PURPOSE: To prevent the erroneous detection of a vehicle by removing the influence of external noise, by applying the detection signal of amplified output to a time constant change-over circuit under a predetermined pressure.
CONSTITUTION: When a vehicle is present on a loop coil 1 at the colsing time of a power source, the inductance thereof is reduced and the output of an amplifier 6 is stabilized on the basis of the reduced inductance value by a feedback loop (time constant change-over circuit 10<integrator circuit 11→V/f control circuit 12→mixer circuit 13). Next, when the vehicle is escaped from the coil 2, inductance increases and the output level of the amplifier 6 is lowered while an AND gate circuit 16 generates output corresponding to the output of a power source closing memory circuit 15 by the output of a level detector circuit 9 and the time constant of a change-over circuit 10 is shortly set through an OR gate circuit 18. When the vehicle is present on the coil 1 for a time longer than the set time of a hold circuit 13, this time elaspse is stored in a memory circuit 14 and, when the vehicle is escaped from the coil thereafter, the time constant of the change-over circuit 10 is shortly set by the output of the AND gate circuit 16.
ASADA MASATOSHI
NISHIYAMA KAZUTO
