Published:2011/7/30 3:09:00 Author:Zoey | Keyword: Fixed Period pulse, 555 Generator , changeable Duty cycle | From:SeekIC
As shown in the figure 3-6,the astable multi-vibrator in the circuit consists of a 555 circuit, VT1,VT2, R1,R2,RP1 and C1.From the picture we can see that the capacitor C1 has a separate charge and discharge looped, the adjustment of RP1 will change its charge and discharge time constant instead of its oscillator frequency. Therefore, before C1 gets a 2/3 threshold level of VDD, pin 3willstayin high level, that is, during t1, VT2 will conduct, and VT1 will be saturated and conducted, thus, RVT1 presents a weak impedance, the charge time
t1=0.693(RVT1+R1+RP1left) C1
t2 =0.693(R2+RP1right) C1
T=t1+t2
=0.693((RVT1+R1+RP1) C1
DK=t1/T
then, adjust RP1 and the duty cycle can be limited in between 2%~98% with an unchanged period.
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