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.

Technical Specifications:

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 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.)

Simulation and Working

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 Diagram

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 Diagram


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