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 projector. The mechanical parts in the autofocus system are assumed to be still functional.

Auto Focus for Slide Projector Fig. 1:


Auto Focus for Slide Projector Fig. 1


Most automatic focusing systems 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 connected 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 outside, 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 moved until the circuit detects that the reflected beam from L falls exactly in between the LDRs (point 1).

Auto Focus for Slide Projector Fig. 2 :

Auto Focus for Slide Projector Fig. 2


The circuit is based on the use of an existing set of  LDRs as part of the optical module in the slide projector. The symmetrical supply shown to the left,  and the motor plus decoupling capacitor, are also part of the projector. The inverting input of opamp IC1  is at ground potential when the above mentioned test beam falls in between the LDRs. The output of the opamp keeps the non-inverting input at 0 V as well, so that no motor voltage is available  at the emitters of power drivers Ti T2. Should the  reflected beam illuminate either one of the LDRs, the circuit arranges for the motor to move the objective glass towards the correct focal position, until no  voltage difference between the LDRs is detected. 

The feedback gain of the circuit has been kept relatively low to keep the motor from continuously moving the objective glass past the target position, causing the system to oscillate slowly. Resistors R3 and R4 may have to be dimensioned differently than shown to achieve optimum response as regards speed and stability.

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