The reticle illumination source of the Telrad sight is a red Light Emitting Diode (LED) powered by two "AA" batteries. The LED is controlled by means of a potentiometer at the right rear corner of the Telrad, rotating the dial varies the intensity of the LED. While this arrangement permits the observer to adjust the brightness of the light source, this is either a continuous on or off arrangement.
Over time, some observers noted that by "blinking" the faint red pattern of a sight or guiding eyepiece this made it easier and less fatiguing to observe the very faint objects in the background. Some developed "blinking" circuits to turn the light sources of their sights on and off automatically, with more sophisticated versions permitting the operator to vary the period of time between the on blinks. When the Pulse control is dialed to "on" then the reticle pattern alternates on and off repeatedly with a frequency that can be controlled by turning the dial. The Telrad Pulser as an accessory for the Telrad sight which may be installed by the user.