Transcript EE311: Junior EE Lab 555 Timer

EE311: Junior EE Lab
555 Timer
J. Carroll
8/26/03
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
Background
• 555 timer IC
– a SR latch
– two analog
comparators
– an totem pole
output stage
– a discharge
transistor
– a voltage divider
network
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
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
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
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
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
– latch is reset,
Q=0,~Q=1
– output goes LOW,
PW=1.1 R C
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)
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.
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 0.001 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
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
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
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