Some places will charge you a small fortune for a little signal convertor to turn your 5v ECU tacho output into a 12v spike to drive an older coil-driven tachometer. Do it yourself for a few pennies and a spare old 12v relay you're got kicking about out of your dad's vintage Metro.

Once upon a time I converted a Lotus 7 style kit car to a modern engine, complete with Emerald K6 engine management. But sitting in the old dashboard was an early 90's vintage ETB tachometer (rev counter) that used to be driven direct from the coil on an old carburettor setup. Emerald wanted the best part of £36 for a little converter but, being moderately savvy in electronics, I figured that can't be anything that awkward to do myself. I set about with my knowledge of transistors and did a bit of an internet search, and turned up a diagram related to the Megasquirt system, that suggested using a coil from an automotive 12v relay, coupled with a suitable transistor and a limiting resistor, to drive a 12v output from your 5v ECU output. And so I figured you could get all of this into a relay casing, thus keeping it rather neat and tidy.


There were some assumptions to make in figuring out the resistance needed on the base side of things (I didn't want to assume 1K was correct), but working out that the tacho was unlikely to pull more than 100mA, and that the Emerald ECU was unlikely to want to supply more than 20mA out of one of it's MCU pins, it came to pass that the transistor used needed a minmum hFE gain value of 25 - the 2N5551 covers this fine, with a minimum gain of 30 at 100mA load. The 1Kohm resistor seemed fine. I figured I'd run with it... So, with a 2N5551 transistor, a 1Kohm resistor and some solder at hand, I cracked open an old 12v 30A relay - and there was the coil I needed. Of course, I didn't need all the switching capability, so I ripped out the paddle and braided wire that formed the actual switch within the relay. I then soldered up across the plates in the relay to achieve my desired wiring - here's a crib sheet on what you solder to where, including what the 4 numbered pins will relate to once you're done...

The whole caboodle fits easily within the original plastic case of the relay, keeping it all rather neat and tidy. You wire pin 86 to a suitable 12v supply, pin 30 to earth, pin 87 is the 5v trigger coming in from your ECU, and pin 85 is your 12v spike out to your old-style tacho. I figured there's no way this could be it, could it? I mean, surely, 36 quids worth of converter does something magical that my <£1 circuit couldn't hope to match? Bollocks does it - plugged it all in, worked a treat, and I now have a rev counter again. DIY... a very fulfilling experience!

Relay used to house the components