Vehicle Speed Limit Alarm Circuit

In our previous articles we have learned comprehensively regarding the ICs LM2907/LM2917 which are fundamentally frequency to voltage converter ICs, and are ideally applicable in all such relevant fields. Here we see how the same chip can be applied for making a vehicle speed limit alarm circuit.

Using a Single IC for Speed Detection

As per the explained example the IC could be used for making a simple speed limit switch circuit which in turn could be applied in vehicles for detecting over-speeds and consecutively alarming or preventing the vehicle from crossing the set dangerous mark.

As shown in the below diagram a single LM2917 is enough for designing the proposed speed limiter circuit along with a few external passive parts.

Referring to the diagram, the input pin#1 receives the signal from the wheels of the vehicle through a magnet and pickup coil arrangement or through a Hall effect sensor circuit.

Circuit Operation

You may refer to this article for knowing the basic set up of the wheel and pickup coil in the given diagram.

The function is pretty much identical to the previously explained speedometer circuit functioning:

The applied speed info in the form of pulses corresponding to the rotating wheel count is sensed by the differential opamp whose inverting input is referenced to ground for maximum sensitivity.

The output from this opamp is fed to the next sage which is the charge pump stage responsible for tracking/holding/boosting the info in the form of a steady DC.

This function relies particularly on the value of C a pin#2 of the IC.

The above info is yet again amplified and compared by the subsequent opamp and common collector transistor stage.

The output is terminated at pin#4 of the IC via the emitter of the internal common collector transistor.

Formula for Setting the Speed Limit

As per the formula, the speed limit alarm can be set and calculated by using the formula:

f(in) = R2/R1+R2 x 1/RC

Once the set limit is reached, the IC detects it and produces a high logic at pin#4 which is equal to the supply voltage of the circuit.

This high logic may be used for sounding an alarm, deactivating the engine or for triggering other similar preventive measures.

Circuit Diagram

A typical implementation of the above circuit can be witnessed in the following diagram.

Here the load or the speed alarm unit is connected at the transistor collector pin#5 output instead of pin#4 as narrated in the above section.

The collector load configuration provides the advantage of better current gain allowing a higher wattage device such as a relay or an alarm to be directly used with the circuit.