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@judenonesuch
As I've just spent a few moments doing this I thought I'd share it with you all: (if you're riding fixed) how to get any old cheap cycle computer to display cadence, without having to attach things to your crank arms or anything (maybe people like attaching things to their crank arms, I don't know).
When you're setting the wheel circumference (assuming for the purposes of this that it is input in mm, if not a further straightforward conversion will be required), input the value given by the following formula:
(Sprocket teeth ÷ chainring teeth) x (number of mm in chosen display unit ÷ 60)
i.e. if your display is in mph then "number of mm in chosen display unit" is 1609344 as there are that number of mm in 1 mile (apparently) and for kms then it's 1000000; the dividing by 60 is the conversion from hours to minutes to give rpm.
Then as your cadence is likely to go into 3 figures, and cycle computers don't have a 3-figure display, you will want to divide this number by 10 so at 120rpm the display will read 12.0 usw.
This has the advantage of being able to easily reset the computer to give speed/distance etc. simply by resetting the wheel circumference to its true value, and you can leave the magnet and sensor where they are on the front wheel (n.b. this only works if your `sensed' wheel and your driven wheel are the same circumference!) It is interesting to note that the figure you input depends only on your ratio (and the unit conversions) and is independent of wheel circumference, which makes sense if you think about it but also takes a bit of getting your head around! (or it did for me at least!)