CRYSTAL_ACTIVITY_TESTER

Published:2009/7/12 23:47:00 Author:May | From:SeekIC

U1 is a 74LS00 quad two-input NAND-gate logic chip.The bo LS version was selected because ofits low current requirement Two of its internal gates are connected as inverters and,In conjunction with R1,R2,and C1,form a crystal oscillator when a crystai is connected between J1 and J2 The third gate,also connected as an inverter,is used as a buffer to provide RF at the crystalfrequency from pin 11,through isolation capacitor C3 and high-pass filter R4 and C6,to J3 so that the crystal frequency can bemonitored by an external-frequency counter The fourth gate,also connected as an inverter-buffer,provides RF at the crystal frequency from puts 8,12,and 13 connected in parallel The RF is fed through isolation capacitor C4 to voltage-doubling rectifiers D2 and D3,and the resulting dc voltage is filtered by C5 and provides a positive bias to the base of Q1,an NPN transistor,which is normally cut off.Q1 goes into conduction, and its collector current flows through LED D1 and R3 in serles,both forming the collector load circuit. The greater the activity of the crystal, the higher the positive bias on the base of Q1 will be, and the higher its collector current will be. This current illuminates D1, and its relative brightness is indicative of the level of crystal activity.



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