycle with the 555; just put a diode (1N914 or similar) with cathode to pin 2 and anode to pin 7. Making the 2 resistor values equal gives a duty cycle of very close to 50% (varies with resistor tolerance, so I recommend using Xicon's 271-XX-RC 1% metal film resistor series).

You can then use a capacitor-based low pass filter, but an LC filter will be much more effective at eliminating harmonics.

But as Dr. Kilovolt said, it's better to make an op-amp based sine wave oscillator. However, I prefer the phase shift oscillator (it's on Wikipedia) because I think it's simpler and easier to get working. It requires only an op-amp and a handful of passives.

Duality

Mon Feb 02 2009, 07:06PM

Registered Member #1951 Joined: Sun Feb 01 2009, 01:59PM
Location:
Posts: 105

really thanks for replying ;P
i asked because i have toooooo much ne555's laying around XD
i have two high-speed comparator's can't i use them instead, or do it need to be op-amps?
if so are the lm386 good for this?

Bored Chemist

Mon Feb 02 2009, 07:41PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

If you run two 555 square waves with similar frequebncies into a comparator then low pass filter the output you get a sine wave with a frequency equal to the difference in frequency between the two 555s. The heterodyne oscilator or beat frequency generator. This is usually a daft way to generate a sinewave- unless you happen to have lots of 555s and lots of comparators

Duality

Mon Feb 02 2009, 07:44PM

Registered Member #1951 Joined: Sun Feb 01 2009, 01:59PM
Location:
Posts: 105

ne555's enough comparators not :(

Dr. Dark Current

Mon Feb 02 2009, 07:51PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Bored Chemist wrote ...

If you run two 555 square waves with similar frequebncies into a comparator then low pass filter the output you get a sine wave with a frequency equal to the difference in frequency between the two 555s. The heterodyne oscilator or beat frequency generator. This is usually a daft way to generate a sinewave- unless you happen to have lots of 555s and lots of comparators

I would think that would generate a triangle wave

Bored Chemist

Mon Feb 02 2009, 09:02PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

You might be right, what about adding the 2 square waves?

Steve Conner

Tue Feb 03 2009, 10:25AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The "industry standard" way of doing this is to use switched current sources to make the 555 generate a triangle wave on its timing capacitor, instead of the usual exponential sawtooth. An overdriven transistor long-tailed pair is then used to "soft clip" the triangle wave into a sine wave.

This is mathematically exact, because the transfer function of a long-tailed pair is tanh something or other. In practice you can make it work with about 1% distortion. It works over a very wide frequency range without adjustment, unlike all of the proposed schemes here using filters.

It's also how the sinewave output works on function generator chips like the ICL8038 and MAX038.

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