Auto Focus For Slide Projector

This circuit is intended as a replacement for the electronics in a partly or wholly defective autofocus driver in a slide pro- jector. The mechanical parts in the autofocus system are as- sumed to be still functional. Most automatic focusing sys- tems in slide projectors are based on the use of an optical module, which comprises a small lamp, a few lenses and mirrors, and a light sensor made from two series-connec- ted light dependent resistors (LDRs), which function as a potential divider. As shown in Fig. 1, lamp La projects a narrow beam onto the centre of the slide, A, whose surface reflects it onto the LDRs. When the slide surface bulges inside or out- side, the projected image on the screen is blurred, and the beam from L is received on the surface of one of the LDRs (point 2 or 3). This is detected by a motor driver circuit, which ensures that the focal distance between the objective, 0, and the slide surface is corrected to maintain a sharp image, i.e., the objective is mo...