In this post we will learn how IC 555 can be used for making various types of monostable multivibrator circuits, such as one-shot type, debounce preventor, retriggerable type, and touch switch monostable circuit.
A monotsable multivibrator circuit is a configuration in which, a short momentary pulse at the input of the circuit causes a one-shot momentary pulse at the output which has a prolonged or an extended duration or extended ON time. This extended pulse output stays ON for a time interval which is determined by a resistor/capacitor network of the circuit.
The image below shows the block diagram of the internal structure of the well-known 555 oscillator /timer, that is composed of a number of transistor stages which represent the upper and lower threshold comparators, a control RS flip-flop, and an output-amplifier stage. The upper and lower thresholds of the comparators are internally fixed at 2/3rd and 1/3rd of the supply voltage using a resistive voltage-divider network
Standard Monostable Multivibrator
In the standard monostable mode, the THRESHOLD and DISCHARGE (pins 6 and 7) pinouts of the IC 555 are hooked up with each other and attached to the junction of a resistor and a capacitor configured like a RC network.
This RC configuration actually decides the output length of time of the IC's one-shot output. Figure 2 below displays a 555 constructed in the form of a one shot monostable timer.
In this mode when the IC is switched ON by briefly connecting pin 2 with the ground line, the output of the IC at pin 3 instantly becomes high, and this high continues to hold for a period of time as fixed by the values of the resistor and the capacitor in the RC network connected at pins 6 and 7.
The timing count could be quickly aborted by temporarily grounding the pin 4 of the IC.
When the circuit is powered ON, the circuit initiates with a low at the output pin 3, and the capacitor of the RC network stays in the discharged condition through pin 7 of the 555. As soon as a timing cycle is started by connecting pin2 to ground or the negative line C1 starts charging exponentially (via R1) towards the supply voltage level.
As soon as it gets to 2/3 voltage level of the supply, the IC 555 output pin3 becomes low yet again. This creates a shorting effect across C1, forcing the capacitor to discharge, which completes and finishes a single timing cycle for the IC.
The timing interval for which the output of the IC remains low does not depend on the supply voltage simply because the C1 charging level, and therefore the voltage introduced to the threshold input pin 6 of the IC, are both directly proportional to the supply voltage. The period of the time interval (T) can be expressed by the formula:
T= 1.1 x R1 x C1
Let's suppose R1 is selected to be a 1 megohm resistors and C1 is 1 uF, the output time for which the IC 555 output pin 3 would be low is: T =1.1 x 1,000,000 x .000,001= 1.1 seconds
The C1 value can be anywhere between 0.001uF to 100 µF; and R1 could be any value between 1k ohms and 10 megohms. It is advisable to have the capacitor C1 a low leakage type to guarantee improved timing accuracy.
As soon as the output of the 555 goes high due to a momentary ground signal at pin 2, applying further ground signal trigger pulses to pin 2 produces no effect on the output condition. But, it may be possible to quickly abort the output ON time by applying a momentary ground or negative signal to pin 4 RESET pin of the IC. The action forces the 555's internal flip -flop to get reset, causing the 555's output to revert to low.
Monostable with Two Outputs
Figure 3 below indicates a 555 monostable having an additional output.
While the IC is in the quiescent or standby state, it output pin 3 stays low and the timing capacitor C1 (which is in effect connected in parallel with R1) remains discharged. As soon as the circuit is triggered by a negative-going signal at pin 2, the output of the IC turns high, which causes the capacitor C1 to start charging via R1.
The moment the voltage across C1 gets to the stipulated upper limit level, the output reverts to low. Due to this C1 begins discharging by means of D1 in the beginning, until the voltage drops to between 0.6 and 0.7 volt (the estimated voltage drop that exists across any silicon diode). When this occurs, C1 starts discharging the rest of its charge a lot more slowly by means of R1.
The length of time necessary for C1 to fully discharge will be higher compared to the normal monostable circuit (the main disadvantage of this type of circuit), and may result in timing issues and /or pulse-width variations when retriggered soon after the finish of a completed output pulse.
The circuit could be enabled by connecting a positive-going voltage (or pulse) with pin 4, or this can be halted simply by a applying ground voltage to pin 4. In case a negative-going pulse is placed on the reset input of the 555, the timing cycle ends, and the action resets the output much earlier than the stipulated timing cycle is able to complete.
IC 555 Monostable with Push Button
One particular well-known undertaking while designing electronic circuits is to integrate switches or push buttons with the electronic circuit.
Since the associated standard switch bounce and noise can easily crank out many or intermittent output pulses, an intermediate buffer circuit (generally known as a "switch debouncer") is frequently utilized to get rid of these complications. The following monostable circuit works great as an intermediate circuit to ensure the switch debounce is eliminated for the subsequent circuit associated with the pin3 of the monostable.
In the one-shot monostable circuit as demonstrated in Fig. 4 above, the 555 timer is triggered using a pushbutton S1. Just before the activation of S1, capacitor C1 is charged to the positive supply rail level through R1.
Next, the moment S1 is pressed, it causes C1 to discharge very fast by means of R4, which generates an instantaneous sharp negative surge voltage. Any intermittent spikes happening due to the switch debounce on pressing S1, can be effectively eliminated due to the presence of R1 and C1, which enables a perfectly clean, negative-going spike.
This spike is subsequently allowed to move through C2 to pin 2 of 555 U1, triggering the monostable to activate, and generate a zero interference, clean square-wave output with a period of T= 1.1 x R1 x C1. Then, as soon as the S1 is released, causes C1 to recharge to the +V potential level so that the circuit now goes in the standby mode, ready for the the next clean push button actuation cycle.
The monostable design allows a single fixed high output pulse when the S1 button is pressed, regardless of how long S1 is held depressed.
However the monostable circuit will be able to retrigger, as fast as S1 may allow to be reactuated. But it has to be ensured that the lowest possible output pulse duration of the design must be longer than the typical debounce period of the switch itself, mostly this should be around 10 ms or less.
A unique form of monostable circuit, configured like a touch switch can be witnessed in Fig. 5 below.
In this design, the switching function is achieved without involving any kind of traditional mechanized switch buttons. The circuit is essentially an old-fashioned 555 monostable, having just one key distinction, which is its style of triggering.
The resistor R1 is used for biasing the trigger input which helps to improve the sensitivity of the monostable circuit.
In this type of circuit, the trigger input requires very small amount of current during the time it remains above its 1/3 of the positive threshold. While using threshold control, resistor R2, is set to ensure the voltage at pin 2 is kept above 1/3 +V the monostable circuit output is held low until triggered.
As soon as the touch plate is contacted, the capacitance of the body (that now becomes effectively parallel with R3) causes the actual impedance between pin 2 and ground to drop. Due to this pin 2 potential is pulled below the 1/3 +V threshold.
This results in the triggering of the IC U1, causing an output pulse to genearte which has an ON time of T= 1.1 x R1 x C1.
It must be ensured that the set time period is longer than the average touch time on the touch plate. If this rule is not followed will result in the the timer getting retriggered even before the first timing pulse cycle is actually completed.
As indicated in the Fig. 5, the timing period of the monostable is approximately 5 seconds.
The touch plate could be any metal plate organized for easy finger touching. It may be also recommended to have some sort of feedback arranged such as an LED or buzzer, that will indicate the user regarding the successful touching and triggering of the circuit.
Retriggerable One-Shot Monostable
The circuit which is displayed in Fig. 6 is just one more version of the IC 555 monostable multivibrator, where we find the standard triggering features of the circuit has been changed.
The circuit is different from the previous monostables since it works with a couple of IC 555's instead of one, Additionally, we do not have to reset the circuit to initiate a fresh timing cycle.
In this concept we can see the 555 timer U2 is wired like the normal monostable, and is triggered by a negative or ground pulse, supplied by 555 U1, which is hooked-up in the form of a basic inverting monostable.
The 555 U1's discharge pin 7 is wired to hold C2 for the time-span determined by the U1's output pulse period. In the normal working conditions, when the trigger pulses tend to be longer than the time period U2, the output pulses appear with the rate of the input cycles, and the width of the output pulses corresponds to 1.1xR2xC2.
When the trigger pulses have shorter time duration than 1.1xR2xC2, the voltage across C2 is forced to come back to zero along with each pulse. As a result, C2 is never able to attain the 2/3 + V threshold as determined by the components R1/C2 for U2; which consequently causes U2 to never time out.
With the above conditions, the output of the circuit continues to be at a constant high DC level, and carries on in this way for so long as the input triggers are obtained with a time period lower than 1.1xR2xC2.
The Fig. 6 concept could be modified to work with just about any rate that corresponds with the appropriate selection of component values for 555.
The IC U1 555's output pulse-width of must be maintained at a certain modest percentage to that of IC 555 U2, ensuring that the pulse-width are sufficiently long and are able to satisfy the full discharge of C2 during the 555 U1's timing period.
Monostable Controlled with Light
The next diagram exhibits a 555 monostable multi-vibrator merged with a couple of light-controlled comparators to configure a circuit which you can use to activate the front door lights of your home for a some fixed period of time, and during the end of this time interval the light is turned off.
Remember that pin 7 of U1b is hooked up with pin 4 (reset) of U2. This is because we know that to ensure that a timing cycle gets initiated, pin 4 has to be held high. If pin 4 were to be held low during the time a negative-going trigger pulse is given to pin 2 of the 555, no timing cycle would be initiated. WE also know that the LDR's possess a resistance of around 0.5 megohm in the absence of light and around 100 ohms with optimal light conditions.
When it is daytime the LDR R5 receives optimal amount of light, which causes its resistance to drop at around 100 ohms. Since the resistance of R6 is much higher compared to the resistance of the LDR during daytime, it causes a far higher voltage to drop across R6, approximately to 5 V, while the drop across R5 is only 0.02V
At this point, with the values the ersistors R1 and R3 have, less than 50 percent of the supply voltage, which is around 1.02 volt, reaches the inverting input of U1b.
With the op amp inverting input of U1b having a increased voltage difference than the non-inverting input, the U1 op amp output becomes low. This low output reaches the pin 4 (reset) of U2, disabling the IC 555 function, which mens the implemtation of a negative or ground trigger pulse fails to reach the at pin 2 which keeps the initialization of the monostable switching cycle disabled.
But as the sun goes down, the R5 resistance goes higher and touches the maximum value of approximately 0.5 megohm. Due to this increase in the R5 resistance, the voltage drop across it as well raises, causing the U1b output to get less negative.
As soon as the R5 resistance goes over the 47k limit, the U1b output instantly turns positive, eliminating the disabling signal utilized to pin 4 of IC 555 U2.
Pushing S1 in this situation can trigger ON the IC 555 monostable timing cycle, causing pin 3 of U2 to go high, and switch ON any device connected to pin3 of the monostable IC 555. For example a relay could be connected across pin3 and ground, and the N/O contacts of the relay wired across a series lamp and AC mains supply, which would cause the lamp to light up in this scenario, and remain illuminated as long as the monostable cycle keeps the relay ON.