Abstract: An electronic control system for a television receiver (12) incorporates a light pen (10) which can be applied to selected portions (16) of the screen (14). These portions (16) are modulated (FIG. 2) at the field scan rate by data inputs such that in successive television fields they are black or white in accordance with the values 1 or 0 of the bits of the data. The data can be used to pre-program the receiver, or alternatively simply stored or printed. The light pen can also be used to read bar codes (36) in a broadcasting periodical (38), and to distinguish whether the light pen is reading the CRT or a bar code a differentiator (42) and asymmetry detector (44) sense whether the light pen output pulses have sharp or decaying trailing edges. In an alternative arrangement the light pen can be used with a light emitting diode such as on a radio receiver or a telephone.