The Monostable configuration of the 555 chip allows us to create sophisticated delays within a circuit using an external RC circuit. In other words, in this configuration, the chip serves to produce a signal of fixed time duration, as will be explained. Please note that this feature can also be used for debouncing. The event of bounce can cause serious instability in a circuit and needs to be eliminated. But what is a bounce? Let us consider a circuit that needs to be controlled by a mechanical switch. When the switch is pressed, the 2 metal parts of the switch come into direct contact, but unfortunately not immediately. The parts are connected and disconnected a few times before a permanent connection is made, as shown in the input part of the following figure.
(Source: referenced)
For that matter, we can use the 555 chip in monostable mode as a proxy between the mechanical switch and the circuit we want to control. In that sense, we could connect the mechanical switch as an input to the 555 chip, and its output to the controlling circuit. Now, pressing the mechanical switch triggers the 555 chip which makes its output long enough to swallow the occurrence of bouncing, as shown in the output part of the figure. Since the 555 is actually triggered with a low input value, this figure shows only the principle, not the true workings of the monostable configuration which will be much clearer after the experiment.
The figure below shows a simplified monostable configuration of the 555 chip.
(Source: referenced)
The explanation is based on the brilliant explanation of Mr.Charles Platt in the book Make: Electronics:
- Pin 1 is connected to the ground, and the Pin 8 is connected to VCC
- Pin 5 is not used and is hence grounded via capacitor C2 to eliminate noise
- Flip-flop is a sequential logic component that is able to preserve the state of one bit and whose work we will get to know later. For now, it is important that we understand that it remembers the state
- If the flip-flop is SET through S input, its Q output value 1 is sent to output pin 3. Also, its inverted Q output value 0 is sent to the base of the NPN transistor
- If the flip-flop is RESET through R input, its Q output value 0 is sent to output pin 3. Also, its inverted Q output value 1 is sent to the base of the NPN transistor
- Comparators 1 and 2 compare voltages and are used to SET and RESET the flip-flop
- The (+) input of comparator 1 is connected to 1/3 of a source voltage by the use of a voltage divider network and serves as a reference voltage. The (-) input of comparator 1 is connected to pin 2
- The (-) input of comparator 2 is connected to 2/3 of a source voltage by the use of a voltage divider network and serves as a reference voltage. The (+) input of comparator 2 is connected to pin 6
- Pin 4 is used to reset the flip-flop with low input. It implicates the fact that its high input enables the flip-flop, so it is tied high on VCC in order to permanently enable the flip-flop
- Pin 7 is connected to the collector of the NPN transistor and it is used to discharge the capacitor C1 when the NPN transistor conducts
- Pins 8, 7, and 6, together with R resistor and C1 capacitor form an RC circuit that will control the behavior of the monostable configuration
Let us try to explain the workings of the monostable configuration, by following the steps:
- If the voltage value on pin 2 is below the 1/3 of the source voltage, the output of comparator 1 is high which sets the flip-flop – this can be accomplished by pressing a pushbutton or by means of some other incoming signal
- Consequently, the Q output value 1 from the flip-flop is sent to the output on pin 3 as a high value and the positive part of the cycle begins
- At the same, the inverted Q output value 0 from the flip-flop is sent to the base of the NPN transistor and disconnects pin 7 from the ground
- Since pin 7 is not grounded, the RC circuit is set in motion. Capacitor C1 starts to charge, at a rate determined by the resistance value R, and the capacitance of capacitor C1
- When the voltage on pin 6 is above the 2/3 of the source voltage, the output of comparator 2 is high which resets the flip-flop
- Consequently, the Q output value 0 from the flip-flop is sent to the output as a low value
- At the same, the inverted Q output value 1 from the flip-flop is sent to the base of the NPN transistor and connects pin 7 to the ground which discharges the capacitor
- The cycle is over
In summary, we can express the work in the following way:
- low voltage at the input causes the high voltage at the output
- after a time defined by R and C values, the circuit returns to its stable state with low output
It is obvious that there is only one permanent stable state, hence is this configuration called monostable.
References:
https://digilent.com/reference/learn/microprocessor/tutorials/debouncing-with-ne555-timer/start
https://www.electronics-tutorials.ws/waveforms/555_timer.html
C. Platt, Make Electronics (Maker Media, 2015)
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