【正文】 that any variation of R2 has negligible effect on the operating frequency of the circuit. In Fig. 215, the duty cycle is determined by C1D1R1 (mark), and by C1D2R2 (space). The pulse frequency is variable between 300 Hz to 3 kHz via R4. Fig. 214 Squarewave generator with variable dutycycle, and frequency. Fig. 215 Variable frequency narrowpulse generator. Resistance activation Notice from the description of the oscillator in Fig. 210 that the output changes state at each half cycle when the C1 voltage reaches the threshold value set by the R2 –R3 voltage divider. Obviously, if C1 is unable to attain that value, the circuit will not oscillate. Fig. 214 shows a resistance activated oscillator that will oscillate only when R4, which is in parallel with C1, has a value greater than R1. The ratio of R2:R3 must be 1:1. The fact that R4 is a potentiometer is only for illustration. Most resistanceactivated oscillators use either thermostats or LDR39。s, which simulate the potentiometer action. Fig. 216 Resistanceactivated relaxation oscillator. Fig. 217 is a precision lightactivated oscillator (or alarm), and uses a LDR as the resistance activating element. The circuit can be converted to a “dark activated oscillator by transposing the position of LDR and R1. Fig, 218 uses a NTC thermistor, RT, as the resistanceactivating element y and is a precision overtemperature oscillator/alarm. The circuit can be converted to an under temperature oscillator by transposing RT and R1. The LDR or RT can have any resistance in the range from 2021 ohms to 2 megohms at the required trigger level, and R1 must have the same value as the activating element at the desired trigger level. R1 sets the trigger level the C1 value can be altered to change the oscillation frequency. Fig 217 Precision lightactivated oscillator Fig. 218 Precision overtemperature oscillator/alarm, Triangle/square generation Fig 219 shows a function generator that simultaneously produces a linear triangular wave and a square wave using two opamps Integrator IC1 is driven from the output of IC2, where IC2 is wired as a voltage parator that39。s driven from the output of IC1 via voltage divider R2–R3 The squarewave output of IC2 switches alternately between positive and negative saturation levels. Suppose, initially, that the output of IC1 is positive, and that the output of IC2 has just switched to positive saturation. The inverting input of IC1 is at virtual ground, so a current IR1 equals＋ VAST／ R1. Because R1 and C1 are in series, IR1 and IC1 are equal. Yet. in order to maintain a constant current through a capacitor, the voltage across that capacitor must change linearly at a constant rate, A linear voltage ramp therefore appears across C1, causing the output of IC1 to start to swing down linearly at a rate of 1/C1 volts per second. That output is fed via the R2R3 divider to the noninverting input of IC2. Consequently, the output of IC1 swings linearly to a negative value until the R2 –R3 junction voltage falls to zero volts (ground), at which point IC2 enters a regenerative switching phase where its output abruptly goes to the negative saturation level. That reverses the in puts of IC1 and IC2 so IC1 output starts to rise linearly until it reaches a positive value that causes the R2R3 junction voltage to reach the zerovolt reference value, which initiates another switching action. The peaktopeak amplitude of the linear triangularwaveform is controlled by the R2 –R3 ratio. The frequency can be altered by changing either the ratios of R2R3 the values of R1 or C1 or by feeding R1 from the output of IC2 through a voltage divider rather than directly from opamp IC2 output. In Fig. 210, the current input to C1 (obtained from R3R4) can be varied, over a 10: 1 range via R1, enabling the frequency to be varied from 100 Hz to 1kHz。 resistor R3 enables the fullscale frequency to be set to precisely 1 kHz, The amplitude of the triangular waveform is fully variable via R5 and the square wave via R8. The output generates symmetric waveforms, since C1 alternately charges and discharges at equal current values determined by R3R4. Basic function generator for both triangular and square waves. Fig. 220 100 Hz 1 kHz function generator for both triangular and square waves. Fig. 221 shows how to modify to make a variable symmetry ramp/rectangular generator T where the slope of the ramp and duty cycle is variable via R4. C1 alternately charges through R3D1 and the upper half of R4, and discharges through R3D2 the lower half of R4. 400 Hz~1kHz function generator with variable slope and duty cycle. 2. 9 Switching circuits Fig 222 shows the connections for making a manually triggered bitable circuit. Notice that the inverting terminal of the opamp is tied to ground via R1, and the n oninverting terminal is tied directly to the output. Switches S1 and S1 are normally open. If switch S1 is briefly closed the opamp inverting terminal is momentarily pulled high, and the output is driven to negative saturation j consequently , when S1 is released again T the inverting terminal returns to zero volts ,but the output and the noninverting terminal remains in negative saturation. The output remains in that state until S1 is briefly closed。 that switches the output to a stable positive saturation state until S1 is closed again. Figure 223 shows how Fig 222 can be modified for operation from a singleended power supply. Fig. 222 Bistable with simple manual triggering, Fig. 223 Single supply bistable. Finally, Fig, 224 shows how to connect an opamp as a Schmitt trigger, which can be used to convert a sine wave into a square wave. Suppose, initially, that the opamp39。s output is at a positive saturation value of 8 volts. Under