Anyone who drives a car inevitably exceeds the posted speed limit on occasions, either deliberately or due to lack of attention. But these days, it’s really not a good idea to speed. Apart from the obvious safety considerations, there are lots of speed cameras about and it’s all too easy to cop a heavy fine and maybe even lose your licence.

You don’t have to be a speed demon either. On a long trip, your speed can gradually creep up as you become used to the road conditions. It’s also quite difficult to stick to the speed limit in a 60km/h zone after you have been driving at high speed on the open road – 60km/h seems agonisingly slow after driving at 100km/h.

In this situation, a speed alarm can keep you on your toes and ensure that you stick within the posted limit.

Another situation where it’s easy to inadvertently exceed the speed limit is if you using a cruise control. Now while cruise controls are a great help when it comes to maintaining a set speed, they do have one inherent limitation – the speed of the car can increase beyond the set limit on downhill stretches. Once again, a speed alarm can instantly warn you when you’ve over-stepped the mark.

Main features

Our new Speed Alarm is quite compact and fits neatly into the smallest available jiffy/zippy box. By contrast, our previous Speed Alarm (described in the December 1997 issue) used a case this size just for the display circuitry. The rest of the circuit was housed in a separate instrument case and while it was OK for large vehicles, it wasn’t all that easy to squeeze into the average family sedan.

All the parts fit on two small PC boards which are housed in a compact plastic case. Note the black cardboard sleeve around the 7-segment displays in the photo above. This prevents light leakage from the LEDs adjacent to the pushbutton switches from spoiling the appearance of the readout.

So how have we managed to shrink the circuitry so dramati-cally? The answer is that we have replaced all the discrete control circuitry with a low-cost PIC microprocessor and come up with the necessary software to control the internal "smarts" of this device. The resulting circuitry all fits on two small PC boards which are stacked inside the case.

It’s also just as easy to drive as before. As shown, the front panel carries a 3-digit LED display, a LED indicator and three pushbutton switches. Two of these pushbuttons let you set the alarm speed in 5km increments between 0km/h and 155km/h (one switch increases the speed; the other reduces it). As soon as you exceed the preset speed, the indicator LED lights and an internal piezo alarm briefly sounds at 10-second intervals to provide a warning.

The third switch selects between three display modes: (1) the alarm speed value; (2) the actual vehicle speed (ie, the unit functions as a digital speedometer); and (3) the alarm off mode. Each press of the switch cycles the unit to the next operating mode – it really is that easy to operate.