s takes on a low value it is capable of immediately triggering the operation of means for terminating the supply of electric current to the cooking appliance depending on specific requirements the functions of the inverting and non inverting inputs of comparator 22 can be interchanged in a manner to reverse the direction of variation of the voltage at output s device 24 thus permits application of a threshold potential to the connection point e which potential is variable from one sampling phase to another and more precisely takes account of dust or fouling of the optical means and or aging of the optical means and the components of electronic circuit 20 self calibration of the system in a device according to the invention is thus effectuated at each start of operation of an appliance provided with a safety device according to the invention this self calibration is necessary taking into account that the safety device is not intended to serve as an optical barrier having an on off mode of operation detection must be carried out in response to the presence of a totally opaque body such as the user 39 s hand as well as bodies or substances such as smoke having varying degrees of opacity such that a certain proportion of radiation emitted by emitter 10 will still reach receiver 11 for detecting the presence of bodies or substances which are only partially opaque it is necessary that the threshold value with which the detector signal is compared corresponding to the presence of such an object or substance must be reproducible even in the event of.