Photo-feedback drive system

Claim 1

A circuit for providing power to an electroluminescent lamp having an output light intensity controlled by a drive signal from a drive source, said output light intensity decreasing over a period of time when said lamp is driven at a constant voltage, comprising: means for detecting the light intensity of the lamp and providing a light intensity signal responsive to said light intensity; first comparison means for comparing the light intensity signal to a first reference signal and providing a first comparing output signal responsive to said first comparison; second comparison means for comparing the first comparing output signal to a second reference signal indicative of the drive signal and providing a second comparing output signal in response to said second comparison; means for providing the drive signal in response to the second comparing output signal; a driver driving the electroluminescent lamp in response to the drive signal, whereby the electroluminescent lamp is driven at a constant intensity; a first potentiometer coupled between a voltage supply and ground, with a first adjustable terminal of the first potentiometer coupled to the first comparison means, wherein adjustment of the first potentiometer affects an adjustment of the first reference signal; and a second potentiometer coupled between the voltage supply and ground, with a second adjustable terminal of the second potentiometer coupled to the first comparison means in a parallel circuit with the first adjustable terminal of the first potentiometer, wherein adjustment of the second potentiometer also affects an adjustment of the first reference signal, whereby the first potentiometer is adjusted to restrict lamp intensity above a desired minimum intensity and the second potentiometer is adjustable by an operator to control the lamp intensity, which is restricted above the desired minimum by the first potentiometer.