# Convert square wave to sine wave

Discussion in 'General Electronics Discussion' started by JamesBourne, Sep 27, 2010.

1. ### JamesBourne

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Sep 26, 2010
Hi everyone,

I am building a circuit that will produce a sine wave as output.

I am using a 555 timer (astable) to create a frequency of 140Hz. This produces a square wave with an amplitude of about +5V.
Note that voltage is 0V and +5V (not -5V and +5V).

My problem is that I do not know how to turn the square wave into a sine wave.
I have read on various sites that you can use capacitors and inductors to filter
the signal to produce a sine wave, but calculations and explanations are nowhere to be found.

2. ### davennModerator

13,808
1,943
Sep 5, 2009
About the best i could find was on this site....
http://electronicdesign.com/article...-circuit-converts-square-waves-to-sine-w.aspx

there's a reasonable explanation of the cct. for your purposes you dont need the left hand part of the cct the RDD104 chip and 1MHz xtal osc. just feed you 555 timer output into the 10nF capacitor and let U2 do its thing

cheers
Dave

3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,489
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Jan 21, 2010
Next question will be: "Where can I find a MSFS5".

I looked and I couldn't find any from the usual suspects..

Another option might be a very narrow band pass filter

Here is a cunningly useful PDF

4. ### davennModerator

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Sep 5, 2009
awww Steve... dont spoil a good plan

But agreed nor could I, I wonder where the cct originator got the IC from ?

Dave

5. ### JamesBourne

6
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Sep 26, 2010
Would it be possible to convert the square wave using analog components (possibly an inductor)? If so, how would you calculate the component values?

Note: I would have used an op-amp or something else instead of the 555IC if I could, but the circuit needs to run from batteries.

6. ### davennModerator

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Sep 5, 2009

I guess its time to ask......

why dont you just produce a 140Hz sine wave for a start ??

Dave

7. ### Militoy

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Aug 24, 2010
That's a very good question. One good reason to start out with a sine oscillation, or to stick with an active filter instead of a passive one, is the size of the components to make up a bandpass filter way down at 140 Hz. I put together a quick, simple "meatball" solution - and ran it on LTSpice. It doesn't give a perfect sine - but pretty close. From the input side - I wound up with a 10 mH inductor in series with a 47 uF cap and a 20 ohm resistor. Then across the line after that, I have a 10 mH inductor in parallel with a 100 uF cap. In series with the cap, I added a 2 ohm resistor for de-queuing. Those numbers haven't been optimized - but you can see for just shaping a low-power signal - you're going to be using up significant real estate if you go with a passive solution.

8. ### JamesBourne

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Sep 26, 2010
After a bit of reading and experimentation I managed to get a good sine wave using a 3 pole RC filter. (See attached photo)
In response to Dave's question, I'm using a 555 timer to get an accurate frequency of 140Hz and will rather filter the square wave than generate a sine wave from scratch.

Thanks for your help.

File size:
61.8 KB
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9. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,489
2,830
Jan 21, 2010
Silly question, but why do you want a sine wave?

Have you considered a simple oscillator made from a Schmitt trigger inverter, a resistor and a capacitor in place of the 555? It will operate from a lower power supply and use less power.

10. ### JamesBourne

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Sep 26, 2010
Why I want a sine wave:
A square wave sounds bad when played through earphones. After I filtered it, it sounded much better.

After searching the internet, I have found a 555 CMOS which requires a very low supply voltage (2V). The circuit will run from a rechargeable battery (one gotten from those shake torches) to make it portable. Unfortunately the maximum voltage is 3.6V and drops to 2V after some hours of usage, which makes standard digital IC's unsuitable.

Luckily this chip draws extremely low current and voltage which is really good for the battery life (40mAh). All that's left to do now is to test and tweak it on the breadboard and to build the circuit.

Thanks for your help.

11. ### davennModerator

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Sep 5, 2009
yup of course it does a square wave is comprised of many harmonics.

D

12. ### NickS

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Apr 6, 2010
I am late to this party but its interesting so I would like to throw some stuff at it anyway.

1: I love filtering with op amps filters at low frequencies because
a) The parts are much much smaller.
b) You don't have to use a coil with bad Q(high loss),
c) Just changing resistors can buy you have so many options(topology/gain).
Look at TI they make a free active filter calc tool that rocks.

2: If its for audio then I hope you didn't already ruin your headphones. Square wave signals have a bad habit of destroying cones.

3: If you wanted a simple solution for multiple tones. You could build a clapp oscillator and pull the frequency around with a variable inductor. But it will drift slightly with temperature.

13. ### foTONICS

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Sep 30, 2011
how about a tank circuit?

14. ### Alec_t

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Jul 7, 2015
Do you realise you've just necroposted in a 5-year old thread?