1. 221308 Engineering Electronics I
Analog Integrated Circuits
(Integrated Circuit Timer)
Asst. Prof. Dr. Montree Siripruchyanun Matheepot Pattanasak Dept. of Teacher Training in Electrical Engineering Faculty of Technical Education King Mongkut’s Institiute of Technology North Bangkok
12/29/2018
Asst. Prof. Dr. Montree Siripruchyanun

2. Asst. Prof. Dr. Montree Siripruchyanun
The Capacitor
The capacitor is a device that stores electric charge
Rislove’s Rules of Capacitors (DC Circuits)
When voltage is applied, capacitors will briefly allow current to flow as they charge
Voltage across a capacitor increases as it charges
Once charged, capacitors no longer allow current to flow and the voltage across the capacitor is constant
Capacitors with large capacitance (measured in Farads) take longer to charge
Capacitors can be discharged by short-circuiting them
12/29/2018
Asst. Prof. Dr. Montree Siripruchyanun

3. Charging and Discharging
Voltage across a capacitor increases as it charges
Large resistor + large capacitor = longer charging time
12/29/2018
Asst. Prof. Dr. Montree Siripruchyanun

4. Asst. Prof. Dr. Montree Siripruchyanun
The RC Time Constant
The time it takes for a capacitor to charge or discharge depends on two things:
How much current is flowing (limited by the resistor R)
How much charge the capacitor can store (limited by the capacitance C)
R x C is called the time constant and determines the charging time
12/29/2018
Asst. Prof. Dr. Montree Siripruchyanun

5. Capacitors as Backup Power
Very large capacitors (a few Farads capacitance) can be connected between VCC and Ground near the power supply
If the power fails, the capacitor can supply current to the circuits for up to a few hours at a time
12/29/2018
Asst. Prof. Dr. Montree Siripruchyanun

6. Capacitors in Digital Circuits
99% of capacitors in digital circuits connect VCC to ground near a chip in order to cut down on noise
The rest do one of the following:
Provide backup power
Provide external capacitors for the clock circuit (known as an astable multivibrator)
12/29/2018
Asst. Prof. Dr. Montree Siripruchyanun

7. Asst. Prof. Dr. Montree Siripruchyanun
Mutivibrators
A multivibrator is a circuit that changes its output between two states continuously or on demand
Three types:
Bistable multivibrators (i.e. flip-flops)
Monostable multivibrators produce a single pulse once triggered by an input signal
Astable multivibrators produce an oscillating train of ones and zeroes spontaneously
12/29/2018
Asst. Prof. Dr. Montree Siripruchyanun

8. Monostable Multivibrators
Principal use: to adjust the duty cycle of a pulse train
Feel free to read more in your text—enough said
12/29/2018
Asst. Prof. Dr. Montree Siripruchyanun

9. Astable Multivibrators
An astable multivibrator is inherently unstable, switching constantly between HIGH and LOW
Astable multivibrators are used principally to generate clock signals in digital circuits
12/29/2018
Asst. Prof. Dr. Montree Siripruchyanun

10. A Simple Astable Multivibrator
The input (and output) constantly switches from HIGH to LOW as the capacitor is charged and discharged
Uses feedback to make the circuit unstable
12/29/2018
Asst. Prof. Dr. Montree Siripruchyanun

11. Asst. Prof. Dr. Montree Siripruchyanun
Timing Diagram
12/29/2018
Asst. Prof. Dr. Montree Siripruchyanun

12. Asst. Prof. Dr. Montree Siripruchyanun
555 Timer
12/29/2018
Asst. Prof. Dr. Montree Siripruchyanun

13. Asst. Prof. Dr. Montree Siripruchyanun
The 555 Timing IC
Note: A comparator outputs a HIGH when the + input is at a higher voltage than the - input
12/29/2018
Asst. Prof. Dr. Montree Siripruchyanun

14. The 555 as an Astable Multivibrator
The discharging time constant is RBC
The charging time constant is (RA+RB)C
In this case, the circuit takes longer to charge than to discharge
A small (~0.01 uF) capacitor should be placed on pin 5
12/29/2018
Asst. Prof. Dr. Montree Siripruchyanun

15. Asst. Prof. Dr. Montree Siripruchyanun
Timing Diagram
tLO = 0.693RBC
tHI = 0.693(RA+RB)C
The output frequency is f = 1/(tLO+tHI)
12/29/2018
Asst. Prof. Dr. Montree Siripruchyanun

16. Asst. Prof. Dr. Montree Siripruchyanun
A 50% Duty Cycle Clock
For a 50% duty cycle clock, the charge and discharge RC time constants must be the same
The circuit charges through RA and discharges through RB = RA
The output frequency is f = 1/(2x0.693xRAC)
12/29/2018
Asst. Prof. Dr. Montree Siripruchyanun

17. Monostable Multivibrator
12/29/2018
Asst. Prof. Dr. Montree Siripruchyanun

18. Monostable Multivibrator
12/29/2018
Asst. Prof. Dr. Montree Siripruchyanun