The 555 Timer
(Process)
0V
1
Trigger
2
Output
3
Reset
4
555
The 555 Timer is a highly versatile integrated circuit (chip) that can be used to produce
time delays and oscillators. We will look at the Astable and Monostable circuits.
8
VCC
7
Discharge
6
Threshold
5
Control Voltage
555 IC
Inside the chip
Astable Circuit
An astable circuit continually switches the output on and off at a rate dependent on the
timing components (R1, R2 and C)
Its operation works on the principle that it takes a capacitor a certain amount of time to
charge up when a source of current is applied. The resistors limit the amount of current
that is allowed to flow into the capacitor and therefore the speed it is allowed to charge
up (the larger the resistor value the longer it takes for the capacitor to charge up.) Also,
increasing the capacitance of the capacitor increases the time it takes to charge up.
+V
R1
2
Output
3
8
555
1
4
7
6
5
R2
C
0V
Frequency
Calculating On and Off times
TON
Frequency is the number of pulses that occur in
one second. For example, if the output was
connected to an LED and it flashed 10 times in 1
second we would say it had a frequency of
10Hertz (or 10Hz)
+V
0V
TOFF
As we know the frequency is dependant on the
resistors and the capacitors in the circuit, the
formula below allows you to calculate the
frequency of the output signal:
Off time (TOFF) = 0.693 x R2 x C
On timer (TON) = 0.693 x (R1+R2) x C
1
F = 0.693 x (R1+2 x R2) x C
EJC
Prototyping a 555 Astable Circuit
(Process)
The circuit below makes the LED flash at a fixed rate. Plan where you are going to put
the components first before using the prototyping board.
+V
R1
(47K)
2
3
4
8
555
1
7
6
R2
(47K)
5
R3
(330R)
C
(10μF)
0V
Exercise:
1.
Count how many flashes occur one
ten seconds?
2.
Calculate the frequency.
3.
Calculate the On Time.
4.
Calculate the Off Time.
Answers:
EJC
Monostable Circuit
(Process)
The monostable circuit allows you to produce accurate time delays. In its normal state
the output is turned off. But when it receives a trigger pulse the output is switched on
for a set period depending on the values of R1 and C.
+V
R1
1
2
3
4
555
10K
8
7
6
5
Output
C
0V
+V
Pin 2 Voltage
(Trigger)
Trigger pulse
0V
+V
Pin 3 Voltage
(Output)
0V
Output pulse (T seconds)
Once the circuit is triggered the period of the output pulse is equal to:
T = 1.1 x R X C (Seconds)
EJC
Prototyping a 555 Monostable Circuit
(Process)
The circuit shown below will make a buzzer sound for a period set by the timing
components R1 and C one the trigger button has been pressed. Plan where you are
going to put the components first before using the prototyping board.
+V
R1
(220k)
1
2
3
4
555
10K
8
7
6
5
C
(100μF)
0V
Exercise:
1.
Once triggered, how long does the
buzzer sound for?
2.
Calculate the time you would
expect the buzzer to sound using
the given formula.
Answers:
EJC