Look here, you might have to order the magazine
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or kit
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If you want to build it yourself you will need something like a PLL to multiply the input from the pickoff to display RPM on a digital counter circuit. The loop constant will have to be fairly long to smooth out the variations in idling speed. You will also need a switch to select various divide by N settings to cater for 4, 6 or 8 cyl engines. This part could be performed by a PIC micro or similar small micro.