The 555 Timer IC (and the related 556 IC)

 

The 555 timer integrated circuit is one of the most popular and useful integrated circuits of all time.  It has been used in hundreds or thousands of applications, for example to:

 

· Generate a single pulse

· Debounce a switch – i.e. create a clean pulse when a “noisy” switch is pressed

· Generate a time delay

· Measure time elapsed between events

· Measure time duration of events

· Measure frequency of a pulse train

· Generate a pulse train of specified duty cycle and frequency

· Create a variable-frequency train of pulses

· Create a fixed-frequency train of pulses having varying width (called pulse width modulation)

· Create a fixed-frequency train of pulses having varying position (called pulse position modulation)

· Create a pulse train of half the frequency of the incoming pulse train (frequency division)

· Generate a voltage which rises linearly (a linear ramp)

· Generate a sawtooth voltage (repetitive linear ramps)

· Detect a condition (e.g. light on or off, switch open or closed, etc.) and sound an alarm

· Generate interesting sounds (sirens, two-tone alternation, dial tone, busy signal, etc.)

 

The timing intervals available in such applications can be as short as microseconds or as long as months!

 

If you use a web search engine such as Google and type in “555 timer” you will be led to hundreds of sites where this IC is used in these and other applications.  The 555 specs are available on the specs page.

 

A lengthy tutorial here on the 555 timer and how to use it would be a duplication of effort since at  http://www.uoguelph.ca/~antoon/gadgets/555.htm  there an excellent tutorial, which you should study to learn how this IC works.  The elegant graphics on the page you are reading were lifted from that tutorial.  You don’t need to understand all of the electronic details in the tutorial in order to get the benefit from this piece, although you should know that:

 

· A comparator is a device which compares two voltages and creates a “high” voltage or a “low” voltage depending upon which of the two being compared is larger.  The input voltages are analog (continuous-valued) but the comparator output is digital (high or low).

· A flip flop is a memory device which stores one bit of information.  It has digital inputs and output.

· The convention used in the tutorial for labeling resistors is that the “K” is placed inside the value rather than after it.  For example what we would write as 4.7K is written there as 4K7.  The letter is placed where a comma would be if the value were written out (as 4,700 Ohms for example), in order to avoid using decimal points.

· The transistors in the 555 are all used as simple on-off switches, behaving pretty much the same way a MOSFET would in the same places.

· The positive power supply is referred to as Vcc.

 

In our circuits lab #4 we will be using two timers.  In the lab you can use two 555  ICs, but you can also use the 556 IC which contains two timers in one package, to save space and a little wiring.  Click here for the 556 pinouts.

 

All of the timing calculations for circuits using the 555 timer are based on the response of a series R-C circuit with a step or constant voltage input, and exponential output taken across the capacitor .  You should understand that standard response very well in order to understand how to use the timer.

The two basic modes of operation of the timer are (1) monostable operation, in which the timer wakes up and generates a single pulse, then goes back to sleep, and (2) astable operation, in which the timer is trapped in an endless cycle – generate a pulse, sleep, generate a pulse, sleep,… on and on forever.

 

The control voltage, usually but not always 2/3 of the power supply voltage, determines the limits of a capacitor voltage in the operations described below.

 

The monostable (one-pulse) operation can be understood as consisting of these events in sequence: 

 

0. (up to t = 0) A closed switch keeps the C uncharged: Vc = 0, Vout is low.

1. (at t = 0) A triggering event occurs: Vtrigger drops below Vcontrol/2, very briefly.  This causes the switch to open.

2. (0 < t < t1) Vc(t) rises exponentially toward Vcc with time constant RC.  Vout is high.

3. (at t = t1) Vc reaches Vcontrol.  This causes the switch to close, which instantly discharges the C.

4. (from t = t1 on) A closed switch keeps the C uncharged: Vc = 0, Vout is low.

 

Monostable operation showing current flows, pin numbers, charging, and output

 

The astable (pulse train) operation can be understood as consisting of these events, starting at a point where Vc = Vcontrol/2:

 

1. (at t = 0) Vc = Vcontrol/2, and the switch opens.

2. (0 < t < t1) Vc(t) rises exponentially toward Vcc with time constant (R1+R2)C.  Vout is high.

3. (at t = t1) Vc reaches Vcontrol.  This causes the switch to close.

4. (t1 < t < t1 + t2) Vc(t) falls exponentially toward zero with time constant R2C.  Vout is low.

5. (at t = t1 + t2 = T) Vc reaches Vcontrol/2.  This causes the switch to open.  These conditions are the same as at step 1, so the cycle repeats every T seconds.  (Go to step 2.)

 

Astable operation, showing currents, pin numbers, charge/discharge cycle, and output

 

Notice that in both modes, the output is high only when the C is charging.

 

Other details:

· There is a reset pin (#4); whenever the voltage on that pin is below 0.7 volts, the output is forced low.  This pin is usually connected to Vcc to prevent accidental resets.

· The negative trigger pulse used in the monostable mode (step #1) must rise back to a voltage higher than Vcontrol/2 before the monostable pulse ends, or another pulse will be generated.

· Usually, Vcontrol is obtained internally from a voltage divider in the 555 and is constant at (2/3)Vcc.  In this case a capacitor is connected between pin 5 and ground, to keep Vcontrol at that voltage.  The Vc(t) in this case ranges between the limits of (1/3)Vcc and (2/3)Vcc.

· The Vcc and Ground pins are often connected by a 0.01uF capacitor (a “despiking” capacitor) to avoid problems from imperfect power supplies and circuit wiring.

· The Vcontrol can be usefully varied from about 2 volts to about 1 volt less than Vcc,  but  not much outside of that range.

· The discharge of the C in monostable mode is not truly instantaneous but requires a few microseconds.  Because of this and other factors, the 555 cannot reliably generate pulses shorter than about 10 microseconds, and cannot generate a pulse train with a frequency higher than about 100 KHz.

· The resistors R1 and R2 in the circuits shown must be greater than 1K and smaller than 3.3Megohm.

· The capacitor C in the circuits shown must be greater than 0.001 uF, or 1 nF.