Photo-lab timer

Depending on the version, the desired duty cycle is set by means of a mechanical dial with scale or a push-button arrangement along with a numerical display.

Manual timers are usually set by turning a dial to the desired time interval, storing energy in a mainspring that operates the mechanism.

They work in a way similar to a mechanical alarm clock, the energy of the mainspring causing a balance wheel to oscillate back and forth.

[3] Short period electromechanical timers for enlargers use a mechanism driven by an AC synchronous motor without any electronic circuitry at all.

There is a type of automatic timer manufactured by Kaiser, Philips, and others that by means of a photoelectric sensor (like the one in the photo or mounted inside the easel), is capable of regulating the exposure time according to the light projected by the enlarger, through the negative, on the photographic paper, magnitude inversely proportional to the darkness of the negative and its the distance to the paper.

Selective or integral exposure measurement using an independent metering head with a silicon optical sensor and converging lens.