| 摘要 |
<p>In a coin detection system a coin passes between conductive plates 7, 9 which form a capacitor, which provides part of the frequency controlling capacitance of an LC tuned oscillator circuit 23. The presence of a coin between the conductive plates 7, 9 alters the capacitance, and consequently alters the output frequency of the oscillator circuit 23. The oscillator output is supplied to the clock of a counter 31 which counts the number of clock pulses received in a 10ms period, and the count value is provided to a microprocessor 75 via a shift register 33. The count value is a measure of the frequency of the output of the oscillator 23. Microprocessor 35 substracts the count value from a pre-stored reference value, and supplies the difference to a look-up table 47 stored in a memory 37. The microprocessor 35 uses the output of the look-up table 47 to determine whether a valid coin has been received, and if so, what the denomination of the valid coin is. The pre-stored reference value represents the frequency of the output of the oscillator 23 when no coin is present. While no coin is present the microprocessor 39 monitors the count values obtained from the counter 31, and updates the stored reference value so that it tracks drift in the frequency of the oscillator 23.</p> |
