Question

Question 8 (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 1uF capacitor and access to any resistor values you want. 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 (i.e 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 RB and then discharges through Re only. Therefore, the duty cycle is controlled by the values of RA and RB This astable connection results in capacitor C charging and discharging between the threshold-voltage level 0.67 x Vcc) and the trigger-voltage level 0.33 x Vcc). As in the mono-stable circuit, charge and discharge times (and, therefore, the frequency and duty cycle) are independent of the supply voltage (5 V to 15 V) See Figure 12 C-0.15 0.01 (see Note A) 5 CONT RESET DISCH Output OUT RB THRES Output Voltage GND 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 operation. This should be evaluated for individual Capacitor Voltage Time-0.5 ms/div 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 tH and low-level duration t can be calculated as follows 0.693 (RA +Rs)C 0.693(RC Other useful relationships are shown below period t0.693 (RA +2RB )C 1.44 Output driver duty cycle Output waveform duty cycle Low-to-high ratio 2 AR A& -上

Answer 1

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