The Keyring Breath Tester is a 3 step alcohol test that (as the name suggests) you can keep on your keyring. It's just the thing you need when you're out on the town, and it's incredibly easy to use. Just hold down the power button until the LED turns green and blow across the top of the grill. If the light remains green, then you're under 0.05% BAC*, and in theory in the clear. If the yellow LED comes to life, then you need to use caution (and probably a bus) as it means you're hovering dangerously close to the legal alcohol limit of 0.08% BAC. If the red and yellow LEDs come on, then you should throw your car keys into the nearest river and start weaving your way home as you've had far too much to drink**. If you were silly enough to take our advice and throw your car keys away then you'll be thankful for the breath tester's built-in torch (unless it was attached to your car keys of course).

If you need to ask whether you're too drunk to drive, then you probably are, but rather than trying to remember how much you've drunk in those unfathomable units, just blow on this gadget to find out.