Welcome to the fourth blog in the '555 - Demystification' series! In this blog, I will attempt to demystify the monostable operation mode of 555. Why another blog on the subject - you might ask? Well, simply put, I couldn't find any on the subject which could paint a comprehensible picture for my early teen son :)
Back to the topic, then! The problem we are trying to address is as follows - We want to design a circuit using the 555 IC, which once triggered will maintain the high output state (Pin 3) for a pre-defined period of time, before resetting the output pin to low. The diagram below explain the ask pictorially.
So, what's the big deal about it? Well, we built the bi-stable circuit using 555 earlier, but that had a CLR button to bring down (reset) the output state. In this case, the reset is accomplished automatically and precisely after a pre-defined period of time. What makes it interesting is that we will be introducing the intelligence of 'sensing' elapsed time in the circuitry.
Incidentally, this type of behaviour is also called 'Monostable', since it has one stable state (0) and tends to return to the stable state after any trigger - though after a period of time.
Before we start assembling the magical circuitry, let's spend a few moments on the concept of 'sensing' elapsed time in the electronic world. Note that I have been using the word 'elapsed time' instead of just time. Just time is absolute, but elapsed time is relative to a marker - in our case the moment we triggered the output to high.
There are many ways of measuring time - rely on a clock, count pulses etc. However, there is another simple and cheap mechanism which is very common when it comes to time measurement. The charging of a capacitor ! A capacitor charges and discharges very predictably when a voltage is applied across it through a resistor. If you want to know more about how a capacitor charges and discharges read this first. In short, the charge build up across a capacitor in an RC circuit is in accordance with the below (mathematically defined and hence precise) curve.
In the above equation, V is the voltage across the capacitor, Vcc is the source (max) voltage, R & C are the values of Resistor and Capacitor respectively and t is the time required for a voltage V to build up across the capacitor.
Great! So, we can precisely predict what will be the voltage across the capacitor after time t, or conversely how much time will it take for a certain voltage to build up across the capacitor. The practical question is - how do we build a circuit to act on the time or voltage trigger? See the circuit below:
With the above circuit, we have accomplished the following:
In essence, the output of the comparator will go to high after the time required to charge the capacitor to the voltage reference set by the divider.
Let's continue building on it further. Now let's build more circuitry to automatically reset the timer after triggering an output. See the circuit below:
The above circuit consists of the following parts:
There is one important point to note though. The moment the capacitor is grounded by the transistor, the output of the comparator goes back to 0. This implies that the high output of OP1 is momentary and we will need some more circuitry to make good use of it. But the circuit above is quite helpful in demonstrating the concepts of setting time thresholds, tracking time, triggering on threshold breach and then resetting the timer.
If you are with me so far, the rest of the journey is going to be downhill from here :)
Before I show you how to build the circuit with 555, I will take a moment more to refresh your memory of 555 internals with special attention to Pin 7, the Discharge pin. The above diagram shows how it is positioned. It is worthwhile to remember that when the Output is LOW, the Discharge pin is grounded and consequently when the Output is HIGH, the Discharge pin is not connected to ground.
Easy way to remember - Output Pin LOW - Discharge Pin LOW
The circuit below unveils the 555 monostable operation circuit (finally!) based on the understanding we have built above. Let's start with the interpretation of the timing sequence diagrams to the right. It contains four time markers:
It is assumed that the time taken to push the switch is quite less as compared to the pulse width. Don't get hung up on this assumption - we will dissect it later, for now just accept it :)
Let's analyse the behaviour of the circuit in reference to the time markers t0-3.
When the circuit is switched on, Pin 2 (trigger) is pulled to high via R1 and Pin 6 (Threshold), Pin 7 (Discharge) are effectively grounded (because C1 is not charged up yet). This forces the output to LOW. Remember Pin 7 is grounded when the output is low effectively preventing the capacitor from charging through R. This state continues till time t1.
At t1, the SET button is pushed and released quickly, effectively sending a negative pulse to the trigger pin (Pin 2). This causes the output to go high and Pin 7 to decouple from ground. From this point onwards, the capacitor C starts charging through R5 and voltage at Pin 6 starts to build exponentially. Remember that Pin 6 is connected to the first comparator (CMP1) within the 555 IC. The reference voltage for CMP1 is maintained at 2/3 Vcc through the three 5k resistor chain. The capacitor charging will continue till it reaches 2/3 Vcc. The equations below show that it will take 1.1 * R5 * C1 seconds for the capacitor to reach a voltage equal to 2.3 Vcc.
As soon as the voltage across the capacitor reaches 2.3 Vcc, CMP1 triggers a high - effectively resetting the output. Hence 1.1 * R5 * C2 defines the width of the monostable pulse.
Now as soon as the output is reset, Pin 7 gets grounded, immediately pulling the capacitor voltage to 0 - resetting the whole mechanism. The monostable circuitry will remain in this state till it is triggered by switching in on via the push button switch.
I hope this explanation helps appreciate (and not just understand) how a monostable circuit using 555 is constructed and works!
Back to the topic, then! The problem we are trying to address is as follows - We want to design a circuit using the 555 IC, which once triggered will maintain the high output state (Pin 3) for a pre-defined period of time, before resetting the output pin to low. The diagram below explain the ask pictorially.
So, what's the big deal about it? Well, we built the bi-stable circuit using 555 earlier, but that had a CLR button to bring down (reset) the output state. In this case, the reset is accomplished automatically and precisely after a pre-defined period of time. What makes it interesting is that we will be introducing the intelligence of 'sensing' elapsed time in the circuitry.
Incidentally, this type of behaviour is also called 'Monostable', since it has one stable state (0) and tends to return to the stable state after any trigger - though after a period of time.
Before we start assembling the magical circuitry, let's spend a few moments on the concept of 'sensing' elapsed time in the electronic world. Note that I have been using the word 'elapsed time' instead of just time. Just time is absolute, but elapsed time is relative to a marker - in our case the moment we triggered the output to high.
There are many ways of measuring time - rely on a clock, count pulses etc. However, there is another simple and cheap mechanism which is very common when it comes to time measurement. The charging of a capacitor ! A capacitor charges and discharges very predictably when a voltage is applied across it through a resistor. If you want to know more about how a capacitor charges and discharges read this first. In short, the charge build up across a capacitor in an RC circuit is in accordance with the below (mathematically defined and hence precise) curve.
In the above equation, V is the voltage across the capacitor, Vcc is the source (max) voltage, R & C are the values of Resistor and Capacitor respectively and t is the time required for a voltage V to build up across the capacitor.
Great! So, we can precisely predict what will be the voltage across the capacitor after time t, or conversely how much time will it take for a certain voltage to build up across the capacitor. The practical question is - how do we build a circuit to act on the time or voltage trigger? See the circuit below:
With the above circuit, we have accomplished the following:
- Set up a mechanism to establish a precise voltage (or time) reference using a voltage divider (R1 and R2). If, R1 = R2 then the voltage reference is 1/2 Vcc etc.
- Used a voltage comparator to output a high when the capacitor charge exceeds the voltage reference.
In essence, the output of the comparator will go to high after the time required to charge the capacitor to the voltage reference set by the divider.
Let's continue building on it further. Now let's build more circuitry to automatically reset the timer after triggering an output. See the circuit below:
The above circuit consists of the following parts:
- Green components (R1 & R2) - Voltage divider used to set reference point
- Blue components (R & C) - RC circuit used to measure time
- The comparator (OP1)
- Orange components (R3 and T1) - Reset machinery
So how do they work in unison?
- OP1 output remains low as long as the capacitor is charging. This causes the transistor T1 to remain off - implying that the voltage at the transitor collector and the comparator +ve pin continues to reflect the voltage across the capacitor.
- After some time the voltage across the capacitor reaches a value above the one set by the resistors (R1 and R2) and the OP1 output goes high.
- As soon as OP1 output goes high, it switches on the transistor effectively grounding the collector thus resetting the timing device.
There is one important point to note though. The moment the capacitor is grounded by the transistor, the output of the comparator goes back to 0. This implies that the high output of OP1 is momentary and we will need some more circuitry to make good use of it. But the circuit above is quite helpful in demonstrating the concepts of setting time thresholds, tracking time, triggering on threshold breach and then resetting the timer.
If you are with me so far, the rest of the journey is going to be downhill from here :)
Easy way to remember - Output Pin LOW - Discharge Pin LOW
The circuit below unveils the 555 monostable operation circuit (finally!) based on the understanding we have built above. Let's start with the interpretation of the timing sequence diagrams to the right. It contains four time markers:
- t0 - The instant of powering up the circuit
- t1 - The instant when the SET switch is pushed
- t2 - The instant when the SET switch is released
- t3 - The time when the output pulse resets itself
It is assumed that the time taken to push the switch is quite less as compared to the pulse width. Don't get hung up on this assumption - we will dissect it later, for now just accept it :)
Let's analyse the behaviour of the circuit in reference to the time markers t0-3.
When the circuit is switched on, Pin 2 (trigger) is pulled to high via R1 and Pin 6 (Threshold), Pin 7 (Discharge) are effectively grounded (because C1 is not charged up yet). This forces the output to LOW. Remember Pin 7 is grounded when the output is low effectively preventing the capacitor from charging through R. This state continues till time t1.
At t1, the SET button is pushed and released quickly, effectively sending a negative pulse to the trigger pin (Pin 2). This causes the output to go high and Pin 7 to decouple from ground. From this point onwards, the capacitor C starts charging through R5 and voltage at Pin 6 starts to build exponentially. Remember that Pin 6 is connected to the first comparator (CMP1) within the 555 IC. The reference voltage for CMP1 is maintained at 2/3 Vcc through the three 5k resistor chain. The capacitor charging will continue till it reaches 2/3 Vcc. The equations below show that it will take 1.1 * R5 * C1 seconds for the capacitor to reach a voltage equal to 2.3 Vcc.
As soon as the voltage across the capacitor reaches 2.3 Vcc, CMP1 triggers a high - effectively resetting the output. Hence 1.1 * R5 * C2 defines the width of the monostable pulse.
Now as soon as the output is reset, Pin 7 gets grounded, immediately pulling the capacitor voltage to 0 - resetting the whole mechanism. The monostable circuitry will remain in this state till it is triggered by switching in on via the push button switch.
I hope this explanation helps appreciate (and not just understand) how a monostable circuit using 555 is constructed and works!
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