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diff --git a/281/CH9/EX9.1/example9_1.sce b/281/CH9/EX9.1/example9_1.sce
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+disp('chapter 9 ex9.1')
+disp('given')
+disp('design a suitable circuit using 741 op-amp with a supply of +or-12volt')
+disp('capacitor coupled zero crossing detector to handle 1kHz square wave input with peak-to-peak amplitude of 6volt')
+Vi=6
+f=1000
+Vcc=12
+disp('I2>IBmax')
+disp('let I2=100*500nA')
+IBmax=500*10^(-9)
+I2=100*500*10^(-9)
+disp('let Vb=0.1volt')
+Vb=0.1
+disp('VR2=Vcc-Vb')
+VR2=Vcc-Vb
+disp('volts',VR2)
+disp('R2=VR2/I2')
+R2=VR2/I2
+disp('ohms',R2)
+disp('use 220kohm standard value and recalculate I2')
+R2=220000
+disp('I2=VR2/R2')
+I2=VR2/R2
+disp('amperes',I2)
+disp('VR3=Vb=0.1volt')
+VR3=0.1
+disp('R3=VR3/I2')
+R3=VR3/I2
+disp('ohms',R3)   //use a 1.8kohm standard value
+disp('let VBE=0.7volt')
+VBE=0.7
+disp('R1=0.1*VBE/IBmax')
+R1=0.1*VBE/IBmax
+disp('ohms',R1)
+disp('use R1=120kohm standard value')
+R1=120000
+disp('Vi(peak)=Vi/2')
+Vipeak=Vi/2
+disp('volts',Vipeak)
+disp('I1=Vipeak/R1')
+I1=Vipeak/R1
+disp('amperes',I1)
+disp('let v=1volt')
+v=1
+disp('t=1/(2*f)')
+t=1/(2*f)
+disp('seconds',t)
+disp('C1=I1*t/v')
+C1=I1*t/v 
+disp('farads',C1) 
+disp('use a 0.015*10^(-6)F standard value to give v<1volt')