Photo film processor circuit

This machine essentially does three things: It controls the process temperature, provides constant agitation of the chemical baths, and performs the timing. The operator must pour in and out the liquids by hand. The heart of the circuit is a Basic Stamp 2p microcontroller. It has a program loaded that contains several film processing algorithms. For each of them, the process temperature, the bath sequence, the bath names, and durations, are stored. The program offers the options via a menu, displayed on a 16x2 character alphanumeric backlit LCD, while user input is through two pushbuttons. A small speaker gives confirmation beeps, and is also used to signal the end of the process time for each bath.
Photo film processor circuit - schematic

The processor reads a Dallas DS18S20 digital temperature sensor, which is immersed in the water bath, and uses this information to display the temperature on the LCD, and to control the phase angle of conduction for the heater. For this purpose, a proportional-integral control characteristic is implemented in software. This routine generates an analog output value, which is output on a digital pin through pulse width modulation, and used in an extremely simple but effective optocoupled phase control circuit. The power device is an insulated-tab TRIAC which can be mounted directly to the metal box of the processor. This TRIAC can easily control de machine's 2000 Watt heater. Rotation control is implemented completely separate from the microprocessor. 555 chips are so cheap that it's not worth the effort to put the rotation control routine into the Basic Stamp, which would generate some difficulties with precise timekeeping, since the Stamp has no realtime multitasking. And anyway the power and interface circuit would be necessary. So I used two 555 timers, connected in such a fashion that the rotation motor runs 30 seconds to one side, then stops for one second, runs 30 seconds to the other side, stops, and so on. To control the rotation speed, the entire "reversomatic" circuit is powered from an adjustable voltage regulator. This IC must be mounted with insulation, as its tab is connected to the internal circuit. A...

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