1. EET 1131 Unit 13 Multivibrators and the 555 Timer
Read Kleitz, Chapter 14.
Lab #13 due next week.
Homework #13 due next week.
Quiz next week.
-Handouts: Quiz #12, Unit 13 practice sheets.
-Fire up Multisim.

2. Multivibrators
A multivibrator is a circuit whose output changes between two digital levels.
Three types:
Bistable: Two stable states (latches and flip-flops)
Monostable (also called “one-shot”): One stable state
Astable (also called “oscillator”): No stable state

3. More on the Three Types
Bistable Multivibrator: When placed in either state, it will stay there indefinitely; won’t spontaneously switch to the other state.
Monostable (“One-shot”): Will stay in its stable state indefinitely, but if forced into the other state it will spontaneously switch back after a certain time.
Astable (“Oscillator”): From either state it will spontaneously switch to the other state after a certain time. 3

4. A Detour: Resistors and Capacitors
For monostable and astable multivibrators, timing is involved. A cheap way to control timing is with an RC circuit. (More precise way is with a crystal oscillator.)
Resistors and capacitors are often used to control the timing of monostable and astable multivibrators. 4

5. A Detour: Resistors and Capacitors
Resistors and capacitors are often used to control the timing of monostable and astable multivibrators.
Therefore the next few slides cover basic information about resistors and capacitors that you’ll study in EET 1150 (DC Circuits.)
A more precise way to control timing is with a crystal oscillator. 5

6. Resistors (From EET 1150, DC Circuits)
Resistors are measured in ohms (Ω).
Typical values range from 10 Ω to 10 MΩ.
Value is coded on the body using a color code. Above, green-blue-red = 5.6 kΩ.
Schematic symbol: 6

7. Capacitors (From EET 1150, DC Circuits)
Capacitors are measured in farads (F).
Typical values range from 10 pF to 1000 F.
Value may be printed directly on the body (as above) or using a numeric code.
Schematic symbol: 7

8. Polarized Capacitors (From EET 1150, DC Circuits)
Most capacitors (above) are not polarized: it doesn’t matter which way you insert them in a circuit.
Some capacitors (below) are polarized. Body markings show which end is negative. 8

9. Capacitor Charge and Discharge
Charge and discharge curves in series RC circuit:

10. Capacitor Charge and Discharge
(From EET 1150, DC Circuits)
For the series RC circuit on the previous slide, the time constant is equal to the product of R times C: Time constant = R  C
This time constant is a measure of how quickly the capacitor charges.
Rule of thumb: The capacitor is fully charged after about 5 time constants.
-99% charged after five time constants. (63%, 86%, 95%, 98%).
Do Practice Question 1.

11. End of Our Detour
This ends our detour on resistors and capacitors. Now, back to multivibrators…. 11

12. One-Shots
The one-shot or monostable multivibrator is a device with only one stable state. When triggered, it goes to its unstable state for a predetermined length of time, then returns to its stable state. +V
REXT
CEXT
For most one-shots, the length of time in the unstable state (tW) is determined by an external RC circuit. Q CX
RX/CX
Trigger
-Example: security light on garage.
-How will it behave if it gets another trigger pulse while it’s in its unstable state? Q
Trigger Q tW

13. One-Shots
Nonretriggerable one-shots do not respond to any triggers that occur during the unstable state.
Retriggerable one-shots respond to any trigger, even if it occurs in the unstable state. If it occurs during the unstable state, the state is extended by an amount equal to the pulse width.
Retriggerable one-shot:
Trigger
Retriggers Q tW

14. One-Shots
An application for a retriggerable one-shot is a power failure detection circuit. Triggers are derived from the AC power source, and continue to retrigger the one- shot. In the event of a power failure, the one-shot is not triggered and an alarm can be initiated.
Missing trigger due to power failure
Triggers derived from ac Q
Retriggers
Retriggers
Power failure indication tW tW tW

15. IC Monostable Multivibrators
74121
Nonretriggerable
Pulse width tW  0.69RC
74122 & 74123
Retriggerable
For C > 1000 pF:
74122: tW  0.32RC ( /R)
74123: tW  0.28RC ( /R)
For C < 1000 pF, use timing chart.
74121: -Pulse width is given by RC ln(2) or approx 0.7RC. Typically want to keep C >= 1000 pF and 1 kOhm < R < 100 kOhm.
-Stable state is LOW output (Q). Triggered by either a HIGH->LOW transition on A1’ or A2’ while B and the other A are high, or a LOW->HIGH transition on B while A1’ or A2’ is low.
-Do practice questions 2 & 3.

16. 555 Timer
The 555 is a popular, versatile chip used in many timing applications.
By itself, it’s neither a one-shot nor an astable multivibrator, but it can be hooked up to perform as either of these.
These applications, and several others, are discussed in its data sheet.
-It’s a “mixed” analog-digital chip. (Not available in Quartus II, but show where to find it in Multisim.)

17. 555 Input and Output Pins
Aside from its power and ground pins, the 555 has five input pins and one output pin.
-Pins 1, 8 are easy. (Vcc can range from 4.5 V to 16 V)
-We won’t use pins 4, 5.
-Pin 3 is output.
Pin diagram
from datasheet
Logic symbol from Multisim (but pins 2 and 4 are active-low)

18. The 555 timer as a monostable multivibrator
The 555 timer can be configured in various ways, including as a one-shot. A basic one shot is shown. The pulse width is determined by RAC and is approximately tW = 1.1RAC.
-1.1 is approx ln3.
-This is a non-retriggerable one-shot, and it’s triggered by a negative pulse on pin 2.
The trigger is a negative-going pulse.
tW = 1.1RAC

19. Example Solution The 555 timer as a monostable multivibrator
Determine the pulse width for the circuit shown.
Solution
tW = 1.1RAC = 1.1(10 kW)(2.2 mF) =
24.2 ms
= 10 kΩ
-Do as practice question 4.
= 2.2 F
tW = 1.1RAC

20. The 555 timer as an astable multivibrator
The 555 can be configured as a basic astable multivibrator with the circuit shown. In this circuit C charges through RA and RB. It discharges through only RB. The time high, time low, and frequency are given by:
𝑡 𝐿𝑂 =0.693 𝑅 𝐵 𝐶
-Duty cycle = (R1+R2)/(R1+2R2) x 100%; with this design, duty cycle is always > 50%
𝑡 𝐻𝐼 =0.693 ( 𝑅 𝐴 +𝑅 𝐵 )𝐶
𝑓= 1 𝑡 𝐿𝑂 + 𝑡 𝐻𝐼

21. The 555 timer as an astable multivibrator
Given the components, you can read the frequency from the chart. Alternatively, you can use the chart to pick components for a desired frequency.
C (mF)
f (Hz)

22. 555 IC Timer Block Diagram

23. 555’s internal components
A three-resistor voltage divider
Two analog comparators
An SR latch
An inverteting buffer
A transistor that acts as a switch

24. 555’s Voltage Divider From EET 1150 (DC Circuits):
Assuming that no external voltage is connected to pin 5, then
The voltage here equals 2/3 of VCC.
The voltage here equals 1/3 of VCC.

25. 555’s Analog Comparators
From EET 2201 (Electronic Devices and Circuits):
If the voltage at a comparator’s + input is greater than the voltage at its  input, its output is HIGH.
If the voltage at a comparator’s + input is less than the voltage at its  input, its output is LOW.
Not to be confused with digital comparators from Chapter 8.

26. 555’s SR Latch
Assuming that Pin 4 is held high, then we can summarize the SR latch’s operation as follows:
S R | Q Q’ | Comments
| |
| Q0 Q0’ | No change.
| | RESET
| | SET
| | Invalid state. c)

27. 555’s Transistor From EET 2201 (Electronic Devices & Circuits):
This transistor acts as a switch.
When the voltage at its base is LOW, the transistor turns OFF, disconnecting its collector from to its emitter.
When the voltage at its base is HIGH, the transistor turns ON, connecting its collector to its emitter.

28. Five Rules of 555 Operation
Whenever Vpin 2 < 1/3 VCC, the latch is set, forcing the 555’s output (pin 3) HIGH.
Whenever Vpin 6 > 2/3 VCC, the latch is reset, forcing the 555’s output (pin 3) LOW.
Whenever neither a) nor b) is true, the latch holds its value, and the 555’s output (pin 3) is constant. 28

29. Five Rules of 555 Operation (Cont’d.)
When the 555’s output (pin 3) is LOW, pin 7 is shorted through the transistor to GROUND, which will discharge an external capacitor connected to pin 7.
When the 555’s output (pin 3) is HIGH, pin 7 is internally open, allowing the external capacitor to charge up. 29

30. 555 Connected as a One-Shot (Monostable Multivibrator)