A piece of photography darkroom equipment it is used to control the length of exposure of light from the enlarger onto the photographic paper.

There are two main types of enlarger timers, manual and automatic ones:

• Manual enlarger timer
  Similar to an high tech egg timer, set it for the time you want, and it will alert you, with this type you will have to manually switch the enlarger off. For skilled photographers it has an advantage of being used as an indicator. 
  For some people who use the enlarger itself as a tool for creating unusual photographic art they use multiple manual timers to time the exchange of different image or light effects in succession.
   
• An automatic enlarger timer
  A digital or mechanical timing device that connects to the enlarger and once the time is reached it will instantly turn the enlarger off, its simple and easy to you with no margin for error. 

Enlarger Timers can be purchased from a Photography Supplies Store or for more information about working in a dark room see: Darkroom Photography Equipment.