Robust low-torque biped walking using differential dynamic programming with a minimax criterion

For use in monitoring a safety device switch through which alternating current flows, monitoring means comprising a rectifier connected across such a switch to provide a direct current output when the switch is opened, a light and a capacitor connected in series across the output of the rectifier, a signal light, and a light-actuated silicon controlled rectifier. The silicon controlled rectifier is positioned and arranged to be actuated or rendered conductive by light from the light which is in series with the capacitor. When the safety device switch is opened, direct current provided by the rectifier charges the capacitor to a predetermined level. During this charging process, the light in series with the capacitor produces light for actuating the silicon controlled rectifier. The power source for the signal light and the silicon controlled rectifier may be a separate, low voltage direct current power source completely separate from the power source producing the current flowing through the safety device switch.