Simple rev counter?

[Matty272]

I am attempting to build a rev counter for an inboard diesel boat engine. The installed rev counter has unfortunately ceased to work and needs to be replaced. The owners of the boat will probably shell out for a new one, but I thought it would be instructional (for myself) to try to build a digital replacement.

What I am thinking of is a simple pulse generator coming direct from the camshaft the original place for the transducer,using something simple like a lit lamp/led passing a reversed led/LDR feeding into a pulse counter of some kind.

I can remember some simple circuits from my HND days, but have no idea where my old course notes have disappeared to in recent years. The intention is to clock a two/four part 7seg display every second with the counted pulses coming direct from the camshaft.

transducer ---> pulse counter ---> binary/decimal converter ---> display

I have no idea what I can use for the pulse counter, and (probably) a dedicated IC for the conversion to decimal and display driver. The reason for using only two 7segs over four would be that it really isn't necessary to have the coxn knowing the exact RPM (and its cheaper), although rounding up might be hard to achieve without using a PIC or similar - I don't know.

The count gets displayed on the clock trigger and the pulse counter gets reset by the same signal (delayed by a couple of useconds perhaps?)

Any help in the form of ideas or URL's would be greatly appreciated.

Thanks in advance,
Matty

[Matty272]

Further thinking on this is that I need to multiply the count by 60 every time I want to display the count, otherwise the coxn of the boat only sees the revs per second.

Maybe the output from the camshaft used as a clock to a circuit which adds 60 to the total.

[audioguru]

Hi Matty,
The revs are stepped-up in frequency by a factor of 60 by using a phase-locked-loop IC like a CD4046 and a divide-by-60 divider. The PLL has a voltage-controlled-oscillator that is sync'd to exactly 60 times higher than the actual revs. Then you display the frequency of the VCO.

[Matty272]

PLL, remember completely failing to understand that for days when I first came across it. Have forgotten how that works, will have to investigate. Thankyou Audioguru