Alternating current with a relay?

Again a newbie question due to lack of electric-fu...

I am trying to reuse my old rotary phone, the ringer in particular. Originally it was ringing with higher voltage AC. While I have found out that 9V is enough for the hammer to give out a (weak) sound, I have no idea how to make the current flowing through the coil alternating with the control of Arduino.

I have thought that maybe a double NC-N-NO relay could work - like this one:


If I connect voltage to one middle pin and ground to the other middle pin and the ends of the coils to the side pins respectively, the current should, I think, flow in one direction when the relays are inactive and in the other when both relays are active - like in the image below:


Does that make any sense?

And if it does, I see two problems: the relays most likely will not be switched exactly at the same time... That would be bad, wouldn't it, as the current would flow unrestricted?

The second problem: can I power Arduino from the same battery?

Naturally, if there is another easy solution for this, I would be glad to hear about it...

That should work. You’re right though… If one relay goes to it’s 2nd position for the other relay breaks the 1st position, you could get a short. But, there’s probably enough time-lag when there’s no connection that you’ll be OK.

A DPDT relay would insure that never happens because both sets of contacts are mechanically connected together. (A standard double pole switch or relay is “break before make” so there is a short period of time when there’s no connection.)

One issue is that you can’t use a diode to prevent the back-EMF (since you are applying power to the coil in both directions). That means you’ll get an arc when the relay contacts break and that can tend to shorten the life of the relay.

I assume the coil takes quite a bit of current, so if you’re using a 9V regular battery it may not last long (and the voltage my drop below 9V even with a fresh battery).

BTW - I believe the standard ring frequency is 20Hz so the bell should operate most efficiently at that rate.

Just use one relay, and pulse the coil of the ringer. I don't think you need to run AC into it. It is not a transformer, just a coil. The circuit you have is more likely to short the battery. With a single relay and pulsing the coil you can use a back biased diode to take the back EMF. Dwight

Does that make any sense?

As others have said it is dangerous. If you must use a relay then use a circuit like this:- Replace your coil with where it says motor.

But you need something like a h-bridge to switch the 40 to 70V you need for a proper ring. The exact voltage depends on who's telephone network the phone was designed to work on.

There is no reason or advantage to driving it with
an AC signal.
A pulsed DC will work just as well.

The problem is that there are regular magnets which hold the hammer that the coil has to overcome (I suppose it strikes harder that way), so pulsing will not help here.

Unfortunately, with the relay I have there is definitely a bit of shorting. The switching window is too small to sync it correctly.

I am not worried too much about the power - the phone will be used by kids as a timer - they dial the minutes e.g. for teeth brushing and the phone rings after the set time. The rotary dial is fun, but I do not think they will use it more than a few times a day and even then the ringing will be short. I remember my vintage doorbell, which could last years on a single battery...

dwightthinker: There is no reason or advantage to driving it with an AC signal. A pulsed DC will work just as well.

When I used to do ringing telephones as stage props I found that pulses DC would not cut it. The UK phones needed the push pull from the electromagnet that you only get with alternating voltage in the coils.

Hi Mike The phone has a capacitor in series with the ringer, I believe he is just driving the coil directly. The coil could care less about the polarity. Dwight

The coil could care less about the polarity.

Not sure what you mean by this. To my ear this says that it is important.

Maybe English is not your first language maybe it is American

The ringers are just coils with an iron armature. The polarity of the signal is not important but the amplitude is. Depending on the inductance, AC peak values can drive more current than a RMS DC value. Switching a DC value for polarity will not make the ringer any stronger. His circuit risk welding the contacts of the relays or damaging the battery if alkaline. Sorry for the meaningless statement. Dwight

Grumpy Mike: It took me a while to understand the solution, but now I get it... In fact, it works great: I am not switching the relays at the same time, but they are moving through a neutral stage, where the coil voltage is disconnected completely, no risk of shorting.

I have assembled the complete phone and it just works. Yes, of course, the ringing is a bit feebler than original, but still nice.

No Telecoms people here then? It seems not! :roll_eyes:

Whilst lesser voltages may suffice - since you are not using long lines with significant resistance, ringing voltage is about 70 V AC and wither 15 Hz or 16.6 Hz.

And it is AC.

The ringer is not "just a coil"; never has been. It is actually a "bridge" configuration of coil and permanent magnet so that the armature moves one way with positive voltage and the other with the opposite voltage. Just like a loudspeaker, it is elegantly designed for efficiency.

That circuit I posted can be used with only one relay providing it has a double pole change over contacts. That is two sets of contacts moved by the same magnet coil. It will work with two relays as well it is just a lot simpler with the right single relay.

I don't recall seeing two bells in my phone( not using land line any more, I'll have to dig it out ). This bell would require an AC signal to strike both bells. Pulsed DC would still strike on bell but might require more voltage because of the inertia of the clapper. I may stand corrected. An alternative to the relays is an H bridge as is often used to drive steppers and pulse DC motors. Dwight