• Arduino And 555 Timer

For example, consider a cascaded timer circuit that uses two separate 555 timer chips. In this circuit, both of the 555 timer chips are configured in monostable mode. The time interval for the first 555 is controlled by R1 and C1. For the second 555, the interval is controlled by R2 and C2. This is part 3 of a series of articles on the 555 timer. Part 1 goes into more detail about the pins and how the chip functions, so you might want to start there if you haven’t read it already: 555 Timer Basics – Monostable Mode. Astable Mode of the 555 Timer The astable mode is what most.

The 555 timer IC is an amazingly simple yet versatile device. It hasbeen around now for many years and has been reworked into a number ofdifferent technologies. The two primary versions today are the originalbipolar design and the more recent CMOS equivalent. These differencesprimarily affect the amount of power they require and their maximumfrequency of operation; they are pin-compatible and functionallyinterchangeable.

This page contains only a description of the 555 timer IC itself. Functional circuits and a few of the very wide range of its possibleapplications will be covered in additional pages in this category.

The figure to the right shows the functional block diagram of the 555 timer IC. The IC is available in either an 8-pin round TO3-style can or an 8-pin mini-DIP package. In either case, the pin connections are as follows:

1. Ground.
2. Trigger input.
3. Output.
4. Reset input.
5. Control voltage.
6. Threshhold input.
7. Discharge.
8. +V_CC. +5 to +15 volts in normaluse.

The operation of the 555 timer revolves around the three resistors thatform a voltage divider across the power supply, and the two comparatorsconnected to this voltage divider. The IC is quiescent so long as thetrigger input (pin 2) remains at +V_CC and the threshhold input(pin 6) is at ground. Assume the reset input (pin 4) is also at+V_CC and therefore inactive, and that the control voltage input(pin 5) is unconnected. Under these conditions, the output (pin 3) is atground and the discharge transistor (pin 7) is turned on, thus groundingwhatever is connected to this pin.

The three resistors in the voltage divider all have the same value (5Kin the bipolar version of this IC), so the comparator reference voltagesare 1/3 and 2/3 of the supply voltage, whatever that may be. The controlvoltage input at pin 5 can directly affect this relationship, althoughmost of the time this pin is unused.

The internal flip-flop changes state when the trigger input at pin 2 ispulled down below +V_CC/3. When this occurs, the output (pin 3) changes state to +V_CC and the discharge transistor (pin 7) isturned off. The trigger input can now return to +V_CC; it willnot affect the state of the IC.

However, if the threshhold input (pin 6) is now raised above(2/3)+V_CC, the output will return to ground and the dischargetransistor will be turned on again. When the threshhold input returns toground, the IC will remain in this state, which was the original statewhen we started this analysis.

The easiest way to allow the threshhold voltage (pin 6) to graduallyrise to (2/3)+V_CC is to connect it to a capacitor being allowedto charge through a resistor. In this way we can adjust the R and C valuesfor almost any time interval we might want.

The 555 can operate in either monostable or astable mode, depending onthe connections to and the arrangement of the external components. Thus,it can either produce a single pulse when triggered, or it can produce acontinuous pulse train as long as it remains powered.

In monostable mode, the timing interval, t, is set by a single resistorand capacitor, as shown to the right. Both the threshhold input and thedischarge transistor (pins 6 & 7) are connected directly to the capacitor,while the trigger input is held at +V_CC through a resistor. Inthe absence of any input, the output at pin 3 remains low and thedischarge transistor prevents capacitor C from charging.

When an input pulse arrives, it is capacitively coupled to pin 2, thetrigger input. The pulse can be either polarity; its falling edge willtrigger the 555. At this point, the output rises to +V_CC andthe discharge transistor turns off. Capacitor C charges through R towards+V_CC. During this interval, additional pulses received at pin 2will have no effect on circuit operation.

The standard equation for a charging capacitor applies here: e = E(1 - ^(-t/RC)). Here, 'e' is the capacitor voltage atsome instant in time, 'E' is the supply voltage, V_CC,and ' is the basefor natural logarithms, approximately 2.718. The value 't'denotes the time that has passed, in seconds, since the capacitor startedcharging.

We already know that the capacitor will charge until its voltage reaches (2/3)+V_CC, whatever that voltage may be. This doesn't give us absolute values for 'e' or 'E,' but it does give us the ratio e/E = 2/3. We can use this to compute the time, t, required to charge capacitor C to the voltage that will activate the threshhold comparator:

2/3 = 1 - ^(-t/RC)

-1/3 = -^(-t/RC)

1/3 = ^(-t/RC)

ln(1/3) = -t/RC

-1.0986123 = -t/RC

t = 1.0986123RC

t = 1.1RC

The value of 1.1RC isn't exactly precise, of course, but the roundofferror amounts to about 0.126%, which is much closer than componenttolerances in practical circuits, and is very easy to use. The values of Rand C must be given in Ohms and Farads, respectively, and the time will bein seconds. You can scale the values as needed and appropriate for yourapplication, provided you keep proper track of your powers of 10. Forexample, if you specify R in megohms and C in microfarads, t will still bein seconds. But if you specify R in kilohms and C in microfarads, t willbe in milliseconds. It's not difficult to keep track of this, but you mustbe sure to do it accurately in order to correctly calculate the componentvalues you need for any given time interval.

The timing interval is completed when the capacitor voltage reaches the(2/3)+V_CC upper threshhold as monitored at pin 6. When thisthreshhold voltage is reached, the output at pin 3 goes low again, thedischarge transistor (pin 7) is turned on, and the capacitor rapidlydischarges back to ground once more. The circuit is now ready to betriggered once again.

If we rearrange the circuit slightly so that both the trigger and threshhold inputs are controlled by the capacitor voltage, we can cause the 555 to trigger itself repeatedly. In this case, we need two resistors in the capacitor charging path so that one of them can also be in the capacitor discharge path. This gives us the circuit shown to the left.

In this mode, the initial pulse when power is first applied is a bitlonger than the others, having a duration of 1.1(Ra + Rb)C. However, fromthen on, the capacitor alternately charges and discharges between the twocomparator threshhold voltages. When charging, C starts at(1/3)V_CC and charges towards V_CC. However, it isinterrupted exactly halfway there, at (2/3)V_CC. Therefore, thecharging time, t1, is-ln(1/2)(Ra + Rb)C = 0.693(Ra + Rb)C.

When the capacitor voltage reaches (2/3)V_CC, the dischargetransistor is enabled (pin 7), and this point in the circuit becomesgrounded. Capacitor C now discharges through Rb alone. Starting at(2/3)V_CC, it discharges towards ground, but again isinterrupted halfway there, at (1/3)V_CC. The discharge time, t2,then, is -ln(1/2)(Rb)C = 0.693(Rb)C.

The total period of the pulse train is t1 + t2, or0.693(Ra + 2Rb)C. The output frequency of this circuit is theinverse of the period, or 1.44/(Ra + 2Rb)C.

Note that the duty cycle of the 555 timer circuit in astable modecannot reach 50%. On time must always be longer than off time, because Ramust have a resistance value greater than zero to prevent the dischargetransistor from directly shorting V_CC to ground. Such an actionwould immediately destroy the 555 IC.

One interesting and very useful feature of the 555 timer in either modeis that the timing interval for either charge or discharge is independentof the supply voltage, V_CC. This is because the sameV_CC is used both as the charging voltage and as the basis ofthe reference voltages for the two comparators inside the 555. Thus, thetiming equations above depend only on the values for R and C in eitheroperating mode.

In addition, since all three of the internal resistors used to make upthe reference voltage divider are manufactured next to each other on thesame chip at the same time, they are as nearly identical as can be. Therefore, changes in temperature will also have very little effect on thetiming intervals, provided the external components are temperature stable. A typical commercial 555 timer will show a drift of 50 parts per millionper Centigrade degree of temperature change (50 ppm/°C) and0.01%/Volt change in V_CC. This is negligible in most practicalapplications.

Arduino And 555 Timer