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555 IC as a Monostable Oscillator

In a previous post we learned how we could use a 555 IC to create an Astable Oscillator in order to generate a continuous wave of alternating signals. In this post, we will take a look at configuring the 555 IC as a Monostable Oscillator.


As the name suggests a Monostable oscillator maintains 1 stable output signal. This is unlike an astable oscillator, which alternates between HIGH and LOW signals. The stable output signal in a monostable mode is a LOW signal. However, leveraging the exposed pins, a 555 can be triggered to produce a HIGH signal for any duration of time. Such a duration can be extremely short, or last as long as several days-no joke.


The image shown below reveals the pins exposed by the IC and labels them by their nature. Moving counter-clockwise, the chip exposes ground, trigger, output, reset, control, threshold, discharge and power.


The behavior in the monostable mode is due impart to the external factors on pin 2 ( trigger) and pin 6 (threshold). A change in voltage from HIGH->LOW on pin 2 (trigger) results in a HIGH signal on pin 3 (output). At this point, if a HIGH signal is observed on pin 6 (threshold), pin 3 (output) outputs a LOW signal. If you read the previous article on the Astable Oscillator, you should already be familiar with the pins on the 555 IC. While the functionality of the pins remain the same, the configuration in which they are designed gives us our monostable mode.


If you were to compare a monostable hook up from that of an astable hookup, you'd see that they don't vary all that much. The most notable differences are the arrangement of pin 2 (trigger), pin 6 (threshold) and pin 7 (discharge). The monostable arrangement of these 3 three pins looks to be the inverse of the astable configuration. Previously pin 7 (discharge) remained independent while pins 2 (trigger) and 6 (threshold) where connected. However, in this configuration it is pin 2 (trigger) that remains independent while pins 6 (threshold) and pin 7 (discharge) are connected. The difference is in the intended use of the oscillating behavior of the 555.


In a monostable timer, the intended use is the generation a HIGH Signal of arbitrary duration. Such a capability is desired for a variety of reasons. One such reason is to specify a window in which a secondary clock remains operational. Another reason is to generate a pulse of unchanging length, which may be required by another component.


So how does one go about designing such a timer? Simply put, we involve a resistor, a capacitor and the RC Time constant. One thing I didn't mention in the previous article, is that when the output signal is LOW on pin 3 (output), pin 7 (discharge) becomes tied to ground, which drains current away from the circuit. However, this is not the case when the signal output on pin 3 (output) is HIGH. By default, the 555 IC produces a LOW signal on pin 3 (output)-meaning that by default discharge is connected to ground. This is a critical piece of our puzzle to build around and represents the singular stable state for which the monostable timer gets its nom de gare.


When pin 7 (discharge) is connected to ground, it prevents the capacitor from charging. It's not until the moment a HIGH->LOW signal on pin 2 (trigger) triggers the change in state on the 555 from drain to source. This causes pin 3 (output) to output a HIGH signal. At this point, pin 7 (discharge) is no longer connected to ground enabling the capacitor to charge through a resistor. Steadily, the capacitor begins to charge at a rate defined by the RC Time constant. As it charges, pin 6 (threshold) continues to monitor the voltage level of the capacitor. The moment the capacitor reaches 2/3 the supplied voltage, the 555 switches state from source to drain once more outputting a LOW signal on pin 3 (output).


The RC constant plays an important part in this because it determines how long the capacitor will take to charge to 2/3 the source voltage. Which, as you may have deduced, is when the drain on pin 7 (discharge) will occur.


You may recall the RC Time constant as

T(seconds)=R(ohms) * C(farads)

You can solve for any of the 3 values by substituting for the other two. In the image shown above, I used a resistor of 1K ohm and a capacitor of 1 µFarad. Substituting these two values in our equation results in a duration of 1000*.001 = 1 second. So for the circuit setup above, we can expect to pulse a HIGH signal for 1 second. An important thing to note, is that a HIGH signal from a 555 timer, can range from 3v to that of 12 volts, which is more than enough voltage to power a secondary timer as seen in the video below.



The video above demonstrates how a monostable timer (lower circuit) can be the power source of an astable circuit (topmost circuit), thereby defining the duration for which the astable oscillation occurs.


Allaboutcircuits.com has a convenient calculator for determining the duration or the pulse given the values of a Capacitor and Resistor. It also has a great read on the 555 monostable timer.


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