555 constitutes an enlarged exposure time automatic controller circuit diagram

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One-piece inductor

As shown in the figure, the automatic control circuit for the amplified exposure time. The circuit is composed of a buck rectifier circuit, a monostable timing circuit and a relay control circuit. The monostable timing circuit is composed of an IC (555) and a photosensitive detecting head R0, capacitors C1 to C4, etc., and the timing depends on the charging and discharging time constant of the circuit. Where R0 is a photoresistor. The toggle switch K2 has a four-speed exposure mode, and the 1, 2, and 3 stops are automatically controlled. The three exposures are “bright”, “normal”, and “dark”, and the fourth gear is manually controlled.

During exposure, the light is irradiated onto the magnifying paper through the film and the lens, and then reflected to the photosensitive detecting head R0. The internal resistance of R0 will become larger and smaller due to the intensity of the light, thus determining the length of exposure time. When the film is thick, the light transmittance is low, the reflected light of the photo paper is weak, and R0 exhibits a large electric resistance, and the timing capacitor C1 is long when charged. When the capacitor is charged until the IC6 pin potential reaches 2/3VDD, 555 is reset, and the low level of the 3-pin output causes relay J to be released. On the contrary, if the film is thin, R0 exhibits small resistance and short exposure time.
With this circuit, it is possible to automatically detect the thickness of the film during the enlargement of the photograph, thereby automatically controlling the exposure time.