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Placing noise on mains?

Some of the household circuits gadgets I made decades ago have recently been getting activated spuriously. I suspect some sort of interference or spikes or whatever on the 240 V mains supply.

One example is a buzzer in the shed that's normally triggered from a push button in the house - primarily my wife's 'come in for dinner' call. (It's essentially a CMOS monostable, NPN transistor and a bistable feeding a small earpiece speaker). But sometimes it gets triggered unexpectedly, perhaps by an appliance in the house (boiler, washing machine, etc). Or maybe by an external source nearby. And it always gets triggered when I switch on the fluorescent lights in the adjacent garage. There are other gadgets which suffer similarly too.

So in order to experiment with fixing this, I'm seeking a reliable way of generating mains noise myself. Any practical circuit ideas would be appreciated please.

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Terry, East Grinstead, UK
Reply to
Terry Pinnell
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AC motor with brushes.

Reply to
Tom Biasi

Use a relay (in the shed) to turn on the buzzer. The low impedance, slow speed of the relay will ignore short term noise that a high impedance circuit will react to.

An ounce of prevention at the source of the noise is worth a pound of circuitry at the sensitive end -

You can also lower the impedance with a resistor across the line, and put a filter cap across the signal and ground too to slow the response. But a relay is cheap dirty and works. (short of a lightening strike on the line)

Old Magnetic style fluorescent ballasts, are really bad for turn on/off transient noise. I had to add filters on each and every one - years ago when I still used them.

Snubbers work good too - 100 ohm flameproof resistor in series with a .1 UF (200+ VAC rated cap) across the switch that turns on the light.

Reply to
default

A suggestion: have you considered rebuilding the gadgets? 20 or 30 years sounds like it will degrade the component parts enough that they won't behave like they used to.

Reply to
Aleksandar Kuktin

One thing to keep in mind is that there's a lot more RF in the world than there was 20 or 30 years ago, particularly at mobile phone and WiFi frequencies. This could be getting into your circuits and causing false activations.

If there are a couple of long wires running from the buzzer in the shed to the button in the house, they might be acting as an antenna.

If your gadgets are powered from the mains, and you think noise is getting in that way, you might try a prepackaged filter module. They have "in" and "out" connections, and (usually) all the safety approvals you need for connecting to the mains. They are available new, but also often available cheaply as surplus. Corcom and Delta are a couple of popular makes but there are several. This is an example of a Corcom one from Digikey:

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Standard disclaimers apply: I don't get money or other consideration from any companies mentioned.

Matt Roberds

Reply to
mroberds

Dont know about generating noise but I think I can figure out how to remove it. Replace your capacitor on the power supply, it sounds like its not holding charge anymore and will cause a ripple in the supply..

Also, put a pull up R on the input line to lower the impedance or pull down, depending on which way you designed it.

Jamie

Reply to
M Philbrook

Electric drills (with cords) work well for this.

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

Your wife is just f%$#ing with you!

Mikek :-)

Reply to
amdx

It's within the realm of possibility. :-)

Reply to
Tom Biasi

Thanks all, appreciate the helpful suggestions and will experiment along those lines.

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Terry, East Grinstead, UK
Reply to
Terry Pinnell

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