10 555 configured as a monostable circuit + is the pos. of the battery=9VResistive dividerRRR+3 VcomparatorFF=flip-flip, depicted as a DPDT switch.+6 V- is the neg. term. of the battery=0V(Assumption: pin #3 is connected to 0V)
11 Comparator We will assume that the FF is in the “up” position when the 555 timer IC is powered up.comparatorEach comparator compares two inputs and delivers an output depending on whether the inputs are similar or different.“UP” and “DOWN” remind you what each comparator does when it is activated.
12 555 configured as a monostable circuit 0 VStep 1: Ground pin 2,The output voltage of comparator A goes upPin 7 will be disconnected from the ground.Step 2:C4 will get charged through R4.The voltage at pin 6 will go up.0 VYou don’t need to memorize this. Just lookat switch drawn in the FF.
13 555 configured as a monostable circuit Step 3:The voltage at pin 6 will take some timeto rise up to 9.Step 4:Note that the voltage at pin 5 is fixed to2/3 of 9 V. When the voltage at pin 6exceeds the voltage at pin 5, theoutput (pin 3) will be grounded and pin 7will again be connected to ground.(You don’t need to memorize this. Justlook at the switch in the flipflop.)0 V+6V
17 555 configured as an astable circuit Explanation on the next slide.
18 Operations of astable timer Let’s assume that C1 is initially grounded.Step 1: A low voltage at pin 2 causes theComparator to send a “down” signal to FF.Step 2: The output voltage at (pin 3) goes upand the pin 7 is disconnected from R2.(Again, look at the switch in the FF)Step 3: C1 gets charged up through R1+R2.Step 4: The voltage across C1 goes up.Step 5: The voltage at pin 6 exceeds the voltageat pin 5.Step 6: The comparator sends a “high” signalto the flip flop.Step 7: The voltage of Pin 7 goes to ground andThe voltage goes to ground.Step 8: C1 discharges through R2.Step 9: Go back to step 1.