Who invented “electric eye” elevator doors that open if you walk through them when they close?

Two of the “fathers of the atomic bomb,” Albert Einstein and Max Planck, are also responsible for the development of elevator doors that reopen automatically if they start to close as a person steps into the car.

The two men explained how light falling on some materials can change them from poor conductors of electricity into good conductors. Research into the nature of electricity that followed from the discoveries of Planck and Einstein when they described the “photoelectric effect” led to the invention of the photocell, the device that cues a motor to reopen the doors without your having to touch them.

As you step into an automatic elevator, you’ll notice a thin beam of electric light shining at about thigh level across the threshold; it leads to a little receptor on the right.

When you put your hand or body in front of that beam, the doors won’t close. The receptor is made of a type of metal called a semiconductor. Semiconductors include silicon, cadmium, arsenic, and germanium; they sometimes resist an electric current and sometimes conduct it efficiently, depending on the frequency of the light striking them.

Electricity is the flow of free electrons. Ordinarily we think of an electron as a tiny particle orbiting a particular atom the way the moon goes around Earth.

In some metals, however, the nuclei of the atoms bond closely with each other in a rigid, regular pattern called a crystal, with loose electrons moving at random through the entire structure. Metals such as iron and copper arrange themselves this way and are good conductors of electricity, because the electrons drifting through the crystal lattice can easily be made to bump into each other and flow from one point to another, through a wire, for example.

Other metals, however, are made differently. When they crystallize, they do not leave any electrons adrift, but keep them bound with the nuclei in particular atoms.

Thus they are poor conductors, since there aren’t free electrons to conduct a flow of current. The element germanium solidifies in this nonconducting form under ordinary conditions; but if the right frequency of light strikes its atoms, electrons can be “kicked” free by the force of the energy and made available to conduct electricity.

Max Planck’s discovery, which Einstein confirmed, was that light consisted of packets or particles of energy called photons, which vibrate at different frequencies. Planck said that the higher the frequency or speed of vibration a photon has, the greater its power to “kick” an electron free from a nucleus. Substances have different thresholds of energy needed for the kick to take effect, below which their electrons can’t be moved.

The sensors in automatic elevator door mechanisms generally respond to frequencies close to visible light. One common arrangement works like this: the elevator doors’ “at rest” position is closed; it requires the action of an electric motor to open them.

The current supply for the door opening monitor is controlled by a conventional switch. Most of the time this switch is off, with no power reaching the doors. Two different mechanisms, however, can throw it to “on” under different circumstances, to make the doors open. The most obvious is a timer that opens the doors for a certain number of seconds when the car reaches each floor; the other is the photocell safety device.

The safety device is a switch controlling another switch: the door switch springs to the “on” position and opens the doors unless it’s held “off” by an electromagnet called a relay. Power for the relay magnet must pass through the germanium crystals in the photocell receptor on the right of the doorway.

While the light across the threshold is reaching the receptor and keeping its electrons “kicked” and excited enough to conduct current, the power to the relay keeps flowing, which keeps the door opener off, which means the doors can close whenever the timer lets them.

When the beam is blocked by a person entering the elevator, however, the electrons in the germanium lose their energy and are recaptured in its crystal structure; the germanium suddenly loses its conductivity and breaks the relay circuit. The electromagnet stops working and no longer holds the door motor off.

The motor therefore switches on, the doors pop open instead of crushing you, and you can step through the door into the elevator.