car speed limit indicator circuit - Car Speed Limit Warning Indicator Circuit

Car Speed Limit Warning Indicator Circuit

The following post describes a simple yet useful car speed limit warning indicator circuit which can be used in vehicles for getting an instant indication of a possible over speed limit conditions. The idea was requested by Mr. Abu-Hafss.

The Circuit Request:

Hi Swagatam

Shall appreciate, if you could design a circuit to warn with some melody that the car has reached preset speed limit. As soon as the speed is reduced below the limit the alarm should go off. 

The circuit should have 2 options for switching between 2 speed limits.

1. The circuit should have a selection switch --- Postion A >> 100Km/h (for ordinary highways) and Position B >> 120 Km/h (for Experessways).

2. When the switch is positioned for example at position B, a sensor which can get current speed from car speed-o-meter and then compare with the selected speed limit (120 Km/h). Whenever, the sensor reads the speed to be 120 Km/h or more, it will start playing a warning melody until the speed is reduced.

The Circuit Design

The designed circuit of a car over speed limit indicator basically consists of two stages. Both the stages incorporate the ubiquitous IC 555.

The stage which includes IC1 is configured as a simple frequency to voltage converter or a frequency dependent voltage generator circuit.

Here IC1 is rigged in the form of a standard monostable multivibrator whose ON time is decided by the resistors R3/R4 and the capacitor C2. These components must be selected appropriately for getting most favorable output response.

As we all know that all modern motor vehicles today incorporate electronic ignition systems which invariably include a CDI or a capacitive discharge ignition network, in contrast to the older circuit-breaker units.

The the CDI unit is responsible for generating the required igniting sparks inside the engine of the vehicle and its rate of firing is directly proportionate to the speed of the vehicle.

It means with an increase in the speed of the vehicle the charge/discharge rate of the CDI capacitor also increases and vice versa.

The monostable built around IC1 exploits this feature of the CDI system and derives a sample potential from the CDI at the base of T1.

T1 effectively switches the varying high voltage pulses from the CDI into low voltage triggering  pulses across C1 and ground.

With response to the above pulses, every time T1 conducts it pulls pin#2 of IC1 to ground initiating the monostable output to go high.

The monostable sustains the output in the high position for a period determined by the values of the respective timing components as explained in the previous section.

However, the continuous train of pulses initiates an appropriately stabilized high output at pin#3 of IC1 (due to the functioning of the monostable which produces almost an exact average DC output proportional to the frequency of the pulses.)

Circuit Description

The output is further stabilized into a perfect measurable equivalent DC by the integrator stage comprising R7/R8/C4/C5 and also P1.

IC2 is wired up as a voltage comparator.

Its pin#2 is allowed to receive the varying voltage from the IC1 output.

P1 is set such that as soon as the output from IC1 rises to a certain limit which may be calculated as the over speed limit value, the potential; at pin#3 rises over 1/3rd Vcc

This instantly prompts the output of IC2 to go low, activating the connected alarm device.

This alarm would stay activated as long as the car speed does not come down below the preset limit.

As soon as the speed is reduced, the alarm stops sounding.

The preset P1 may be suitably replaced with a properly calculated potential divider ladder type network along with a selector switch for enabling the selection of different speed limits for different free-ways.

Circuit Schematic

Car Speed Limit Alarm Circuit


Parts List for the proposed Car Speed Limit Warning Indicator Circuit
R1 = 4K7

R2 = 47E


R4 = 3K3,

R5 = 10K,

R6 = 330K
R9 = 1K,

R7 = 1K,

R8 = 10K,

R10 = 100K,

C1 = 47n,

C2 = 100n,

C3 = 100n,

C4 = 100uF/25V,

C5 = 10uF/25

VP1 = 10k PRESET


T1 = BC547

IC1, IC2 = 555,

D1,D2 = 1N4148


Improving the Design By using an Opamp

The accuracy and reliability of the above explained car speed warning indicator can be further improved by replacing the 555 buzzer stage with an opamp/relay stage as shown below:

Circuit Schematic

speedlimit 1 - Car Speed Limit Warning Indicator Circuit


24 thoughts on “Car Speed Limit Warning Indicator Circuit

  1. Have questions? Please feel free to post them through comments! Comments will be moderated and solved ASAP.
    • Yes that's right, but when speed is increased, RPM would also increase proportionately..and in cases where RPM is accelerated without any change in the speed, still the buzzer would sound indicating a misuse of the engine, and unnecessary wastage of fuel.

    • Hi Swagatam

      Your circuit will be definitely okay but, we have no proper solution to sense car speed.

      In analogue speed-o-meters, I think it will be impossible.
      In EFI cars, might be possible if we can interface with ECU (the car CPU).

    • Swagatam Majumdar, thank you very much for your efforts. how ever, i will appreciate very much if you can shade more light on 1. the function of Diode D1 and Diode D2. and also can you shade more light on the function of P1. thanks you.

    • Thank you Abdul, the D1/D2 are optional and are included to control negative spikes or transients that might possibly enter the circuit causing disturbances in the reading.

      The P1 is used to set the threshold speed or the input frequency where the buzzer needs to to sound

    • Thank you Swagatam for the prompt response. P1 appears to be a potentiometer. Now in your beautiful write up above you say, "P1 is set such that as soon as the output from IC1 rises to a certain limit which may be calculated as the over speed limit value, the potential; at pin#3 rises over 1/3rd Vcc" how is the calculation done to achieve a speed of say 120km/hr. i plan to installed the device in the car of my father. thank again.

    • Abdul, you will need to set the circuit by experimenting it practically, meaning you will need to practically cause the engine to attain a speed proportionate to 120km/hour limit and then at this point carefully adjust the preset such that the buzzer start buzzing, however I think the 555 is not an appropriate device for sensing the threshold, an opamp would work better.

      I'll try to upadte the new diagram soon.

  2. Thanks a million Mr Swagatam, i understand how to set the speed. As i eagerly await the update of the cct using the Opamp, i will have to do some revision of the Opamp device so that i will be able to flow with you in your explanation. It is great to know that someone out there is willing to give you a helping hand. People like you are rear nowadays.

  3. i think i have responded to your last posting however i am not sure. anyway just in case i didn't, i want to appreciate you and your efforts. and to let you know i am still studying the amendment you make using opamp. i will get back to you as soon as i understand how the combination of the opamp and the transistor will acrually operate. how ever why do you chose to use a PNP transitor instade of an NPN considering Vcc is a positive supply?

    • thanks abdul, you can use an NPN at the the output instead of PNP, depends how the input of the opamp are configured.

      for NPN just swap the inputs of the opamp with each other, meaning now pin3 gets connected with the preset and pin2 with zener reference.

      on the output side, the 4k7 gets connected with ground, and the NPN needs to be configured in the usual manner with relay across its collector and positive.

  4. Nice circuit sir, please I wish to ask, where is CDI located inside the car?I can assemble the circuit but don’t know where to locate the CDI thanks n waiting ur help

    • Thanks Nkwenti,

      you can consult any auto mechanic in your area he will be able to tell where exactly the CDI may be located in your car.

      But please build the circuit only after you have understood the stages and functioning perfectly, and first try it on your work bench bfore the final installation

    • Thanks for the reply, i will try to contact any local mechanics, i though i just have to built the circuit as given in the diagram, test it and then install it, anything next should be the speed settings

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