I would have thought parallel. If one goes faulty, and they're in series, the other won't ring. I would also have thought that series would mean a quieter ring from each due to voltage drop.
You probably want them parallel so they each get the full voltage from the transformer. With the switch making the circuit to both in one go when pressed.
I would suggest parallel, or you will be halving the voltage. Wire one up as usual, then take another pair of wires from the two terminals of unit 1 to unit 2 - make sure the transformer can supply enough current for two of them though.
I'd have thought parallel, but it might depend on the sort of chime involved.
The traditional 'bing bong' ones work by having the current power a solenoid which moves a bar to one side to hit one chime, and at the same time that breaks the circuit powering the solenoid. The bar is sprung and slams backwards to hit the other chime (and also completes the circuit again).
I'm not certain that two such chimes in series would actually sound together as the interruption of the circuit caused by one unit's solenoid could prevent the second one from energising.
IME conventional solenoid operated chimes energise the coil on button press making the first chime and the second chime is made by spring release of the sol on button release without the complexity of a switch in the chime unit.
Given the former, this should not apply.
Series or parallel operation depends on the rating of the chimes and the transformer.
If the transformer is beefy enough, parallel operation would be more reliable given the risk of a break in the longer wiring. On the other hand, a 9V tap off a multitap bell transformer would permit 2 4.5V chimes to run in series.
Personally, I'd try 2 in parallel and see if it works.
Are these chimes designed for use on batteries? If so you might have a bit of a prob on ac? I seem to remember my Dad had a problem doing this. I think he got the ding but was missing the dong. We can't have that :D
I'd try it and see. My standard underdome bells work better in series. I'd run both 'bell' cables back to the transformer and do the connections there so you have the choice. I've not tried it with chimes.
Depending on the resistance of the circuits to the bellpush and each of the "bells" you might have to experiment to get the better effect, especially if the "bells" are different. One possibility, if the wire run to the bellpush is long, is to operate a relay near to the power source, to operate both "bells" from a similar supply."in parallel". If one (or both) of the bells use an interruptor as in a trembler bell you'll likely cause strange effects if you wire the "bells" in series.
The Medway Handyman laid this down on his screen :
They have to be in parallel. In series you will get all sorts of weird effects, depending upon the type of sounder from not working very well, to one not working at all and so on.
That is true if the transformer can deliver the required current and the wiring is of sufficient gauge. But with a standard transformer and bell wire, I'd experiment and see what works best. I have two underdome bells - wired in series. They work better that way. They are AC only devices - universal ones may not be happy like that.
It may also be that there isn't a suitable 'bell' transformer easily available which will run two chimes, in which case a higher current one will have to be sourced from an electronics supplier.
download the Friedland catalogue (MK & Friedland seem both to be Honeywell). Specifically, mini chimes cannot be wired in parallel. But I really did not read very far through the detail.
I concur that if it is going to work at all, it would need to be parallel. To make them work properly in series you would need higher voltage and that would cause the light on the bell push to fail rather more quickly. (Unless you start playing with relays, etc.)
Having read the knowledgeable responses, I would go for two bells and two transformers but both sets wired back to the same switch. I have the opposite problem two doors with bells, so I have them with different chimes, so I know which bell is being pushed.
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