Electronic rev counter?

Looking at fitting a rev counter to my latest build, Fancied an old type off the chain case but saw these with the white face etc, Anyone used one of these type rev counter, I presume you connect it to a battery and one lead piggy backs onto the pick up lead from the stator plate?
Did it work or did it interfere with the Lambretta basic electronic ignition system?
Any thoughts greatly received

Did it work or did it interfere with the Lambretta basic electronic ignition system?
Any thoughts greatly received
