lightpen - helps a computer to get to the point

helps a computer to get to the point A light pen is a tool that allows the coordinates of a point on the screen to be entered into a computer. It is based on the principle of sending a pulse to the screen control circuit at the precise moment when it sweeps the spot just in front of the lightpen. In the case of the Elektor VDU card the screen is controlled by a 6845; when this IC"s LPEN input (pin 3) goes from "0" to "1" it loads the ad- dress of the character it is writing into registers 16 and 17. We will see later what can be done with this information. The sensor in the lightpen is a partly. covered light dependent resistor (LDR), the exposed part of which forms a window the same size as a character on the screen. When the electron beam in the screen passes in front of this window the LDR"s resistance reduces drastically. This causes transistor T1 to conduct, followed by T2, with the result that a pulse suitable for the LPEN input of the CRTC appears at the collector of the BC...