Electric and Magnetic locks

   Electronic and magnetic systems are very common for use in locking doors in public places, such as entrances to cooperative housing, hospitals, and secure installations. Anybody who wants to enter these places must typically enter a number into a keypad to open the door or alert a guard inside the installation by using a bell or an intercom. Sometimes it is possible to leave the installation using a one-way handle, but at many sites the only way to leave also requires operating a buzzer. In the past, we suggested that the way to operate such systems on Shabbat was to install a separate buzzer for Shabbat operating on the technique of gramma, indirect action. The problem with this solution is that it has various halachic problems. The only cases for which electrical devices using gramma may be used on Shabbat are in situations of a large financial loss, for a sick person who is not in mortal danger, or to avoid great suffering.

   A new technology that has been developed in the Zomet Institute allows opening exit doors electronically without any problems. This is based on the technique of "modified current" which is permitted on Shabbat without any limitations!

   The change-sensor operates through the use of a capacitance sensing device. Touching the cover of the device with a finger changes the capacitive properties and the frequency of the sensor, because of the electronic properties of the human body. There is no need to open or close an electric circuit. The halachic approval for this action is based on the fact that the prohibition of using electricity on Shabbat is to open or close a circuit (this is a violation of the action of Boneh – construction – or Molid – "giving birth" to something new). But modifying a property setting (current, voltage, frequency, or capacitance) of an existing current is permitted. The change-sensor applies a voltage to electrical devices connected to it. When the capacitance of the sensor changes, the change-sensor increases or decreases the voltage, depending on how long the hand maintains contact with the surface.

    This technique is suitable for exit doors which are operated through the use of a magnetic or electronic lock.

Magnetic lock – A very strong electromagnet keeps the door pressed tight to the doorpost, exerting a force of hundreds of kilograms. The button that is normally used to leave the installation is replaced by a change-sensor. When a hand comes close to the control box, the current passing through the electromagnet will be gradually reduced so that a light push on the door will open it. In this way, there is no electric circuit that is created or broken (and the pull of the magnet continues all the time). A few seconds after the hand is removed from the control box, the current is increased and the magnet returns to full strength.


Electronic lock – The electronic locks have a built-in magnet. Usually if there is no current flowing, the door remains locked. Pressing a switch closes a circuit and sends a current through the magnet, releasing the tongue of the lock. In this case, the usual button will be replaced by a change-sensor. Putting the hand near the control box or touching it causes the electric current in the electromagnet to increase slowly, until a small push on the door will open it. No electric circuit is opened or closed. A few seconds after the hand is removed the current is decreased, once again reducing the pull of the magnet and locking the door.

Ringing a bell – Sometimes a locked door is opened from the inside by a guard who must be alerted by ringing a bell or talking through an intercom. The button normally used to ring a bell will be replaced by a change-sensor. Putting a hand close to the sensor will cause the current to gradually increase in a buzzer or a LED display, alerting the guard. In this technique, no electric circuit is opened or closed. When the hand is removed, the current gradually decreases and the buzz or the LED light at the guard's station returns to its initial very weak state.




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