Speed Checker for Highways

Dipanjan Bhattacharjee


IC CD4026 is a 5-stage Johnson decade counter and an output decoder that converts the Johnson code into a 7-segment decoded output for driving DIS1 display. The counter advances by one count at the positive clock signal transition.

The carry-out (Cout) signal from CD4026 provides one clock after every ten clock inputs to clock the succeeding decade counter in a multidecade counting chain. This is achieved by connecting pin 5 of each CD4026 to pin 1 of the next CD4026.

A high reset signal clears the decade counter to its zero count. Pressing switch S2 provides a reset signal to pin 15 of all CD4026 ICs and also IC1 and IC4. Capacitor C12 and resistor R14 generate the power-on-reset signal.

Power supply

The seven decoded outputs ‘a’ through ‘g’ of CD4026s illuminate the proper segment of the 7-segment displays (DIS1 through DIS4) used for representing the decimal digits ‘0’ through ‘9.’ Resistors R16 through R19 limit the current across DIS1 through DIS4, respectively.

The above circuit shows the power supply. The AC mains is stepped down by transformer X1 to deliver the secondary output of 15 volts, 500 mA. The transformer output is rectified by a bridge rectifier comprising diodes D3 through D6, filtered by capacitor C14 and regulated by IC11 to provide regulated 12V supply. Capacitor C15 bypasses any ripple in the regulated output. Switch S3 is used as the ‘on’/‘off’ switch. In mobile application of the circuit, where mains 230V AC is not available, it is advisable to use an external 12V battery. For activating the lasers used in conjunction with LDR1 and LDR2, separate batteries may be used.

Speed checker construction and working

Assemble the circuit on a PCB. An actual-size, single-side PCB layout for the speed checker and its component layout is shown below.

Fig. 4: Actual-size, single-side PCB layout for the speed checker
Fig. 5: Component layout for the PCB

Download PCB and component layout PDFs: click here

Before operation, using a multimeter check whether the power supply output is correct. If yes, apply power supply to the circuit by flipping switch S3 to ‘on.’ In the circuit, use long wires for connecting the two LDRs, so that you can take them out of the PCB and install on one side of the highway, 100 metres apart. Install the two laser transmitters (such as laser torches) on the other side of the highway exactly opposite to the LDRs such that laser light falls directly on the LDRs. Reset the circuit by pressing switch S2, so the display shows ‘0000.’ Using switch S1, select the speed limit (say, 60 kmph) for the highway. When any vehicle crosses the first laser light, LDR1 will trigger IC1. The output of IC1 goes high for the time set to cross 100 metres with the selected speed (60 kmph) and LED1 glows during for period. When the vehicle crosses the second laser light, the output of IC2 goes high and LED2 glows for this period.

Piezobuzzer PZ1 sounds an alarm if the vehicle crosses the distance between the laser set-ups at more than the selected speed (lesser period than preset period). The counter starts counting when the first laser beam is intercepted and stops when the second laser beam is intercepted. The time taken by the vehicle to cross both the laser beams is displayed on the 7-segment display. For 60kmph speed setting, with timer frequency set at 100 Hz, if the display count is less than ‘600,’ it means that the vehicle has crossed the speed limit (and simultaneously the buzzer sounds). Reset the circuit for monitoring the speed of the next vehicle.

Note. This speed checker can check the speed of only one vehicle at a time.



  1. All the time i used to switch on my speed checker circuit using Ne555, the circuit did not work…Pin 7 and Pin 8 of 555 get shorted,,, thus the circuit is not working. Could anyone please tell me what is the reason behind it? and also provide me the suggestion about what to do so that the circuit can work properly… as soon as possible..

  2. Can u plz send me the pic of the project after completion ……….i dint understand the connection of ldr and laser light

    • I have completed the project on bread board because it’s not working properly on vero board. The connection for ldr and laser is simple. You have to use ldr on the main circuit. The necessity of Laser is here to reduce the resistance of the ldr. You can use any light source, it’s not mandatory you only have to use laser. Make the circuit. For, working video you can use youtube because it would be better than Circuit Image. Good Luck.

      • hi…. im using this circuit as my project but the output is not showing my teacher said that i have to check part by part and each part should be a square wave but the output ive got was a sine wave … im sure that my connections r right ( im using breadboard as well) so, can u tell me please what was the output you have got for each part ??

  3. dude we thought you tested the components before u send them from now on i am never going to recommend any one this.your kits cost only 1000 rs but you take much higher amount.very bad.u put us in a difficult situation

  4. Hi Akhil,
    Regret for inconvenience. We are checking components, but randomly. You are requested to let us know the name of componets to send replacement. Also proivde your Order ID for the same.

  5. Hello
    i wanted to know whether the switch used here is 2 pin or 3 pin. Also how do i check if my connections are right part by part because if i apply power supply at one go the whole thing could burn due to improper connections.

    And is there video demonstrating the working according to the given circuit diagram?

    thank you.

  6. You tricked us!!
    we asked for a 555 timer and nand gate based circuit but, were delivered an entirely different circuit (microcontoller based). Though, all the specification of that other circuit were mentioned but , they were incorrect too.
    We request you to please rectify your blunder mistake!
    Waiting in anticipation.

  7. I have rectified the problem. Input voltage (Vdd) and ground (Gnd) for “ALL” IC 4011 should be given for the circuit to work I hope the circuit diagram is changed. Thank me later just do the connections and see your circuit work!

  8. 1 more problem I used ir transmitter led and receiver led instead of the ldr.
    I’m having problem setting the resistance values for the variable resistors VR1,VR2,VR3. Please help me EFY Team!

  9. Hii, my circuit connection are comple as per circuit diagram but only seven segment display is on.no speed measure when two lasser are intrupt.no one result.

  10. For practical application, if we want to use the circuit on highways,will the circuit work?
    I am asking because LDR will get different intensity in day and night.

  11. Sir please explain each part of block diagram clearly ,timers and counters and all parts in project speed checker to detect Rash driving warning system for highways using 555 timer.
    Why their is no supply for one LED?

    • The details on circuit and its working have been explained in text. Each LED will get supply voltage whenever the corresponding output pin 3 of 555 IC becomes high. So there is not need of separate power supply in any of the LEDs in the circuit.

  12. hey can i know why u have used those capacitors and resistors with that particular values .I would like to know as i am doing this project now as a mini project and also i would like to know LDR receiving signal procedure ..its very important for me to do this..i would like to modify it as well ..hope u respond thank u

  13. I can pay you through PayPal I need whole information regarding this project, I have made the circuit on breadboard but circuit just switches on and segments show zeros nothing else.


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