Monostable Multi vibrators or “One-Shot” pulse generators are used to generate a single output pulse, either “High” or “Low”, when a suitable external trigger [[wysiwyg_imageupload::]]signal or pulse T is applied. In this project, this trigger signal initiates a timing cycle which causes the output of the monostable to change state at the start of the timing cycle and remain in this second state, which is determined by the time constant of the Capacitor C and the Resistor R until it automatically resets or returns itself back to its original (stable) state. This circuit uses NOT gate to generate the monostable mode of 555 timer. Keep on reading to find out how the circuit is made and how it works.
555 timer IC is commonly used in monostable mode to generate a pulse every time a trigger is given. The project described here uses NAND gate for [[wysiwyg_imageupload::]]monostable operation and thereby avoiding the need of 555 timer.The circuit in the project uses IC 7400, which has four inbuilt NAND gates. Here we have used the first and second NAND gate of the IC 7400. Suppose, initially the trigger input T is High at a logic level “1” so that the output from first gate (U1) is Low at logic level “0”. The resistor, R will cause the capacitor, C to charge or discharge. The output from the second NAND gate (U2), which is connected as an inverting NOT gate is also fed back to the input of U1,which is at logic level “1”. Since both junction V1 and the output of U1 are both at logic “0” no current flows in the capacitor C and this results in the circuit being Stable and will remain in this state until the trigger input T changes.
555 timer IC is very commonly used to produce time delays in monostable mode. The IC produces a time delay every time its trigger pin 2 is given an active low signal. The time of the delay is decided by the value of resistance and capacitance. However, in many applications it is required to trigger 555 whenever the output goes high. This circuit based project addresses to the aforesaid problems. The circuit in this project uses an npn transistor T1 to achieve the above objective. 555 timer in monostable configuration is shown in the figure. Pin 2, which is the trigger pin of the IC is connected to the collector of T1. Continue reading to find out how the circuit is connected and how it works.