Ripple control noise suppressor details


Theory of operation

The ripple control frequency used here is quite high, 1050Hz, and at our house is around 4-5V peak-to-peak. Because we want to pick out a continuous burst of this from a background of other noise, I use an LM567 tone decoder. This can’t do the whole job though, because if you read its specs it can’t cope with other signals that are more than 6dB above the signal its trying to detect. In this case the 250V 50Hz mains is at least 60 times larger than the signal we want to detect. We would like to be able to detect down to 1V, so we need a high-pass filter with at least 20dB rejection at 50Hz. This isn’t terribly difficult to do. Here I use an LM358 to build a two-pole high-pass Butterworth filter. The -3dB point is 720Hz, which gives us better than 40dB rejection at 50Hz. The 22nF capacitor on the input of the LM567 forms a third high-pass pole with a -3dB point of about 360Hz, so we have tons of margin. This is a Good Thing because there will be all kinds of other noise on the mains which we don’t want going into the detector.

The other half of the LM358 is used as a pulse stretcher with a time constant of about ten seconds so we don’t start and stop the fan on every ripple control pulse. Switching the mains on and off silently is handled by a MOC3063 which is a clever optically isolated zero-crossing triac driver, coupled to a suitable triac. The circuit reacts within a few cycles of the 1050Hz tone starting, which doesn’t give the fan enough time to make any noise other than a very faint click before it coasts to a halt.

At power-on, both LEDs should be off. The green LED should come on after ten seconds or so. The red LED should come on and the green go off when ripple control signals occur, and the green should then take ten seconds or so to come back on.

Construction notes


This circuit operates on mains voltages, which are potentially lethal. If you do not have experience in mains-rated construction and know how to comply with your local electricity regulations, do not attempt to build this.
I built the main circuit on Veroboard in a small plastic box. It looks like this:

Ripple control signal detector

The triac switch and MOC3063 were mounted seperately in another very small box inside the fan housing, with a cable to the detector which is plugged into a wall socket in the attic. Seperating the detector from the EMF noise created by the fan motor might be important; I don’t know because I didn’t try an all-in-one installation.

The power supply is not critical; any small transformer with one or more 9-12V windings will do. The LM2930T8 (which I think is obsolete) was used because I happened to have one – an LM317L with appropriate programming resistors for an 8V rail would do fine. The transformer can be the smallest you can get, because the whole circuit draws no more than 50mA.

VR1 should be a multiturn cermet type for stability and accurate adjustment. Adjust it to set the free running frequency of the PLL in the absence of a signal. This can be measured at pin 5, and should be the same as the frequency you are trying to detect. Adjusting VR1 requires that you (a) know what your local ripple control frequency is and (b) have a scope or a DFM. I used the frequency meter built into my cheap multimeter and it worked fine. I found out what our ripple frequency was by looking at the relay in our fuse box, which helpfully had 1050Hz written on it.

Testing the device is a bit tricky, as for a real test you need to wait for the ripple control to come on and see if LED1 comes on. If it doesn’t there isn’t much time to make changes. You can however test with a signal generator by disconnecting C3 from the transformer and feeding say 100mV of 1050Hz into the front end. My prototype triggered at 35mV input, which is just under 1V sensitivity on the mains.

The triac requires no heatsink to handle the load of the fan, higher-powered appliances might well want one. A mains-rated relay would probably do just as well.


This circuit will almost certainly work as-is for frequencies other than 1050Hz, assuming you tune the PLL appropriately. Below 700Hz it might be advisable to move the low-pass filter down by scaling C3 and C4 (e.g 15nF will give 480Hz cutoff).

  1. #1 by lachlan on December 3, 2010 - 5:15 pm


    Thanks for the invaluable write up and technical specifications of you ripple signal filter. Unfortunately i am not electrically skilled and need your device. I have confirmed with the electrical company that we have a 750Hz signal on our electrical system. Can you tell me if there is anyone making these devices commercially that i can purchase?

    Thank you,


    • #2 by johnarthur on December 4, 2010 - 7:53 am

      I’m afraid I don’t know of any commercially available units.

  2. #3 by Chris on December 18, 2011 - 7:31 am

    Hello ,

    thanks you for your great schematic. Is it possible to set the ripple control frequency at 175Hz ?

    I calculated for the LM567 the following values:
    VR1=52 Ohm
    C6 = 100nF
    C8 = 100nF.

    I´m not sure which values the filter parts should be . Is it possible to do the filtering with only one op-amp?

    Many thanks !


    • #4 by michaelhamel on December 20, 2011 - 9:54 am

      Thats a ratio of 6 to the 1050Hz I was using, and quite a bit closer to the mains fundamental. I would scale the capacitors first: change the C3/R1, C4/R2 22K/10nF pairs in the filter to 27K/47nF, change the C5 22nF input capacitor to 100nF, bump C6 to 470nF and use the same value for VR1. I’m honestly not sure if this design will handle a frequency that much closer to 50Hz, but it has plenty of margin and is certainly worth a try.

      The filter stage is a single op-amp, I’m not sure what you mean by that.

  3. #5 by David Quayle on February 26, 2013 - 8:56 pm

    Rather than start & stop the fan, is it possible to just pass the frequencies above say 360hz to ground using a capacitor ?

    • #6 by michaelhamel on February 27, 2013 - 7:30 am

      It might be, but its a bit tricky: a passive LC filter that would attenuate the ripple enough to be effective and operate at mains voltages and currents would be fairly massive – and I suspect would cost more to build. I was unable to find anyone marketing a passive filter, which suggests its difficult.

  4. #7 by David Quayle on February 28, 2013 - 9:01 pm

    I want to stop the 1050 Hz getting to my audio system as it is causing issue & as you can imagine the Stop/start thing wouldn’t work for me 🙂


    • #8 by justin on June 30, 2013 - 1:15 am

      I think this signal is playing havoc with my led downlights either in conjunction with the particular dimmer, or the lights themselves. They flicker at the same times each day.
      I wonder if a 1050hz circuit filter is still unobtainiun ?

      • #9 by michaelhamel on July 1, 2013 - 9:48 am

        As far as I know, yes, but I haven’t been searching. LED downlights shouldn’t be affected like this; they use switching supplies that should track mains variations very well. I wonder if you are actually seeing brief brown-outs – any large industrial concerns nearby?

      • #10 by Ed Klein on September 8, 2013 - 3:07 pm

  5. #11 by Tim on January 5, 2014 - 3:24 pm

    Hi Michael,

    Thanks for publishing this extremely helpful article.

    Fortunately I’m sufficiently learned in such electronic things that I designed a PCB based on your schematic and had one (well, 10. Not sure what I’m doing with the other 9) made and a few dollars in components later, I’ve likely violated several bylaws and my insurance but successfully circumvented the deceptively sharp elbow of my wife connecting with my face every 3 hours during the night.

    An elegant and extremely effective design. Thanks again!


    • #12 by michaelhamel on January 5, 2014 - 4:19 pm

      Glad to hear it worked for you; mine is in service to this day and makes sleeping in the rather warm and damp weather we are having that much easier…

  6. #13 by Nigel on December 29, 2014 - 11:20 pm

    Hi Michael. Very clever. Excuse my obvious ignorance but I am curious as to why the offending frequency cannot be effectively filtered, directly on the 50hz line? I have several banks of dimmers on separate supplies, plus 4 fans, again on separate supplies, all displaying symptoms of a ripple control signal at various fixed times. Turning lights off is obviously no better than watching them pulse. I think for a fan it’s a good solution, but clearly not so good for lighting. Any ideas?

    • #14 by michaelhamel on December 30, 2014 - 8:52 am

      Its possible, but the trouble is that a filter would use passive components (inductors and capacitors) that are reasonably close tolerance (depending on how good you want the filter to be) and capable of handling mains voltages and currents. These are expensive and physically large, up to very expensive and very large if you are talking about anything near 1000W. It can certainly be done, but few applications are willing to pay for it, which is why filters are difficult to find.

  7. #15 by Geoff on February 7, 2015 - 1:09 pm

    hi, found this device which may be useful to some people.

    this will be an LC notch filter design so it is specific to the exact tone frequency in use.

    I have flicker issues caused by LED lighting and a dimmer, I need to confirm it’s caused by ripple but since it only happens after 10pm I suspect so. If the flicker is caused by Zellweger tone then I will try filtering it.

  8. #16 by Ken Morison on March 1, 2015 - 6:18 pm

    Hi, At last I’m getting somewhere. I get a flicker on LED lights supplied by Halcyon in New Zealand, model r755, 14 W 350 mA. After weeks of testing I’ve finally found times of flicker correspond to times given by our local power lines company Orion in Christchurch. They use 175 Hz Telenerg ripple with 1.2 to 4 V on a 240 V supply. Now I’ll be looking for a 175 Hz filter to remove this noise. You can search for RippleSignalGuide.pdf published by Thanks for the leads.

  9. #17 by john Whittingham on March 4, 2017 - 1:44 pm

    gayrad manufacture filters like the ZF10-1050.. hunter pacific have a unit that fits in ceiling fans or behind the switch plate. google it

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