need help designing a circuit.

[mkr]

hello all... I'm working on an art project... and I need help designing an electrical circuit. What I want to do is to have an old rotary telephone that will ring by itself, once every three or so hours. It's outdoor, there is no AC power. I'd like to use a battery, but since this is a long term installation, perhaps one that can be recharged via a small solar panel. Also, when the phone rings... I'd like it to ring about 4 or 5 times... just like as though someone were calling.

I've done a little research and found out that the solenoid that operates the bell mechanism needs to run off a somewhat complicated and high voltage circuit. An easier alternative might be to replace the solenoid with a small motor capable of reproducing the 20 Hz ring.

Any takers want to take a crack at this?? ... monetary compensation is not out of the question...

thanks

Mark

[torbjorn]

It is very simple to arrange a 20 or 25 Hz AC voltage for ringing a telephone bell from DC. Normally, the telephone network delivers around 60 - 100 V, but most phone bells are required to work down to, say 30 V. The exact voltages differ for various countries with different telecommunications regulations.

If you look at, for instance, a "line splitter", "fax exchange" or similar, then you will find a 25 Hz square wave oscillator driving a H bridge of four power transistors (just like a bridged audio amplifier), and the output of the H bridge is connected to a step-up transformer (typically a small mains transformer of a few VA) to get the required output voltage. Remember that you cannot use a transformer rated for 50 or 60 Hz for more than half its rated voltage at so low frequencies, as the core would be saturated. And because of the square wave (instead of sine), there is still more saturation. But a 230 V winding will be very suitable to generate, say, 80-90 V in this circuit.
You should put in a resistor in series with one of the transformer windings in order to limit the short circuit current to a harmless value. Also, it would be a good idea to fit some kind of overvoltage protector (varistor, transil or similar) across one of the windings to protect the H bridge from inductive overvoltages.

Using more recent technology, there are several ICs performing this task without transformers now, examples of manufacturers are Maxim, Linear, Analog Devices.