2. CIRCUIT DESCRIPTION This circuit has been designed assuming that the maximum permissible speed for highways is either 40 kmph or 60 kmph as per the traffic rule. The circuit is built around five NE555 timer ICs (IC1 through IC5), four Cd4026 counter ICs (IC6 through IC9) and four 7-segmint displays (DIS1 through DIS4). IC1 through IC3 function as monostables with IC1 serving as count–start mono, IC2 as count–stop mono and IC3 as speed-limit detector mono, controlled by IC1 and IC2 outputs. Bistable set-reset IC4 is also controlled by the outputs of IC1 and IC2 and it (IC4), in turn controls switching on/off of the 100Hz (period=0.01 second) astable IC5 The time period of timer NEE555 (IC1) count–start monostables multivibrator is adjusted using preset VR1 or VR2 and capacitor C1.For 40kmph limit the period is set for 9 seconds using preset VR1, while for 60kmph limit the time period is set for 6 seconds using preset VR2 .Slide switch S1 is used to select the time period as per the speed limit (40 kmph and 60 kmph, respectively). The kmph and 60 kmph, respectively) .The junction of LDR1 and resistor R1 is coupled to pin 2 of IC1. Fig. 2: Circuit of speed checker for highway. 3. COMPONENTS USED IN THE CIRCUIT 1. 555 timer 2. LDR(light dependent resistance) 3. LASER 4. 7 segment display 5. NAND gate 3.1. 555 timer: The 555 timer integrated circuit (IC) has become a mainstay in electronics design. A 555 timer will produce a pulse when a trigger signal is applied to it. The pulse length is determined by charging then discharging a capacitor connected to a 555 timer. A 555 timer can be used to denounce switches, modulate signals, create accurate clock signals, create pulse width modulated (PWM) signals, etc. A 555 timer can be obtained from various manufacturers including Fairchild Semiconductor and National Semiconductor.555 is an IC used to generate a clock. The two attributes of a clock are: a) Frequency. b) Duty cycle. Both of these can be changed using this IC, however the duty cycle is always <50%. There are two modes in which 555 can run: 1) Astable mode 2) Monostable mode 3.1.1. MONOSTABLE MODE As the name suggests; in this mode the output is stable in only one (mono) state i.e.~ ‗off‘ state~ Thus it can stay only for a finite time, if triggered, to the other state i.e.~ ‗on‘ state~ This time can be set choosing appropriate values of resistances in the formula: T=1.1*R1*C1