with reference to the figures , the glass ceramic cooking hob 1 comprises a conventional glass ceramic plate 2 on which a cooking utensil p ( for example a metal pan ) is to be rested . the pan p is rested on a cooking region 3 comprising an electrical heat source consisting , for example , of a conventional resistance element 4 ( or other equivalent heating element ) which , as can be seen in fig2 is positioned in a container 5 for example of insulating material , glass fibre or the like , open upwards towards the lower face of the glass ceramic plate with which it is in contact . the container 5 has a perimetral wall 6 comprising a substantially annular groove housing a coil turn 7 of conducting metal . the ends of the turn terminate in terminals 7 , the ends of the heating element 4 terminating in terminals 8 . the heating element 4 is connected via its terminals 8 to a control circuit 9 to which the turn 7 is also connected . in its sensing part concerning the invention , the control circuit 9 comprises a voltage - controlled oscillator indicated by vco , which generates a square wave signal with a frequency , for example , of 1 mhz . the square wave signal is applied to the input of a low pass filter l / r the inductance l 1 of which forms part of the turn 7 , and an impedance adapter x . however in the limit , using a suitable vco the turn could be sufficient alone . the filter output signal , which differs depending on whether the pan p is or is not positioned on the heat source ( resistance element 4 ), is applied to a peak rectifier ds 1 which transforms the signal leaving the filter into a continuous signal , this enabling the highest possible effective value to be obtained . the signal leaving the peak rectifier ds 1 is applied to the inverting input of an operational amplifier lm ( feedback via the resistor r 19 ), to its non - inverting input there being applied the non - filtered square wave signal transformed into a continuous signal by a peak rectifier ds 2 identical to the said peak rectifier ds 1 . the action of the amplifier is such that its output signal is different according to whether the presence of the pan p is sensed or not sensed by the turn 7 . if the pan is absent , the signal leaving the operational amplifier lm can be used by the remaining electronic circuit 9 in such a manner as to not allow the resistance heating element 4 to be powered even if the operator wishes to switch on the resistance element 4 by operating a conventional knob . it will however be switched on if , having positioned the pan p on the correct point of the cooking hob , the signal leaving the operational amplifier lm enables powering of the resistance element . removing the pan results in automatic switch - off of the resistance heating element . as is apparent , the invention is based on variation in the cut - off frequency of a low pass filter l / r due to the change in the value of l which occurs on resting the pan p on or removing it from the glass ceramic surface . with reference to fig3 in which the horizontal axis indicates frequency and the vertical axis indicates the ratio vu / vi , i . e . the alternation ( output voltage = vu , input voltage = vi of the l / r filter ), if the pan is absent the filter cut - off frequency is ft , whereas if the pan is present the cut - off frequency assumes the higher value ftp . at the oscillator frequency fvco there is , in the absence of the pan , a value vu for the same vi which is substantially lower than that with the pan , and hence a variation δ which is utilized to obtain the control signal , for example for the aforesaid control in switching - on the resistance heating element or another equivalent heating element . as already stated , according to the invention the turn 7 is fed with a square wave signal , for example of 1 mhz frequency , by a voltage - controlled oscillator vco . this , given the high harmonics content of this particular wave form , allows maximization of the variation in the filter output signal vu when the pan is rested on the plate . it should be noted that a square wave oscillator is easy to form using a few simple components , as is apparent from fig4 and from the ensuing analytical description . the use of the peak rectifier ( ds 1 and ds 2 ) both at the output of the l / r filter and in the comparison branch to the non - inverting input of the amplifier lm enables a signal to be obtained having the highest possible effective value , and hence very easily handled . the use of a voltage - controlled oscillator enables the cut - off frequency of the l / r filter to be automatically found , hence enabling the circuit to be adapted to the variabilities introduced by the various sensor components ( coil turn , impedance adapter , etc . ), which vary from circuit to circuit . the impedance adapter x enables a relatively low vco frequency to be used . in actual fact , the very low inductance of the coil turn 7 means that the vco frequency should be greater than 1 mhz . however , with this adapter the value of l “ seen ” by the circuit is l ′, which is directly proportional to n 2 ( where n is the primary to secondary turn ratio , i . e . n = n 1 / n 2 . if n is 40 / 1 , n 2 = 1600 , hence l ′= 1600l . this correspondingly reduces the cut - off frequency which in an l / r filter is given by ft ′= r /( 2πl ′). moreover , a single vco oscillator is able to handle a large number of sensor turns 7 ( i . e . a large number of resistance heating elements ) by selectively injecting the signal of one and the same vco oscillator into the various filters of one and the same cooking hob , with multiplexing of the various sensors 7 . this enables substantial technical advantages to be obtained deriving from the fact that having only one oscillator obviates component tolerances , and also results in cost advantages because of the reduction in the number of components . again , using a voltage - controlled oscillator means that its oscillation frequency can be changed by replacing the fixed voltage source ( indicated by v 1 in fig4 ) with a variable voltage source , for example by using a pmw ( pulse width modulation ) source so as to adapt the cut - off frequency to the specific components of the circuit . in detail , the circuit of fig4 comprises a d . c . power supply source represented by a battery v 4 , the vco oscillator voltage - controlled by the source v 1 , a first circuit branch comprising the coil turn 7 of the low pass filter l / r , the peak rectifier ds 1 , and the feedback connected amplifier lm , of which the inverting input is connected to said first circuit branch and the non - inverting input is connected to a second circuit branch connected to the output of the vco oscillator . the vco oscillator ( of usual type ) comprises two identical transistors q 5 and q 6 connected between the battery v 4 and earth via resistors r 10 and r 11 ( not necessarily identical ). the transistor bases are connected to the source v 1 via identical resistors r 8 , r 9 , the collector of one being connected to the base of the other via capacitors c 8 , c 9 ( not necessarily identical ). the l / r filter comprises the inductance l 1 and the resistor r 1 . the peak rectifiers ds 1 and ds 2 comprise respectively the diodes d 1 . d 2 and d 3 , d 4 , the resistors r 2 , r 16 , and the capacitors c 2 , c 12 . the other not specifically described components ( resistors r and capacitors c ) are provided for circuit calibration , filtration , protection and signal level optimization purposes .