Speed limit Alert Circuit

Here’s a design circuit that can be used to alert the vehicle driver that he/she has reached the maximum fixed speed limit (i.e. in a motorway). It eliminates the necessity of looking at the tachometer and to be distracted from driving. There is a strict relation between engines RPM and vehicle speed, so this device controls RPM, starting to beep and flashing a LED once per second, when maximum fixed speed is reached. Here’s the figure of the circuit;

In this circuit, IC1 forms a differential amplifier for the electromagnetic pulses generated by the engine sparking-plugs, picked-up by sensor coil L1. IC2A further amplifies the pulses and IC2B to IC2F inverters provide clean pulse squaring. The monostable multivibrator IC3A is used as a frequency discriminator, its pin 6 going firmly high when speed limit (settled by R11) is reached. IC3B, the transistors and associate components provide timings for the signaling part, formed by LED D5 and piezo sounder BZ1. D3 introduces a small amount of hysteresis.

D1 is necessary at set-up to monitor the sparking-plugs emission, thus allowing to find easily the best placement for the device on the dashboard or close to it. After the setting is done, D1 & R9 can be omitted or switched-off, with battery savings. During the preceding operation R8 must be adjusted for better results. The best setting of this trimmer is usually obtained when its value lies between 10 and 20K. You must do this first setting when the engine is on but the vehicle is stationary. The final simplest setting can be made with the help of a second person. Drive the vehicle and reach the speed needed. The helper must adjust the trimmer R11 until the device operates the beeper and D5.

Reducing vehicles speed the beep must stop. L1 can be a 10mH small inductor usually sold in the form of a tiny rectangular plastic box. If you need an higher sensitivity you can build a special coil, winding 130 to 150 turns of 0.2 mm. enameled wire on a 5 cm. diameter former (e.g. a can). Extract the coil from the former and tape it with insulating tape making thus a stand-alone coil. Current drawing is about 10mA. If you intend to use the car 12V battery, you can connect the device to the lighter socket. In this case R20 must be 330R. Depending on the engines cylinders number, R11 can be unable to set the device properly. In some cases you must use R11=200K and R12=100K or less. If you need to set-up the device on the bench, a sine or square wave variable generator is required. To calculate the frequency relation to RPM in a four strokes engine you can use the following formula:

Hz= (Number of cylinders * RPM) / 120. For a two strokes engine the formula is: Hz= (Number of cylinders * RPM) / 60. Thus, for a car with a four strokes engine and four cylinders the resulting frequency @ 3000 RPM is 100Hz. Temporarily disconnect C2 from IC1 pin 6. Connect the generator output across C2 and Ground. Set the generator frequency to e.g. 100Hz and trim R11 until you will hear the beeps and LED D5 will start flashing. Reducing the frequency to 99 or 98 Hz, beeping and flashing must stop.