The pulse detector is categorized as a mono-stable multi-vibrator because it has only one stable state. By term stable, we mean to say a state of output where the device is able to latch or hold to forever, without any external prodding. A latch or a flip-flop, being a bi-stable device, can hold in one state either "set" or "reset" state for an in-definite period of time. Once it is set or reset, it will continue to latch in that state unless impelled to change by way of an external input. A monos table device, on the other end, has the only capacity to hold in one particular state indefinitely. Its other state can only be held momentarily when triggered by an external input.
A mechanical analogy of a mono-stable device would be a momentary contact pushbutton switch, that spring returns to its ordinary (stable) state when pressure is removed from its button actuator. Likewise, a standard wall (toggle) switch is a bi-stable device. It can latch in one of the two modes: on or off. All mono-stable multi-vibrators are timed devices. That is, their unstable output state will hold only for certain lowest amount of time before returning to its stable state. With the semiconductor mono-stable circuits, this timing function is naturally achieved through the use of capacitors & resistors, making the use of the exponential charging rates of RC circuits. A comparator is sometimes used to perform comparison of the voltage across the charging or dis-charging capacitor with a steady reference voltage, and the on (off) state of output of the comparator used for a logic signal.
With ladder logic, time delays are achieved with the time-delay relays, which can be constructed with semiconductor/RC circuits like that is just mentioned or by making use of mechanical delay devices which impede the immediate motion of the relay’s armature. Note here the design and operation of the pulse detector circuit in ladder logic. Regard less of how long the input signal stays high (1), the output will remain high for just one second of time, after this it returns to its. Normal (i.e. stable) low state. For some of the applications, it is necessary to have a mono-stable device that outputs a longer pulse than the input pulse which activates it. Take under consideration the following ladder logic circuit.
On the closing of input contact, the TD1 contact closes immediately, and stays closed state for ten seconds after the input contact opens. The output will stays high to 1 for exactly 10 seconds after the input drops low again. This type of mono-stable multi vibrator is known as a one-shot. More particularly, it is a re-triggerable one-shot the timing begins after the input drops to a low state, pointing that multiple input pulses within 10 seconds of each other will maintain a nonstop high output. One application for a retriggerable one-shot is that of a single mechanical contact de-bouncer.
What we only want a 10 second timed pulse output from a relay logic circuit, no matter how many input pulses are received or how long-lived they may be? In that case, you'd have to couple a pulse detector circuit to the re-trigger able one-shot time delay circuit, same asc below:
Time delay relay TD1 gives an "on" pulse to time delay relay coil TD2 for an arbitrarily short moment (for at least 0.5 second each time). As soon as TD2 is energized, the normally-closed, timed-closed TD2 contact in series with it prevents coil TD2 from being re-energized as long as its timing out (10 seconds). This as a result makes it effectively unresponsive to any more actuations of the input switch during that 10 seconds. Only after TD2 times out does the normally-closed, timed-closed TD2 contact in series with it allow coil TD2 to be energized again. This type of one-shot is called a non-re-trigger-able one-shot. The 1-shot multi-vibrators of both the re-triggerable and non-re-triggerable range find wide industrial application for siren actuation and for machine sequencing, where an intermittent input signal creates an output signal of a set time.
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