Inventors have come up with a Bluetooth device for the Israeli Defence Forces, which can make any soldier's mobile phone permissible for use on Shabbat.

Where lives are at risk, such as in the army and in hospitals, special Sabbath innovations are permitted that are not allowed in other contexts.

The new phone device will eliminate the main transgressions associated with telephoning on Shabbat, making it permissible in the army and in hospitals. But the inventors stress that it does not make telephoning for normal social or business purposes acceptable.

The device is being developed by Zomet, a modern-Orthodox institute that finds high-tech solutions to halachic problems, on commission of the army. It will go in to production within a year.

In every mobile phone on the market, each button is linked to an electric circuit which is closed when the button is pressed. One of the main reasons that controlling electrical devices is problematic on Shabbat is that this act of closing a circuit is prohibited.

In Zomet's Bluetooth device, each button will be linked to an electric circuit. But unlike in the mobile phone itself, each button's circuit will constantly have a small electric current running through it. Pressing a button will not close the circuit - eliminating that Sabbath transgression - but simply increase the amount of current passing through it, or in technical terms modulate the current.

Any number dialled using these special buttons will be stored in the Bluetooth device until the person using it presses "send" to transmit the information to the mobile phone. This is deemed preferable to transmitting the numbers one by one as, halachically-speaking, the fewer times a person uses the Bluetooth transmitter, the better.

The Shabbat Bluetooth uses technology developed for a recent Zomet innovation: the Shabbat-compliant computer keyboard.

It is a keyboard with built-in mouse on which the circuits attached to the keys and mouse are not closed but simply modulated.

Like the Sabbath Bluetooth, it is only permissible in contexts where lives are at risk. Rabbi Dan Marans, executive director of Zomet, said that in addition to doctors and soldiers he hopes that the keyboards will prove popular with diaspora synagogues which might use computers on Shabbat for security purposes, to identify people.

"Just take our keyboard, plug it in to any computer in the world, and it becomes shomer Shabbat [Sabbath-observant]," said Rabbi Marans.