What is that little
black arachnid
that was in your
baggie?
Monday July 14, 2014
IC 555
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IC 555
• I can calculate the charging and discharging
time for a capacitor.
• I can calculate the period and frequency for a
555 Timer Oscillator.
• I can describe how the 555 Timer Oscillator
can be used to produce temperature
information.
Monday July 14, 2014
IC 555
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What is the IC 555?
The IC 555 is an 8-pin
DIP chip (Dual-Inline package)
Integrated Circuit (IC)
that is capable of
producing accurate time
delays and/or
oscillations.
Monday July 14, 2014
IC 555
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IC 555 – Why 555?
Vcc (8)
Discharge (7)
5 kΩ
Comparator 1
Control Voltage (5)
-
Threshold Voltage (6)
+
5 kΩ
T1
Flip-Flop
RESET
Q
SET
Q
Comparator 2
Output (3)
+
-
Trigger Voltage (2)
5 kΩ
Ground (1)
Monday July 14, 2014
Reset (4)
IC 555
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4
IC 555 timer
Time Delay Mode
Oscillator Mode
• In the time delay mode, the
delay is controlled by one
external resistor and
capacitor.
Example: Turn a light on in a
delayed amount of time.
(Just turn on or off once)
• In the oscillator mode, the
frequency of oscillation are
controlled with two external
resistors and one capacitor.
Example: Can make a light
flash a specific rate.
(Can turn on and off
repeatedly)
This presentation will discuss how to use a 555 timer in the
oscillator mode.
Monday July 14, 2014
IC 555
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IC 555 timer
Oscillator mode = astable multivibrator mode
Translation: in this mode the IC 555 will continue to
put out pulses until you remove the battery.
(your choice of resistors and capacitors
determines the vibration frequency)
The 555 timer is a two state device: HI and LO
You can be a two state device too……
Monday July 14, 2014
IC 555
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IC 555 timer
Astable multivibrator mode schematic
Notice:
2 resistors and 1 capacitor
OUTPUT is square wave pulses
LESSON 15
“How the 555 Timer Works”
Monday July 14, 2014
IC 555
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Thermistors
Temperature dependent Resistors
R = resistance
T = temperature
k = temperature coefficient
What do you predict will
happen to the resistance of
the thermistor as it ascends
into the atmosphere?
Monday July 14, 2014
Two types of Thermistors
k = positive (PTC)
K = negative (NTC)
IC 555
Our resistor is an NTC
thermistor with a range of
10 kΩ – 80 kΩ
8
Experiment a bit
Experiment by using the
Thermistor in place of R2.
You may use the spray coolant to cool the
thermistor – but be careful it can get super cold!
1. How does reducing the temperature of the
thermistor change the output of the IC 555
timer?
2. What evidence do you have to support this?
Monday July 14, 2014
IC 555
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NTC thermistor
Monday July 14, 2014
IC 555
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Output = High
tHIGH : Calculations for the Oscillator’s HIGH Time
THE OUTPUT IS HIGH WHILE THE CAPACITOR
IS CHARGING THROUGH RA + RB.
t HIGH  0.693 R A  R B C
Monday July 14, 2014
IC 555
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Output = Low
tLOW : Calculations for the Oscillator’s LOW Time
5v
THE OUTPUT IS LOW WHILE THE CAPACITOR IS
DISCHARGING THROUGH RB.
3.333 v
Vc
1.666 v
0v

tLOW

HIGH
Output
LOW
t LOW  0.693R
Monday July 14, 2014
B
C
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IC 555 timer
Visit this link to view a simulation of the IC 555 in astable mode
http://www.williamson-labs.com/pu-aa-555-timer_slow.htm
How does the
charge and
discharge of
the capacitor
relate to the
blinking LED?
Monday July 14, 2014
IC 555
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Period
The Period is the total time of an
on/off cycle and depends on the
values of RA, RB, and C
t HIGH  0 . 693  R A  R B  C
t LOW  0 . 693 R B C
T  t HIGH  t LOW
Calculate the period of the flashing light.
T  0 . 693  R A  R B  C   0 . 693 R B C 
T  0 . 693  R A  2 R B  C
Monday July 14, 2014
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Frequency
The frequency of an oscillation
(or anything that exhibits a
repeating pattern) is inversely
proportional to the period
F 
1
Unit of Measure:
cycles/second = Hertz (Hz)
T
1
FCalculate

the frequency (or blinking rate) of the
0 . 693
flashing
light.R A  2 R B  C
Monday July 14, 2014
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Practice
Calculate the
period and
frequency of the
blinking LED.
μ = 10-6
Monday July 14, 2014
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Solution
Period
Frequency
T  0 . 693 R A  2 R B  C
T  0 . 693 390   2  180    6 . 8  F
T  3 . 534 mSec
F 
F 
1
T
1
3 . 534 mSec
F  282.941 Hz
Monday July 14, 2014
IC 555
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Practice
Calculate the period and frequency of the blinking
light assuming the resistance in the frequency is
10 kΩ.
Assume Elsa from frozen just walked in and room
temperature just dropped to 0 ̊C and the
resistance on the thermistor is 30 kΩ.
PREDICT – will the light blink faster or slower?
Calculate the period and frequency of the blinking
light now.
Monday July 14, 2014
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CricketSat Schematic
Monday July 14, 2014
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Key Points
The 555 timer works with 2 resistors (one of which
is temperature dependent) and 1 capacitor to
establish an oscillation in the circuit.
The 555 timer output has two modes: ON and OFF
and it turns whatever device it is connected to
ON and OFF at a rate that depends on the
changing resistance of the thermistor (which
depends on temperature)!!!
Monday July 14, 2014
IC 555
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Monday July 14, 2014
IC 555
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