Download Lab 2 Applications of the 555 Timer

Lab 2:
Applications of the 555
Timer
Physics 352L Section: 450
Friday, January 30th, 2014
Dr. Hugon
Harris Pharr
I have neither given nor received any unauthorized aid
on this assignment:
Abstract: The purpose of this lab was to investigate and analze the 555 timer and it’s
ability to create oscillation functions. The timer created a square and saw-tooth waves
and are produced by the input voltage of 5V on the blackbox and the resulting charge and
discharge from the capacitor and resistor combination. The frequency is dependent on the
values of each respective resistor and capacitor value; in the final section we sautered out
own person clock with a speed around 5Hz.
Section A: The 555 Timer IC as One-Shot (OS):
The purpose of this section of the lab was to construct and analyze the 555 timer
as a one-shot system according to the diagram below in figure 1. When the 555 timer is in
its one-shot (OS) state, the 555 acts as a monostable multivibrator in that it is off in it’s
stable position however can be triggered to switch on thus the “one shot”. Using a
EXCEL XL830L multimeter, the two 100kΩ resistors read 99.4kΩ and 99.5kΩ. As for
the 1MΩ resistor, the value on the multimeter read 1.003MΩ. The capacitance values
unfortunately were unable to be measured using the multimeter used.
The output of the OS was connected to the LED on the black box and initially the
LED would light on when you click the switch which would illuminate the LED for a
short period of time before turning it off and could be reset but flipping the switch again.
When the trigger input was temporarily grounded, the LED would remain on and the
duration of the pulse was measured to be around 1.05s which was close to the calculated
value according to the formula (also depicted in figure 1) T=1.1*R1*C1=1.1 ± 0.1s. The
reason for this was due to the trigger voltage being less than one third the input voltage of
5V and causes the circuit to become temporarily grounded. When the pulse rate and
trigger rate were connected the pulse rate was then measured and found to be 25.0ms ±
0.5ms which was measured using cursor on the oscilloscope (GWINSTEK GDS 1052U). The current drawn by our design was then calculated using Ohm’s law and was
temporarily connected to a +12V power supply in order for comparison to the online data
sheet. Using the formula below and Ohm’s laws with Icc=2.58mA, Vout=8.70V,
Vcc=+12V, VEE=-12V, and Iout=Vout./R=8.70V/4.650kΩ=1.87mA the power consumption
was calculated to be 106.8mW which was close to the data sheets consumption at 12V
being 135mW. The formula used with this data is shown below.
𝑃𝐶 = 𝐼𝑐𝑐(𝑉𝑐𝑐 − 𝑉𝑒𝑒) + 𝐼𝑜𝑢𝑡(𝑉𝑜𝑢𝑡 − 𝑉𝑒𝑒) = 106.8𝑚𝑊
When the 555 timer was first triggered for a long period and then grounded, the reset pin
was then tested which then stopped the clock impulses and made the LED remain
illuminated.
Figure 1: Circuit Diagram based off 555 Timer
Section B: The 555 Timer as an Oscillator:
In this section of the lab, a timer-based oscillator was controlled by the charging
of the capacitors by the resistors R1 and R2 displayed in figure 2 below. This complex
circuit functions by when the capacitor reaches 2/3 the Vcc voltage, the capacitor then
discharges through pin 7 due to an internal trigger switches this pin to ground. On the
other hand, when the capacitor discharges to about 1/3 Vcc, pin 7 is disconnected from the
ground and the capacitor recharges. An LED on our black box was also connected to the
output of the circuit.
Figure 2: Astable Oscillator Circuit Diagram constructed in section B
Using the same multimeter as section A, the two 100kΩ resistors measured 99.6kΩ and
99.4kΩ and the capacitor value was 100uF along with a 10pF used for pin 5 to ground as
recommended in the lab manual. Initially the voltage across the 100uF capacitor was
measured using a GWInstek GDS-1022 oscilloscope and the max voltage was found to
be 3.52V with the lower level voltage was found to be 1.80V. The approximate frequency
was calculated according to the equation below and produced a period (inverse of
frequency) of 20.6 sec .
1
= 0.69𝐶(𝑅1 + 2𝑅2)
𝑓
We then used a stopwatch to determine the frequency of the circuit by taking 3 time trials
from when the LED initially illuminated, turned off, then turned back on and produced an
average period of 24.7 ± 0.3sec which is around a 20% error in comparison to the
predicted values. The Ton and TOff of the LED was then recorded in order to calculate the
duty cycle of the circuit. Using the same method taking three trials for each TON and TOFF
the values were 13.38 ± 0.31s and 6.38 ± 0.06s. The duty cycle was then calculated using
𝑇
18.38±0.31s
the equation 𝐷𝐶 = 𝑃 ∗ 100 = 24.71±0.29s ∗ 100 = 74.4 ± 1.3%. This agreed with our
expectations due to the sum of Ton and Toff being 24.67s which was very close to our
measured period with the same percent error for the calculated value. The resistors R1
and R2 were then replaced with 1MΩ potentiometer and adjusted until the LED’s blinking
was indistinguishable from by the human eye. The potentiometers were then adjusted to
find the range of frequencyes that circuit could produce with the lowest frequency being
450Hz and the highest frequency around 13.5kHz which was close to the MultiSim
values only with the higher value in Multisim being around 14kHz. At high and low
frequencies, the output signal was similar to a DC voltage or a true or false such as
Boolean. Any frequencies past that seemed not to oscillate most likely due to the limiting
factors of the capacitors ability to charge and discharge which as it happens at a higher
frequency, the voltage becomes relatively smaller. When you apply a negative voltage to
the reset the LED turns off while a positive voltage causes the LED to remain on. The
prices of the components were then calculated in order to build 30 oscillators. The Red
LED’s cost approximately $1.69*30=$50.70 with the capacitors costing $0.62*60=$37.2.
The resistors cost $1.09 each with 60 being used so a total of $65.40 and the 555 Timer
IC costing $1.99*30=$59.70 totaling around $213.00.
Section C: Light-sensitive Oscillator
In section C we initially measured the photoresistor’s resistance value in ambient
light using an EXCEL XL830L multimeter that measured a resistance value of 336Ω in
ambient light at 8:41pm in the electronics lab. It required a few seconds to stabilize its
value depending on any movement of my hand or anyone around me however, only Jeff
Powell was also in the lab working at a station further from me so it stabilized easily.
When I placed my hand completely over the photoresistor in a cone shaped arrangement
in an attempt to block all light without touching the photoresistor, the resistance value
ranged from 10.6-10.8kΩ and was less stable as it was difficult to block all ambient light.
The charging resistor in the previous oscillator was then replaced with PDV-P8104 and
the R2 measured 99.6kΩ using the same multimeter used throughout the lab. A speaker
was then attached in order to hear the changing frequency as a response of IL and started
squealing and the pitch was able to be manipulated by altering the amount of ambient
light transmitting to the photoresistor. A video of the sounds produced was recorded and
will be publicly available on my personal website https://harrispharr.web.unc.edu within
the next week.
Section D: The 555-based saw-tooth oscillator
This section was very similar to the circuit built with the 555 timer-based
oscillator however, with a 2N3906 transistor added to the circuit which permits the
continuous charge of capacitor C2=100nF. A circuit was constructed according to figure
3 below. Using the same multimeter, the 3.3kΩ resistor measured 3.28kΩ and the 20kΩ
resistor measured 19.78kΩ with the 150Ω resistor measuring 147Ω. Using a 10X probe
on the oscilloscope, the waveform was observed across the capacitor. Since the capacitor
was being charged linearly by a constant source, it also charged linearly. In the same
manner, the resistor also acts as a linear-component which maintained a linear growth of
the sawtooth output. This continuous charge had a current of about 1mA with a voltage of
around 1.41mV which was greater than expected resulting in the saw tooth oscillations.
This negative to positive, charge to discharge rate creates this saw tooth when the voltage
across capacitor C2 was greater than 2/3 VCC then capacitor voltage falls immediately to
0. Ti create a frequency of 65*B with B=8 and thus f=520Hz, a potentiometer was
adjusted to around 510Ω which produced a frequency of around 516.Hz. By varying the
potentiometer, the allowed frequency ranged from 100Hz to about 2.44kHz with a rise
time of 1.75ms and fall time of 5.04uS. A diode was connected to pin 5 instead of ground
and forms a constant current source which charged the capacitor; when VC reached 2/3 of
Vcc the internal comparator of the 555 turns on and the capacitor discharges. When it
goes below 1/3 VCC, the comparator goes off and the capacitor stops charging. This
switch causes the sawtooth wave across the capacitor and the diode makes output voltage
0 during the discharging stage. In order to increase the maximum frequency of
oscillations by a factor of 25, one may decrease the value of Vcc by a factor of 2.5 or by
using a larger capacitor of 22.5uF to increase the frequency by 25x. There are many
applications of this circuit, one may be used to connect speakers and LEDs of
ambulances in order to synchronize to alert pedestrians and other drivers nearby.
Figure 3: Saw-Tooth oscillator constructed in section D based on 555 timer
:
Section E: 555-Based Personal Security Blanket
In this section of the lab, we solder together a personal circuit which was
constructed in section B however with components to make a frequency of around 5Hz
and a duty cycle that was less than 50%. The resistor values used in the picture before
measured 9.94kΩ for RA and RB=21.8kΩ using the same multimeter. A diode was added
across pin 7 and 6 which ultimately allowed the duty cycle to range between 40-41% at a
frequency of 4.437Hz on the oscilloscope.
Conclusion:
This lab demonstrated multiple uses of the 555 timer and its multiple functions
illuminating why some say it’s the greatest electronic component ever created. I learned
how easily frequency could be altered to create both square and saw tooth waves which
are generally more difficult to create without the 555 timer. The values in sections A,
B,C, and D were accurate however not always precise which probably has to do with the
capacitance and resistance values due to the oscilloscope and probe.
Appendix:
MultiSim Diagrams:
Section A/B:
Section D:
Section E:
Sawtooth Waveform: