Question

ans,

RA = 721.5 kohms, RB = 360.8 kohms, R3 = 3.5kohms as an example solution.

Question 7 (6 marks) You have been given a 555 Timer, an LED with Von 2V, a 9V battery that can comfortably supply 5mA of current without loss of voltage, a 1 μF capacitor and access to any resistor values you want. You are to design a circuit that causes the LED to flash periodically once per second with a duty cycle of 75%. Ensure that it does not draw more than 5mA of current from the battery (Le 3mA of charging current and 2mA of current flowing through the LED). The following extract from the data sheet is provided: 8.3.2 A-stable Operation As shown in Figure 12, adding a second resistor, Rs, to the circuit of Figure 9 and connecting the trigger input to the threshold input causes the timer to self-trigger and run as a multi-vibrator. The capacitor C charges through RA and Rs and then discharges through Re only. Therefore, the duty cycle is controlled by the values of RA and This astable connection results in capacitor C charging and discharging between the 0.67 x Vcc) and the trigger-voltage level0.33 Vcc). As in the mono-stable circuit, charge and discharge times (and, therefore, the frequency and duty cycle) are independent of the supply voltage evel ( RA-5K Re-3k C-0.15 (5 V to 15 V) See Figure 12 0.01 uF see Note A) 5 CONT RESET DISCH OUT THRES TRIG Output Voltage Pin numbers shown are for the D, JG, P, PS, and PW packages NOTE A: Decoupling CONT voltage to ground with a capacitor can improve aperation This should be evaluated for individual Capacitor Voltage Time-0.5 msdⅣ Figure 12. Circuit for Astable Operation Figure 13. Typical Astable Waveforms Figure 12 shows typical waveforms generated during astable operation The output high-level duration low-level duration t can be calculated as follows and t 0.693 (RA +R)C い0.693(Re )C Other useful relationships are shown below period t0.693 (RA +2RB)Oc 1.44 Output driver duty cycle Output waveform duty cycle1 Low-to-high rato- 7.1) (3 marka) Explain how thc RC circuit intcracta with thc 555 timcr to crcatc the output PWM signal. 7.2) (3 marks) Calculate the values of R1, R2 and Ra required for your design to meet the specified requirements. ev R1 R2 R3 C1 LED D1 R3-

Answer 1

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output ou be 9w LEO raximum ament hou 5m 5m