Wireless Tachometer schemetic and questions on the Pickup?

[thudpucker]

I see a Wireless Tachometer. Also an Infra-red Tachometer.
Both on Ebay.

What 'pickup' do they have? I thought I'd make one, if it's not too complex.
Thanks for any advice (other than buying one) that I may use.

 

[bsprajc]

Maybe they use an inductive pickup on the coil wire, feed into a clamping diode and a rectifying diode, fed into an opto-isolator... ?

 

[jiripolivka]

Without details it is impossible to answer your question. Some tachometers use an induction coil and a magnet on the wheel, others use IR sensors to trigger tachometer circuit.
The common in most tachometers is that their sensors generate pulses, some once per wheel revolution, others many pulses per revolution. You can buy the cheapest and try your sensor with it. I would start on my friend's tacho, disconnect the sensor and try how it operates, Then buy a similar type.