Seat belt warning system using basic AND & NOT gates
1 INTRODUCTION
Combinational logic circuit is an interconnection of logic gates to generate a specified logic function, where the inputs result in an immediate output. This can be easily understood by the following example. The following English expression describes the way a logic circuit should operate in order to drive a seat-belt warning indicator in a car.
''If the driver is present AND the driver has NOT buckled up AND the ignition switch is on, then the warning light should turn ON.''
The Boolean expression for this application can be written as:
This expression can be easily implemented by using a single- three-input AND gate along with a single NOT gate. This expression could also be implemented by multi-level logic , wherein one gate drives an another gate. In general, we may use two - dual-input AND gates and a single NOT gate. Students need to be cautious and judicious while selecting the gates. As the number of levels increases, the response time of the circuit also increases. Multi-level logic leads to the introduction of more time delay as compared to single-level logic with multiple inputs. If it takes τ seconds for a gate to respond to a change in its input and that gate drives a succeeding gate, then it will take 2τ seconds for the entire circuit to respond. It is desirable to minimize the number of levels in the logic. If one needs to construct a truth table for the same, it can be easily prepared by entering 1's and 0's in that expression for various input combinations.
TRUTH TABLE:
| D | B | I | Y= D. B'. I |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 0 | 0 | 1 | 0 |
| 0 | 1 | 0 | 0 |
| 0 | 1 | 1 | 0 |
| 1 | 0 | 0 | 0 |
| 1 | 0 | 1 | 1 |
| 1 | 1 | 0 | 0 |
| 1 | 1 | 1 | 0 |
1.1 APPLICATION: SEAT BELT WARNING INDICATOR IN A CAR
A seat belt is a safety feature installed in modern vehicles to protect the occupants in case of an accident. However, in some instances, the occupant may start driving the vehicle without fastening the seat belt. This may lead to the violation of safety rules as well as pose risk to the occupants. To indicate that the seat belt has not been buckled, while the ignition switch is turned on, the AND-NOT based indicating system provides a good solution. 1.2 CONCEPT: When the driver turns on the ignition switch without fastening the seat belt, then the warning light glows. This indicates that the seat belt is not buckled or fastened.
Figure1. (a) Two Level Logic and (b) Three -Level Logic
Seat Belt Warning Light Indicator in a Car