A protective system on a machine has a beam producing apparatus and a device to give an output signal in response to it. The impulse generator (0) works from the light emitter (D1) which lights a photo-electric device (D2) with impulsed beams. There is a discriminator (G1) at the photo-electric device output so that a signal is only produced when the photo-electric device is illuminated. There is also time delay (H) connected between the impulse generator and the discriminator. The impulse generator is a unijunction device connected by controlled rectifier (S) controlling the light emitter. There are monostables assisting in time delay and oscillating functions.
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