555 Timer Pro 3 Key
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The 555 timer has been with us since 1972 - that's a long time for any IC, and the fact that it's still used in thousands of designs is testament to its usefulness in a wide variety of equipment, both professional and hobbyist. It can function as an oscillator, a timer, and even as an inverting or non-inverting buffer. The IC can provide up to 200mA output current (source or sink) and operates from a supply voltage from 4.5V up to 18V. The CMOS version (7555) has lower output current and also draws less supply current, and can run from 2V up to 15V.
There are many different manufacturers and many different part number prefixes and suffixes, and they are available in a dual version (556). Some makers have quad versions as well. The 555 and its derivatives come in DIP (dual in-line package) and SMD (surface mount device) packages. I don't intend to even attempt to cover all the variations because there are too many, but the following material is all based on the standard 8 pin package, single timer. All pin numbers refer to the 8-pin version, and will need to be changed if you use the dual or quad types, or choose one of the SMD versions that has a different pinout. Note that the quad version has only the bare minimum of pins, reset and control voltage are shared by all four timers, and it has no separate threshold and discharge pins (they are tied together internally, and called 'timing').
Figure 1B shows a complete circuit diagram for a 555 timer, based on the schematic shown in the ST Microelectronics datasheet. Schematics from other manufacturers may differ slightly, but the operation is identical. There's really not much point in going through the circuit in detail, but one thing that needs to be pointed out is the voltage divider that creates the reference voltages used internally. The three 5k resistors are shown in blue so you can find them easily, and the main sections are shown within dotted lines (and labelled) so each section can be identified.
As mentioned above, the 555 can be used as an oscillator or timer, as well as to perform some less conventional duties. The basic forms of multivibrator are the astable (no stable states), monostable (one stable state) or bistable (two stable states). Unfortunately, operation as a bistable is not very useful with a 555 because of the way it's organised internally. However, it can be done if you accept some limitations. A 555 circuit that functions as a bistable is described in Project 166, where the 555 is used as a push-on, push-off switch for powered equipment.
The timing is fairly stable with temperature and supply voltage variations. The 'commercial grade' NE555 is rated for a typical stability of 50ppm (parts per million) per degree C as a monostable, and 150ppm / °C as an astable. It's worse as an oscillator (astable) than a timer (monostable) because the oscillator relies on two comparators but the timer only relies on one. Drift with supply voltage is about 0.3% / V.
Most of the circuits shown below include an LED with its limiting resistor. This is entirely optional, but it helps you to see what the IC is doing when you have a slow astable or timer. The circuits also show a 47µF bypass capacitor, and this should be as close to the IC as possible. If the cap is not included, you may get some strange effects, including a parasitic oscillation of the output stage as it changes state.
As many readers will have noticed, I will generally use an opamp, a comparator or even some discrete circuitry in preference to a 555 timer. This isn't because I don't like the 555 IC, but simply because so few applications I normally work with need the flexibility it offers. It's certainly not a precision device, but it is handy, and countless circuits (many of them hobbyist designs) have used it - often because the designer doesn't know how to get a time delay by any other means.
You may well wonder where the values of 1.44 and 0.69 come from. These are constants (or 'fudge factors' if you prefer) that have been determined mathematically and empirically for the 555 timer. They're not perfect, but are close enough for most calculations. If you need a 555 circuit to oscillate at a precise frequency you'll need to include a trimpot so the circuit can be adjusted. It still won't be exact, and it will drift - remember that this is not a precision device and must not be used where accuracy is critical.
A monostable (also known as a 'one-shot' circuit) has one stable state. When triggered it will go to its 'unstable' state, and the time it spends there depends on the timing components. A monostable is used to produce a pulse with a predetermined time when it's triggered. The most common use of a monostable is as a timer. When the trigger is activated, the output will go high for the preset time then fall back to zero. While we tend to think of timers being long duration (several seconds to a few minutes), monostables are also used with very short times - 1ms or less for example. This is a common application when the circuit needs pulses with a defined and predictable width, and having fast rise and fall times.
With the values shown, the output will be high for 1.1ms. If C1 were 100µF, the time would be 1.1 seconds. As noted, the trigger pulse must be shorter than the delay time. If the trigger were to be 5ms long in the circuit shown in Figure 5, the output would remain high for 5ms and the timer has no effect. Apart from timers, monostables are commonly used for obtaining a pulse with a predetermined width from an input signal that is variable or noisy.
The most common use of the monostable 555 circuit is as a timer. The trigger might be a push-button, and when pressed the output goes high for the preset time then drops low again. There are countless applications for simple timers, and I won't bore the reader with a long list of examples.
Hint: If you happen to need a timer that runs for a long time (hours to weeks), use a variable 555 oscillator circuit that then drives a CMOS counter such as the 4020 or similar. The output of the 555 oscillator might be (say) a 1 minute/ cycle waveform, and that can act as the clock signal for the counter. The 4020 is a 14 bit binary counter, so with a simple circuit you can easily get a delay (using a 1 minute clock) of 8,192 minutes - over 136 hours or a bit over 5½ days. Still not long enough? Use two or more 4020 counters. Two will allow a timer that runs for about 127 years! Note that you will have to provide additional circuitry to make any of this work, and it may be difficult to be certain that a 127 year timer works as expected.
Here's an example (but it's not a monostable), and depending on the output selected from the 4020 counter you can get a delay of up to 20 minutes. If C1 is made larger the delay can be much greater. With the resistor values given for the timing circuit, increasing C1 to 100µF will extend the maximum time to 3.38 hours (3h 23s), using Q14 of U2 as the output. If C1 is a low leakage electro, the values for R1 and R2 can be increased, so it will run for even longer. The drawing also shows how many input pulses are required before the respective outputs go high (Vcc / Vdd). The counter advances on the negative-going pulse. To use higher value timing resistors, consider using a CMOS timer (e.g. 7555).
As shown, the minimum period for the 555 is 20.83ms (48Hz) with VR1 at minimum resistance, and at maximum resistance it's 145.7ms (6.86Hz). When power is applied the timer will run for the designed time period until the output goes high. Pressing the 'Start' button will set the output low and the time period starts again. All outputs from the counter are set low at power-on by the reset cap (C3) and/ or when the 'Start' button is pressed. The 555 runs as an astable, and continues pulsing until the selected output from U2 goes high. D1 then forces the voltage across C1 to 0.7V below Vcc and stops oscillation. Therefore, when the 'Start' button is pressed the output goes low, and returns high after the timeout period.
Additional circuitry is needed if you don't want the timer to operate after power-on, or if you want the 'Start' button to make the output high, falling to zero after the timeout. I leave these as an exercise for the reader. The above is simply an example - it's not intended to be a circuit for any particular application.
There are many uses for 555 timers apart from the basic building blocks shown above. This is an article and not a complete book, so only a few of the possibilities will be covered. They have been selected based on things I find interesting or useful, and if you have a favourite that isn't included then that's just tough I'm afraid.
There are dedicated SMPS controllers that may be no more expensive than a 555 timer, but it's still a useful application and means you don't need to search for an obscure part. It's greatest advantage is that it can often be built using parts you already have in your junk-box, with the added benefit that it doesn't rely on SMD parts and can be built on Veroboard.
One quite common use for 555 timers is as a missing pulse detector. If you expect a continuous train of pulses from a circuit, should one go 'missing' for any reason that may indicate a problem. Being able to detect that a pulse is missing or delayed can be an important safety function, raising an alarm or disabling the circuit until the fault has been corrected.
Input pulses are used to switch on Q1 and hence discharge C1. As long as the pulses keep arriving in an orderly manner the output of the 555 stays high. The time constant of R1 and C1 must be selected so that the timer can never expire as long as the input pulses keep arriving as they should. If the time is too short C1 will charge to 2/3 Vcc before the next input arrives. If it's too long, a single missing pulse won't be detected and it will require several pulses in a row to be missing (or the pulse train may stop altogether) before the timer will operate. You may also need to take precautions to ensure the timer will always operate, even if the incoming pulse train gets stuck at the high voltage level. This will involve adding a differentiator, similar to that shown in Figure 6. 2b1af7f3a8