| 摘要 |
PCT No. PCT/GB81/00014 Sec. 371 Date Oct. 2, 1981 Sec. 102(e) Date Oct. 2, 1981 PCT Filed Feb. 5, 1981 PCT Pub. No. WO81/02354 PCT Pub. Date Aug. 20, 1981.A coin testing apparatus comprises transmitting and receiving coils on opposite sides of a coin passageway. The transmitting coil is connected to high and low frequency oscillators. In the high frequency channel the signal is amplitude controlled by a voltage controlled amplifier rectified by a rectifier and smoothed by a long time-constant circuit. The initial rise in level caused by a coin entering between the coils is detected by an instantaneous level change comparator which responds to the rate of change of signal level at the output of the long time-constant circuit becoming equal to a preset threshold and causes a normally closed switch to be opened. When the switch is closed a comparator compares the signal with a reference value from a source and adjusts the gain of the amplifier until the signal corresponds to the reference value. Upon the arrival of the coin the switch is opened and a long time-constant circuit causes the gain of the amplifier to be maintained at the level before the arrival of the coin. A window comparator compares the difference in voltage with voltage ranges for acceptable coins. A similar arrangement is provided in the low frequency channel but with two differences. The switch in the low frequency channel is operated by the same instantaneous level comparator as is used for the high frequency channel and instead of a rectifier a novel sample and hold technique is used for providing a d.c. signal from the output of the amplifier.
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