When driving on highways, drivers shouldn't go faster than their vehicle's top speed limit. But drivers keep getting into accidents because they don't pay attention to their speedometers. This speed checker will be useful for highway traffic police because it will not only show the speed of a car on a digital screen, but it will also sound an alarm if the car goes faster than the speed limit for the highway. The system is mostly made up of two sets of a laser transmitter and an LDR sensor. Each set is put on the highway 100 meters apart, with the transmitter and the LDR sensor on opposite sides of the road for each pair. Fig. 1 shows how lasers and LDRs are put together. The system shows, with a resolution of 0.01 second, how

long it takes the vehicle to go from one pair of sensors to the next. From this, you can figure out how fast the vehicle is going: Based on the above equation, the display will show 900 (or 9 seconds) for a speed of 40 km/h and 600 for a speed of 60 km/h (or 6 seconds). Note that the LSB of the display is 0.01 second and that each digit after the first one is ten times the one before it. You can also figure out the other readings in the same way (or time). Speed checker circuit diagram In the picture below, you can see how the speed checker works. It was made with the assumption that the speed limit on highways is either 40 km/h or 60 km/h, depending on the rule. Installation of lasers and LDRs on highway

Five NE555 timer ICs (IC1–IC5), four CD4026 counter ICs (IC6–IC9), and four 7-segment displays make up the circuit (DIS1 through DIS4). IC1 through IC3 all work as monostables. The outputs of IC1 and IC2 control IC1 as a count-start mono, IC2 as a count-stop mono, and IC3 as a speed- limit detector mono. The outputs of IC1 and IC2 also control the set-reset bi-stable timer IC4, which in turn controls the on/off switching of the 100Hz (period = 0.01 second) astable timer IC5.

The preset VR1 or VR2 and capacitor C1 can be used to change the time period of the count-start monostable multivibrator timer NE555 (IC1). For a speed limit of 40kmph, preset VR1 sets the time limit to 9 seconds. For a speed limit of 60kmph, preset VR2 sets the time limit to 6 seconds. Slide switch S1 is used to set the time according to the speed limit (40 kmph and 60 kmph, respectively). Pin 2 of IC1 is connected to the point where LDR1 and R1 meet. Usually, the laser light keeps shining on the LDR sensor, so the LDR has a low resistance and pin 2 of IC1 is high. When a car blocks the light from getting to the LDR, the LDR resistance goes up, which makes pin 2 of IC1 go low and set off the monostable. Because of this, output pin 3 goes high for the amount of time that was set (9 or 6 seconds), and LED1 lights up to show this. When the power is turned on or when the reset switch S2 is pressed, pin 4 is controlled by the output of NAND gate N3. For IC2, the monostable is set off in the same way as for IC1 when the vehicle crosses the laser beam hitting LDR2. This makes a small pulse that stops counting and is used to figure out how fast the vehicle is going. LED2 lights up for as long as pin 3 of IC2 is high. The outputs of IC1 and IC2 are connected to NAND gate N1's input pins 2 and 1. When the outputs of IC1 and IC2 both go high at the same time, which means the car has gone over

the set speed limit, pin 3 of gate N1 goes low, which turns on monostable timer IC3. The output of IC3 is used to power the piezobuzzer PZ1, which sounds an alarm if the driver goes over the speed limit. The piezobuzzer sounds for a certain amount of time, which is set by the resistor R9 and the capacitor C5. How a circuit works Circuit of speed checker for highway At the beginning of the count-start pulse, the output of IC1 turns on the bistable (IC4) through gate N2. When IC4's pin 2 goes low, the high signal on its pin 3 turns on IC5's astable clock generator. Since IC2's count-stop pulse output is linked to IC4's pin 6 through diode D1, it resets IC5's clock generator. IC5 can also be reset by diode D2 when the power is turned on or by pressing reset switch S2. IC5 is set up as an astable multivibrator, which means that VR3, R12, and C10 are used to set the time period. Using preset VR1, the astable multivibrator is set to have a frequency of 100 Hz. The

output of IC5 goes to the clock pin 1 of IC6 CD4026, which is a decade counter/7-segment decoder. The IC CD4026 is a 5-stage Johnson decade counter and an output decoder that turns the Johnson code into a 7-segment decoded output for driving the DIS1 display. At each positive clock signal, the counter moves forward by one. In a chain of decade counters, the carry-out (Cout) signal from the CD4026 sends one clock to the next decade counter after every ten clock inputs. To do this, pin 5 of each CD4026 is connected to pin 1 of the next CD4026. When the reset signal is high, the decade counter is reset to 0. When the switch S2 is pressed, pin 15 of all CD4026 ICs, as well as IC1 and IC4, receive a reset signal. The power-on-reset signal is made by capacitor C12 and resistor R14.