Posted on Mar 28, 2007    11839

How it works: the circuit as shown in FIG. VT1, VT2 and resistor R1 constitute a constant current source for charging the capacitor C2 for linear; VT3, VT4 and resistor R2 constitute a constant current source for

discharging the capacitor C2 to achieve linear. Circuit is first turned on, the voltage on C2 is zero, 555 IC 2,6 feet less than 1 / 3Vcc, its 3 feet high output (approximately 10.8V), forward-biased diode VD4, VD3 reverse bias; VD1 positive partial, VD2 reverse bias. VT2 collector current charging C2 through VD1, when the voltage on C2 linear growth 2 / 3Vcc (ie 8V) when the 555 trigger 6 feet, 3 feet so that output low (approximately 0V). At this time VD1, VD4 reverse bias, and VD2, VD3 forward biased, the charge on the capacitor C2 through VT3 collector discharge, when the voltage on C2 linearly down to 1 / 3Vcc (ie 4V), the trigger 555 2 feet, leaving 3 pin reset output high, again and again, in 555 feet of 2,6 can get very high linearity of the triangular wave, and in 3 feet can get 50% duty cycle square wave. The frequency of this circuit is 0 ~ 200kHz and above range, and high stability. Frequency of the circuit by calculation:. F F0.1375 / (R1 + R2) C2 given by the circuit R1, R2 value, f F208kHz. Schematic VT5 high B output buffer stage, triangle pole removed from VT5 shot, by about 4V at the same time, the circuit can work application: change R1 or size R2, you can change the triangle wave rise or fall time, a corresponding change in the square wave duty cycle, or the control voltage 555 5 feet, 3 feet square wave can change the frequency (linear change).

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