1. BE Lesson 8: Using the 555 IC timer chip .
What is a 555 integrated circuit (IC) timer?
Make a blinking LED circuit using a 555 timer.
Make a railroad signal light using a second LED.
Vary the oscillation period.
Make an English police siren.
Make an electronic canary.
© 2012 C. Rightmyer, Licensed under The MIT OSI License, 20 July 2012 1

2. The 555 timer IC (integrated circuit)
Control voltage 5 4
Reset
Threshold 6 3
Output
555 timer IC
Discharge 7 2
Trigger
schematic symbol
This is our first lesson using an integrated circuit (IC). Manufacturers are constantly developing new ICs that are designed to perform one or more circuit functions really well. The “555 timer” was one of the first ICs developed and it is still one of the most often used. The 555 IC can be used in four different modes: (1) astable, (2) monostable, (3) bistable, and (4) buffer. In this lesson, we will experiment with circuits that operate only in the in the “astable” mode. In this mode, the 555 generates an oscillating square-wave output. The period of each square-wave cycle is determined by the input voltages from our host circuit. If you would like to learn about the other 555 modes of operation, read the first three pages of the following website: [
Most 555 ICs have 8 pins. As it happens, these pins fit nicely into the holes of our breadboard. They must, however, be installed across the center valley on the breadboard. See if you can figure out why that is so. Each of the eight pins have a name. The battery terminals are hooked to pins 1 and 8. The (+) terminal goes to pin 8 (Vcc), and the (-) terminal goes to pin 1 (GND).
Input pins 2 and 6 are complementary. Pin 2 is called the trigger input. When the voltage level on this pin is less than 1/3rd of the supply voltage, the pin 3 output is turned off (connected to ground). When the voltage at pin 2 is greater than 1/3rd but less than 2/3rd the supply voltage, the pin 3 output pin is turned on (high). Pin 6 is called the threshold input. When the voltage at pin 6 exceeds 2/3rd the supply voltage, output voltage at pin 3 is again turned off. When Pin 3 is off (low), it can sink a current from the load. When pin 3 is on (high) it can source a load. Pins 4 (reset) and 5 (control) are used with the other operating modes.. In the astable operating mode, Pin 4 is usually connected to the positive supply voltage (along with pin 8). Also, pin 5 is usually shorted to ground through a capacitor (to avoid the possibility that noise spikes might trigger pin 5). 8 1
Vcc
GND
physical appearance 2

3. BE ckt 8-1. Using IC 555 to blink an LED
6.8K W R1 5 4
LED
555
220 W 6 3 + R3
9 volts
16K W R2 on
off
Pin 3 voltage observed
using an oscilloscope 7 2 8 1
We have used resistors and capacitors several times now to generate an oscillating sine wave. In this experiment, the frequency of the sine wave depends on the combination of R1 and R2, and C1.
Here’s how the circuit works. Note the trigger and threshold pins (2 and 6) are connected together. Both pins are reacting to the same voltage that appears at the node in the circuit between R2 and C1. When the battery voltage is first connected, C1 begins to charge up C1 through R1 and R2. At the point when C1 is charged to about 3 volts (about 1/3s of the 9 volt battery), the 555 enables pin 3. That is, pin 3 (output voltage) goes high (9 volts), and in this state, the LED is turns off. (This is because Pin 3 is now at the same voltage level as the (+) terminal of the battery.) The capacitor continues to charge and at the point when the C1 voltage is above 6 volts (2/3rd of supply), pin 3 is returned off (low), and therefore the LED turns on.
For those situations when pin 3 is turned off (low), the 555 connects pin 3 to pin 7, which allows C1 to begin to discharging through R2. This discharging continues until the C1 voltage returns to 3 volts (1/3rd of the 9 volt battery). At this point, the charging and discharging cycle repeats (indefinitely). +
10 m F C1 [ 3

4. Hookup diagram for ckt 8-1
220
LED1 + +
9.0
volts 8 7 6 5
16K
555 1 2 3 4
Here’s how I hooked up circuit 8-1 on the breadboard. Note that the long lead of LED1 connects to the bottom of the 220 ohm resistor. +
10 m F 4

5. BE ckt 8-2. Adding a second LED to ckt 8-1
555 1 2 3 4 5 6 7 8
6.8K W R1
LED1
220 W + R3
9 volts
16K W R2
220 W R4
This circuit is a slight modification of the circuit It simulates the operation of a railroad track warning light. The only difference between this circuit and circuit 8-1 is that we have added another LED to the output (pin 3). From an electronics perspective, we need to know that the output pin (pin 3) can either source or sink a load. That is, when pin 3 is high, it is providing 5 volts DC to the LED (LED #2 in this case). When pin 3 is low, it is providing ground (0 volts) to the connected component (LED #1 in this case). In this situation, the battery provides the 5 volts, and the pin sinks this voltage.
In other words, note that since LED 1 is connected in a forward direction from the (+) battery terminal to pin 3. When pin 3 is off (low), it is in effect connected to ground, and LED 1 turns on. We say that pin 3 is sinking the load.
Also, note that since LED 2 is connected from pin 3 to ground, when pin 3 is high (5 volts presented to the load). (This is called sourcing the output.) Therefore, when pin 3 is high, LED 2 is forward biased from pin 3 the (-) battery terminal (ground), and therefore LED 2 turns on..
Since the output pin becomes alternately on and off during each cycle, the LEDs flash alternately on and off in synch with the timer’s square wave cycle. +
10 m F C1
LED2 5

6. Hookup diagram for ckt 8-2
220 + +
LED1
9.0
volts 8 7 6 5
16K
555
LED2 1 2 3 4 +
Here’s how I hooked up circuit 8-1 on the breadboard. +
10 m F
220 6

7. BE ckt 8-3. Change the flashing cycle for ckt 8-2
555 1 2 3 4 5 6 7 8 R1
LED1 1K R2
220 +
100K pot R3
9 volts R3
220 R4
This circuit is a simple modification of the earlier railroad flashing light circuit such that the flashing rate can be varied. We have replaced the R2 fixed resistor with a smaller R2 resistor in series with a variable R3 resistor (a 100K pot). By adjusting the pot’s resistance value from 0K to 100K, we cause the timer’s output cycle to slow down.
If we start up the circuit when the pot’s resistance is set at zero, the newly installed R2, with its fixed resistance lessor in value than the R2 resistance in circuit 8-2, the timer cycle responds to a smaller RC time constant by speed up the square-wave cycle.
So, if you need a variable clocking input for one of your own experiments, you now know how to achieve that goal. +
10 m F C1
LED2 7

8. Hookup diagram for ckt 8-3
220
555 + 1K
LED1
9.0
volts 8 7 6 5
LED2 1 2 3 4
100 k pot
Here’s how I hooked up circuit 8-3 on the breadboard. + R4 C1
10 m F
220 8

9. BE ckt 8-4. An English police siren10 R1
555 1 2 3 4 5 6 7 8 + NO
9 volts R2
120K
470K R4 R3
220 C
This experiment illustrates a new idea, and reinforces another idea that you have already heard about. The new idea involves using the 555 timer as a tone generator. The second idea, which you may remember, involves placing two resistors in parallel.
When we route the 555’s oscillating square wave output through a transistor to a speaker, we will hear a very distinct single-frequency tone. The 555 timer helps to insure that the period of each cycle is pretty much constant.
When you push the normally open switch, resistors R2 and R3 become connected in parallel. You might remember that when two resistors are connected in parallel, the total resistance becomes equal to the product of the two resistances divided by the sum of the two resistances. In this case, we have:
Rparallel = (470K * 120K) / (470k + 120K) = (470 * 120) K / 590 = 96K ohms
So, the resulting 96K ohm parallel resistor, being smaller than the origial 120K ohm resistor, will form a smaller RC time constant, which in turn, causes the speaker tone to increase in pitch. If you alternately push and release the switch, you will hear two alternating tones that sound like an English police car’s siren. The me, the siren sounds like: neee….naww….neee….naww ...
When I visited Croatia recently, their emergency vehicles made a sound much like this circuit. But, to me and my traveling party, the sound was interpreted as “peevo, peevo, peevo”…..the Croatian word for beer. So, we drank a lot of beer. + B
3904 E
0.01 m F C1 9

10. Hook up diagram for ckt 8-410 + 1 2 3 4 5 6 7 8
9.0
volts
470K
120K
555
3904 b c e
Here’s how I hooked up circuit 8-4 on the breadboard. +
220
0.01 m F 10

11. BE ckt 8-5. A chirping canary?
6.8KW R1 R5
3.3K W
555 1 2 3 4 5 6 7 8 C3 R3 C R7 B
.1 mF
3906
33K W
47 W R2 B C E E
3904 R6
33KW
100K W
pot + C1
One last experiment mostly for fun. Not all students will be quick enough to get to this circuit. My intent here is to provide an opportunity for the students to complete a more complex experiment while the slower students have time to chance to catch up.
Explaining how this circuit works is difficult. There are some similarities with previous circuits. For instance, the right hand part of this experiment is similar to the first tone generation circuit. If you look closely, you will notice that pins 2, 6, and 7 are connected as they were before. The resistance and capacitance values have changed somewhat.
One obvious difference to this circuit is the parallel RC circuit that connects from pin 3 of the 555 timer. Pin 3 is the output pin, to the speaker. What’s happening here is that this RC circuit causes a second sine wave to be added to the tone that is input to the 555 timer. Later, when you take more advance classes in electronics, you will learn that the second sine wave is modulating the first sine wave
There is one more aspect to this circuit worth mentioning. In the four previous experiments, the 555 timer has been configured to provide a continuously repeating square wave. In this circuit, the 555 is configured to generate a single pulse for each push of the normally open switch S1.
By randomly pushing S1, and by varying the 100K pot, the circuit will sound somewhat like the chirping of canary bird. +
10 m F
1K W +
9 volts R4 C2
0.01 m F NO 11

12. Hook up diagram for ckt 8-5b c e
100K pot
3906 +
9.0
volts 8 7 6 5
3904
33K
33K 47 b c e 1 2 3 4
.1 mF +
Here’s how I hooked up circuit 8-5 on the breadboard. I think that the easiest way to construct this circuit is to imagine dividing the breadboard into four quadrants from left to right. At the middle of each of the first two quadrants, insert the two transistors. Note that the 3904 transistor faces down, while the 3906 transistor faces up. Then, place the 555 timer in the middle of the third quadrant. Note the circular mark at the bottom left of the IC circuit. Finally, connect the speaker wires and associated RC circuit components in the last quadrant. Finish up by adding all the other components and hookup wires .
(Note that this circuit will not make a sound at all until you push the NO switch.)
That’s it for this lesson. You have now been exposed to the often used 555 timer IC circuit. Almost every book on electronic experiments have additional experiments using the 555 timer. There are a couple of good books written by Forest Mims that are available at some of the Radio Shack stores. Check them out.
10 m F + 1K
.01 m F NO 12