1. EE311: Junior EE Lab 555 Timer
J. Carroll
8/26/03

2. Background The 555 timer can operate as
a monostable multivibrator, i.e., a ``one-shot’
an astable multivibrator, i.e., an oscillator
behavior dependent on external R, C values
Various analog and digital applications
voltage ramp generator, missing pulse detector, pulse width modulator
power supply in the range of 5-18 volts
with 5-volt supply, compatible with TTL or CMOS digital devices

3. Background 555 timer IC a SR latch two analog comparators
an totem pole output stage
a discharge transistor
a voltage divider network

4. Set-Reset (SR) Latch Operation
SR latch has has two stable states
State 1  latch is set, i.e., Q=1,~Q=0
State 2  latch=reset, i.e., Q=0,~Q=1
note: circuit can remain in (hold) either state
Trigger pulses can be applied to either S or R inputs to change the latch state
S=1 (high) “sets” the latch , i.e., Q=1 (high)
R=1 (high) “resets” the latch , i.e., Q=0 (low)
note: the first input to go high has priority
Timer output tracks Q  output high when Q=1

5. Role of the Analog Comparators
A comparator compares two input voltages and determines which is larger
if V+>V-, the comparator output is high or “1”
if V->=V+, the comparator output is low or “0”
The 555 timer uses two comparators to generate the S and R inputs to the latch
a voltage divider network of three equal valued resistors is used to generate V+=Vcc/3 for COMP1 and V-=2Vcc/3 for COMP2
COMP2 compares the THRESHOLD to 2Vcc/3
COMP1 compares the TRIGGER to Vcc/3

6. Role of the Analog Comparators
A comparator compares two input voltages and determines which is larger
if V+>V-, the comparator output is high or “1”
if V->=V+, the comparator output is low or “0”
The 555 Timer uses two comparators to generate S (COMP1) and R (COMP2) inputs to the latch
a voltage divider network of three equal valued resistors is used to generate V+=Vcc/3 for COMP1 and V-=2Vcc/3 for COMP2
COMP1 compares the TRIGGER (Pin 2) to Vcc/3
COMP2 compares the THRESHOLD (Pin 6) to 2Vcc/3

7. Role of the Discharge Transistor
A discharge transistor is used to clamp an external capacitor (Pin 7) to ground
the transistor is ON (conducting) when ~Q=1, discharging the capacitor at a rate determined by the associated time constant, i.e., R and C
when ~Q=0 the transistor is OFF (open) leaving the capacitor unclamped, i.e., holding its voltage

8. Monostable (One-shot) Circuit
At power up
latch is reset, Q=0,~Q=1
output is LOW
Pin 2 goes LOW
latch is set, Q=1,~Q=0
output goes high
Cx charges to Vcc, with t=RxCx
Pin 6 goes HIGH
output goes LOW, PW=1.1 RxCx

9. Astable (Oscillator) Circuit
Two quasi stable states are possible
latch is set, Q=1,~Q=0 
PW1=0.695(RA+RB)CX
latch is reset, Q=0,~Q=1 
PW2=0.695RBCX
output frequency
f = 1/(PW1+PW2)
= 1.439/((RA+2RB)CX)
waveform duty cycle
D = PW1 /(PW1+PW2)
= (RA+RB)/(RA+2RB)

10. Preliminary Lab Questions
1. Lookup up and record the physical location of the pins on the 555 timer chip
2. The 555 timer is designed so that the timing characteristics, e.g., frequency, pulse width and duty cycle, are independent of the power supply voltage
justify this statement theoretically, e.g., using circuit analysis, etc.

11. Preliminary Lab Questions
3. Experiment with the demo VI's associated with the LabVIEW 555_Monostable.llb library
based on these applications, describe in detail how a 555 timer IC could be used to create an inexpensive meter to measure an unknown capacitance in the range to 1000uF
4. For step 1 of the Procedure, calculate the expected time that the LED will remain lit after the switch is released when pin 4 is connected to Vcc

12. Preliminary Lab Questions
5. Experiment with the demo VI's associated with the LabVIEW 555_Astable.llb library
observe the timer output waveform and duty cycle when RA>RB, RA=RB, and RA<RB
based on these applications, describe in detail how a 555 timer and a thermistor could be used to create an inexpensive temperature transducer
6. For step 2 of the Procedure, calculate the expected frequency and duty cycle of the output at pin 3

13. Preliminary Lab Questions
7. For step 3 of the Procedure, use PSpice to show the expected waveforms at pins 2 and 3 of the 555 timer chip when Vcc=12V
determine the frequency, duty cycle and the amplitude of the output at pin 3
repeat the above with Vcc=6V

14. Discussion of the Lab Procedures
Bonus points will be awarded on the lab report to those groups that implement and document the required lab measurements using LabVIEW