I'm using a 110VAC industrial signal bell, like a fire alarm bell, controlled by a 555 timing circuit of my own design. The bell is isolated from the timer through two relays. The first relay switches 12VDC to the second relay, which switches 110VAC to the bell.

The problem is the timer circuit is re-triggered indefinitely by EMF or other type of noise from the bell.

It could be line noise passing through the relays, or EMF released inside the aluminum enclosure. The bell is physically located 6 feet from the enclosure.

I've tried other AC loads, such as a high power fan that do not cause this. The AC bell is just very aggressive.

I would appreciated any suggestions to isolate the EMF from the timer circuit.

Could an AC line filter be used at the entry point of the bell receptacle to the enclosure? I'm not familiar with AC line filters, and I don't want to degrade the performance of the bell.

 

Break this down in steps. Make the 555 work well with a minimal load, like an LED.
Next, make it work with the first low voltage relay, then with both relays and no bell. If all of this works OK, then focus on the bell.

Your bell probably has a set of interrupter contacts inside of it that open and close as fast as the bell clangs. They are probably sparking a lot radiating noise everywhere. A capacitor across the contacts may fix this.

You could eliminate both relays and the noisy bell contacts by adding another 555 an opto coupler and a triac. Use both 555's (or 0ne 556) as a toneburst generator. That feeds the opto which isolates you as well as triggering the triac.
You can simplify the toneburst generator considerably by using a cheap 8 pin microcontroller in lieu of the 555's. This would also provide a simple way to control the bell's ringing frequency and its on/off time.

 

You could also consider getting rid of the bell and getting an electronic noisemaker that doesn't create the nasty noise. I agree with Jaguarjoe that it's probably the root of your troubles.

If not, also remember to put some good bypassing caps on your power supply and right next to the 555 and other consumers of digital signals to keep the noise down.