It looks like a pocket...

It looks like a pocket calculator and is wired into the ignition circuit. Before the driver can start the vehicle, they must watch the LCD screen, which asks them to enter a sequence of numbers within five seconds. It can also get them to operate controls such as the clutch and indicators in a certain order. If they complete the task correctly, the car will fire up. But if they make a mistake, they may be asked to try a new exercise. The engine will not start if they fail repeatedly.

To stop passengers completing the tests on behalf of the driver, the gadget can set a new test every time the vehicle comes to a standstill, such as at red lights and junctions.

Invented by Sonny Persson, of Karlskoga, Sweden, the device has been dubbed the "drug-o-lock". It follows on from the "alco-o-lock" system, which prevents drunks starting their motor if they fail an in-car breath test.