UNIVERSITY OF MASSACHUSETTS DARTMOUTH
DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
ECE 201
CIRCUIT THEORY I
INTRODUCTION TO THE 555 INTEGRATED CIRCUIT TIMER
BACKGROUND
The 555 integrated circuit timer is a device used to generate accurate time delays or
oscillations. It is packaged in an 8-pin MINI-DIP with the pin connections shown in Figure 1.
Figure 1. Package and pin outline for the 555 IC timer
“Under the hood” of the 555 timer are more than 20 transistors, 15 resistors, and 2
diodes. However, at this point, it is not necessary to understand how any of these individual
devices operate in order to make simple use of the 555 timer!
It turns out that we can think of the 555 as a SPDT switch that provides an output which
is either a DC voltage (VCC) or 0 volts (GROUND). The position of the switch is determined by
the voltage at pin #6 (THRESHOLD). This is illustrated in Figure 2.
Figure 2. A simplified representation of the 555 timer
If the voltage at pin #6 is 2/3VCC, the switch is connected to GROUND, and the OUTPUT
at pin #3 is 0 volts (LOW). If the voltage at pin #6 is 1/3VCC, the switch is connected to VCC, and
the OUTPUT at pin#3 is VCC volts (HIGH). The voltage across a capacitor in a simple RC series
circuit will be applied to the THRESHOLD terminal (pin #6) of the 555 to control the state of the
OUTPUT.
TIME-DELAY CIRCUIT
Generally, we think of activating a device or a circuit by closing a switch. Consider the
case of “arming” an alarm in your car or home. It is beneficial to provide a time-delay which
allows the person arming the system to leave the vehicle or premises without setting off the
alarm.
In the circuit of Figure 3, a 555 timer is used to close a relay which provides power to a
load (simulated by a buzzer) after a certain length of time (the time-delay).
OnOff
EDR201A12
Key = S
12VRelay
RV1
8
Key = A
100kOhm
Prim aryPow er
12 V
U1
VCC
50%
4
RST
7
DIS
6
THR
2
TRI
5
CON
K
OUT 3
Buzzer
GND
LM555CN
1
LoadPow er
12 V
200 Hz
C
1uF
Figure 3. A time-delay circuit using the 555 timer
The circuit operates as follows. When the On/Off switch is closed, the initially uncharged
capacitor C will charge towards the 12 volts provided by the Primary Power battery via the
potentiometer RV1. The OUTPUT of the 555 will be HIGH (12 volts) and the relay coil will not be
energized until the voltage across the capacitor is equal to 8 volts (2/3 of 12 volts). This will occur
at approximately 1.1RC seconds (the time-delay) after the switch is closed.
The 555 OUTPUT will change to LOW (0 volts) and the relay will be energized (the 12
volt Primary Power battery will now be connected across the coil terminals). The 12 volt Load
Power battery will now be connected to the buzzer via the closed relay contact.
The time-delay is adjustable by changing the value of either the potentiometer RV1 or the
capacitor C (or both).
2
PRELIMINARY WORK /DESIGN
Design a time delay circuit using a 555 integrated circuit that will activate (supply 12 volts
DC to) a burglar alarm by closing a relay 15 seconds after a key switch is closed. Prior to key
switch closure, illuminate a Green LED to indicate that the system is ready to be armed. When
the relay closes, turn the Green LED off and illuminate a Red LED to indicate that the system is
armed. Use two separate 12 volt power supplies to operate the 555 and the alarm load. Use a 1
kΩ resistor to represent the loading of the alarm circuitry. Include current-limiting resistors to
protect the LEDs.
Simulate your design in MultiSim and make any modifications that you think are
necessary.
LABORATORY PROCEDURE / RESULTS
Construct the time delay circuit that you designed and simulated in MultiSim. Check that
the circuit operates the relay in 15 seconds (more or less). When you are satisfied that your final
circuit meets the design specifications, show it to the instructor or TA for their approval.
If time permits, investigate the possibility of making the delay time variable from 5 to 30
seconds.
3