Actual pin arrangements (below)
| There is more information about 555 timers and their circuits on the Electronics in Meccano website. |
Next Page: Counting Circuits
Also See: ICs (chips) | Capacitance |
AC, DC and Electrical Signals
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| Example circuit symbol (above) Actual pin arrangements (below) | |
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A popular version is the NE555 and this is suitable in most cases where a '555 timer' is specified. The 556 is a dual version of the 555 housed in a 14-pin package, the two timers (A and B) share the same power supply pins. The circuit diagrams on this page show a 555, but they could all be adapted to use one half of a 556.
Low power versions of the 555 are made, such as the ICM7555, but these should only be used when specified (to increase battery life) because their maximum output current of about 20mA (with a 9V supply) is too low for many standard 555 circuits. The ICM7555 has the same pin arrangement as a standard 555.
The circuit symbol for a 555 (and 556) is a box with the pins arranged to suit the circuit diagram: for example 555 pin 8 at the top for the +Vs supply, 555 pin 3 output on the right. Usually just the pin numbers are used and they are not labelled with their function.
The 555 and 556 can be used with a supply voltage (Vs) in the range 4.5 to 15V (18V absolute maximum).
Standard 555 and 556 ICs create a significant 'glitch' on the supply when their output changes state. This is rarely a problem in simple circuits with no other ICs, but in more complex circuits a smoothing capacitor (eg 100µF) should be connected across the +Vs and 0V supply near the 555 or 556.
The input and output pin functions are described briefly below and there are fuller explanations covering the various circuits:
Trigger input: when < 1/3 Vs ('active low')
this makes the output high (+Vs).
It monitors the discharging of the timing capacitor in an astable circuit.
It has a high input impedance > 2M
.
Threshold input: when > 2/3 Vs ('active high')
this makes the output low (0V)*.
It monitors the charging of the timing capacitor in astable and monostable circuits.
It has a high input impedance > 10M.
* providing the trigger input is > 1/3 Vs,
otherwise the trigger input will override the threshold input and hold the output high (+Vs).
Reset input: when less than about 0.7V ('active low') this makes the output low (0V),
overriding other inputs. When not required it should be connected to +Vs.
It has an input impedance of about 10k.
Control input: this can be used to adjust the threshold voltage which is set internally to be 2/3 Vs. Usually this function is not required and the control input is connected to 0V with a 0.01µF capacitor to eliminate electrical noise. It can be left unconnected if noise is not a problem.
The discharge pin is not an input, but it is listed here for convenience.
It is connected to 0V when the timer output is low and is used to discharge the timing
capacitor in astable and monostable circuits.
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To switch larger currents you can connect a transistor.
The ability to both sink and source current means that two devices can be connected to the output so that one is on when the output is low and the other is on when the output is high. The top diagram shows two LEDs connected in this way. This arrangement is used in the Level Crossing project to make the red LEDs flash alternately.
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may be connected to the output of a 555 or 556 astable circuit but a capacitor (about
100µF) must be connected in series. The output is equivalent to a steady DC of about
½Vs combined with a square wave AC (audio) signal. The capacitor blocks the DC, but
allows the AC to pass as explained in capacitor coupling.
Piezo transducers may be connected directly to the output and do not require a capacitor in series.
However, the 555 and 556 require an extra diode connected
in series with the coil to ensure that a small 'glitch' cannot be fed back into the IC.
Without this extra diode monostable circuits may re-trigger themselves as the coil is
switched off! The coil current passes through the extra diode so it must be a 1N4001 or
similar rectifier diode capable of passing the current, a signal diode such as a 1N4148
is usually not suitable.
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| 555 astable output, a square wave (Tm and Ts may be different) |
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| 555 astable circuit |
The time period (T) of the square wave is the time for one complete cycle, but it
is usually better to consider frequency (f) which is the number of cycles per second.
| T = 0.7 × (R1 + 2R2) × C1 and f = | 1.4 |
| (R1 + 2R2) × C1 |
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The time period can be split into two parts: T = Tm + Ts
Mark time (output high): Tm = 0.7 × (R1 + R2) × C1
Space time (output low): Ts = 0.7 × R2 × C1
Many circuits require Tm and Ts to be almost equal; this is achieved if R2 is much larger than R1.
For a standard astable circuit Tm cannot be less than Ts, but this is not too restricting because
the output can both sink and source current. For example an LED can be made to flash briefly with
long gaps by connecting it (with its resistor) between +Vs and the output. This way the LED is on
during Ts, so brief flashes are achieved with R1 larger than R2, making Ts short and Tm long.
If Tm must be less than Ts a diode can be added to the circuit as explained under
duty cycle below.
| 555 astable frequencies | |||
| C1 | R2 = 10k R1 = 1k |
R2 = 100k R1 = 10k |
R2 = 1M R1 = 100k |
| 0.001µF | 68kHz | 6.8kHz | 680Hz |
| 0.01µF | 6.8kHz | 680Hz | 68Hz |
| 0.1µF | 680Hz | 68Hz | 6.8Hz |
| 1µF | 68Hz | 6.8Hz | 0.68Hz |
| 10µF | 6.8Hz | 0.68Hz (41 per min.) | 0.068Hz (4 per min.) |
to 1M.
It is best to choose C1 first because capacitors are available in just a few values.
| R2 = | 0.7 |
| f × C1 |
min.)
unless you want the mark time Tm to be significantly longer than the space time Ts.
in series
With the output high (+Vs) the capacitor C1 is charged by current flowing through R1 and R2.
The threshold and trigger inputs monitor the capacitor voltage and when it reaches 2/3Vs
(threshold voltage) the output becomes low and the discharge pin is connected to 0V.
The capacitor now discharges with current flowing through R2 into the discharge pin. When the voltage falls to 1/3Vs (trigger voltage) the output becomes high again and the discharge pin is disconnected, allowing the capacitor to start charging again.
This cycle repeats continuously unless the reset input is connected to 0V which forces the output low while reset is 0V.
An astable can be used to provide the clock signal for circuits such as counters.
A low frequency astable (< 10Hz) can be used to flash an LED on and off, higher frequency flashes are too fast to be seen clearly. Driving a loudspeaker or piezo transducer with a low frequency of less than 20Hz will produce a series of 'clicks' (one for each low/high transition) and this can be used to make a simple metronome.
An audio frequency astable (20Hz to 20kHz) can be used to produce a sound from
a loudspeaker or piezo transducer. The sound is suitable for buzzes and beeps.
The natural (resonant) frequency of most piezo transducers is about 3kHz and this will
make them produce a particularly loud sound.
For a standard 555/556 astable circuit the mark time (Tm) must be greater than the space time (Ts), so the duty cycle must be at least 50%:
| Duty cycle = | Tm | = | R1 + R2 |
| Tm + Ts | R1 + 2R2 |
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| 555 astable circuit with diode across R2 |
Tm = 0.7 × R1 × C1 (ignoring 0.7V across diode)
Ts = 0.7 × R2 × C1 (unchanged)
| Duty cycle with diode = | Tm | = | R1 |
| Tm + Ts | R1 + R2 |
Use a signal diode such as 1N4148.
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| 555 monostable output, a single pulse |
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| 555 monostable circuit with manual trigger |
The duration of the pulse is called the time period (T) and this is determined by resistor R1 and capacitor C1:
| time period, T = 1.1 × R1 × C1 |
T = time period in seconds (s)
R1 = resistance in ohms ()
C1 = capacitance in farads (F)
The maximum reliable time period is about 10 minutes.
Why 1.1? The capacitor charges to 2/3 = 67% so it is a bit longer than the time constant (R1 × C1) which is the time taken to charge to 63%.
to 1M, so use a fixed resistor of at
least 1k in series if R1 is variable.
The timing period is triggered (started) when the trigger input (555 pin 2) is less than
1/3 Vs, this makes the output high (+Vs) and the capacitor C1 starts
to charge through resistor R1. Once the time period has started further trigger pulses are ignored.
The threshold input (555 pin 6) monitors the voltage across C1 and when this reaches 2/3 Vs the time period is over and the output becomes low. At the same time discharge (555 pin 7) is connected to 0V, discharging the capacitor ready for the next trigger.
The reset input (555 pin 4) overrides all other inputs and the timing may be cancelled
at any time by connecting reset to 0V, this instantly makes the output low and discharges the
capacitor. If the reset function is not required the reset pin should be connected to +Vs.
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| Power-on reset or trigger circuit |
The capacitor takes a short time to charge, briefly holding the input close to 0V when the circuit is switched on. A switch may be connected in parallel with the capacitor if manual operation is also required.
This arrangement is used for the trigger in the Timer Project.
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| edge-triggering circuit |
The monostable can be made edge triggered, responding only to changes of an input signal, by connecting the trigger signal through a capacitor to the trigger input. The capacitor passes sudden changes (AC) but blocks a constant (DC) signal. For further information please see the page on capacitance. The circuit is 'negative edge triggered' because it responds to a sudden fall in the input signal.
The resistor between the trigger (555 pin 2) and +Vs ensures that the trigger is normally high (+Vs).
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| 555 bistable circuit |
It has two inputs:
Example projects using 555 bistable: Quiz |
Model Railway Signal
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| 555 inverting buffer circuit (a NOT gate) |
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| NOT gate symbol |
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so it requires only a few µA, but the output can sink or source up to 200mA.
This enables a high impedance signal source (such as an LDR) to switch a low impedance output transducer (such as a lamp).
It is an inverting buffer or NOT gate because the output logic state (low/high) is the inverse of the input state:
If high sensitivity is required the hysteresis is a problem, but in many circuits it is a helpful
property. It gives the input a high immunity to noise because once the circuit output has
switched high or low the input must change back by at least 1/3 Vs
to make the output switch back.
