LED is a kind of part driven by current, its illuminant features only correspond to the extent of current passing by. The ordinary drive parts have no features of constant current. The voltage of TTL circuit is only a threshold, although the hair-like change will not affect the logic states, yet it will affect the output current, so will affect the illumination of LED. The only way-out for this problem is to apply the constant current source. The use of constant current source parts can ensure the invariableness of output current, eliminate the serial limiting resistance of the transforming parts and components, reduce the power consumed on resistance and reduce the heating of system so as to achieve the constant current.