Photo Electric cell or Photo Cell, device whose electrical characteristics (e.g., current, voltage, or resistance) vary when light is incident upon it. The most common type consists of two electrodes separated by a light-sensitive semiconductor material. A battery or other voltage source connected to the electrodes sets up a current even in the absence of light; when light strikes the semiconductor section of the photocell, the current in the circuit increases by an amount proportional to the intensity of the light. The current from a photocell can easily be used to operate switches or relays, it is often used in light-actuated counters, automatic door openers, and intrusion alarms. Photocells in such devices are popularly known as electric eyes.

Vactec Vactrol resistive opto coupler, neon lamp - light source. Photocell 80Ω (full bright) to 9MΩ (full dark). Used in Fender and Gibson amps. 3/8" x 1-1/8". Made in 1976. NSN: 670-0028-000.

In the early 1960s, Gibson and Fender started using resistive opto-couplers to modulate the tremolo effect in guitar amplifiers. Both companies were assembling their opto-couplers (RO) from discrete lamps, photoresistors and coupling tubes. While Gibson used cheap but slow incandescent lamps as light sources, Fender replaced them with neon lamps. This Vactec RO uses a neon lamp as used by Fender in the late 60's. It increased the maximum frequency response and reduced controlling currents. At the same time, nonlinear modulation resulted which was desired by some as a sound effect.

The photovoltaic type of photoelectric cell, when exposed to light, can generate and support an electric current without being attached to any external voltage source. Such a cell usually consists of a semi-conductor crystal with two zones composed of dissimilar materials. When light shines on the crystal, a voltage is set up across the junction between the two zones. Photovoltaic cells are also used to make solar cells.