In my younger Electronic Engineer days, I was forever building and designing circuits, either for work or for my own use. Nothing too complicated (unless you cound the serial to midi interface for the PC which I finished but never got round to testing) but mostly stupid things that were more fun than practical.

So I had an idea that rather than spend 70-odd quid on buying a pre-built indicator, I've decided to have a go at making my own. I've done this before for a Scalectrix track as part of a training project, believe it or not, so it shouldn't be that hard.

This is a pre-built DataTool rev light which will cost you about
70 quid and what I'm hoping my home-built version will look like...

On this page I'll document exactly what I've done to research (pah!) and build my own LED tacho so that you can copy it and save yourself a few quid as well.

25th Nov 2002
Right, I've finally got motivated and put this page together after doing some initial Google searches, but not coming up with anything that looks promising. As mentioned above, I have done a similar project before and I know I've got the write-up of this stashed away in a box in the garage which I can't be arsed to go and get at the moment because (a) it's nearly 11pm and (b) it's cold outside and (3) the car is blocking the garage and I'd have to go upstairs and get the cars keys then move it and then find the box and then find the right folder and then put the car back and I'm not even sure if it would work with the output for the Ducati tacho yet, so I'm not going to do it.

Plan for tomorrow: take multimeter out into the garage (remembering not to park the car so that it blocks the garage) and have a fiddle with the feed for the Ducati tacho to see what it does. It may be that it gives me a nice variable voltage (rather than a varying pulse) which would make this *very* simple, but I doubt that!

26th Nov 2002
More intarweb Googling has found that the DataTool version is quite sophisticated, much to my dismay! It looks like it's not only programmable (which it would need to be to take into account different rev ranges, but I can live without) but that it also has some different modes as well - accumulative (build-up lights), sequential (one light at a time), flashing, etc. It also has a dimming function, but that shouldn't be too hard to build in with a variable resistor to control the LEDs. I hope.

Luckily, the searching also came up with this great link which has exactly what I'm looking for - a complete circuit diagram for an LED tacho! This guy has taken the time to create his own circuits using common components and calculate everything, including some tips for making the circuit more adjustable using a variable resistor. This was something I was thinking of doing anyway (for fine-tuning) so at least it proves I was on the right track.

So I will source some components (hopefully from my parts bin) and build a prototype to see if I can get the basic tacho working, then I will modify it from there.

28th Nov 2002
Searching through my parts bin has come up with enough LEDs for the tacho - 3x Green for 3,4,5K, 3x Yellow for 6,7,8K, 3x Red for 9,10,11K and a ultrabright blue for the final 12K+ as the bike redlines at 12,500rpm. The idea would be to use the ultrabright as the shift light out on the track. These ultrabrights are really noticeable, so it should look quite good. I've also got some of the parts and some nice stripboard to mount it all on, but first I need to map out the circuit onto the stripboard and source the remaining parts.

4th March 2003
Ok, I've been really slack about updating the page, but an email from Kevin in the US (of all places!) has prompted me to write up what I've done since the last update.

I started to lay out the components to put them onto the strip-board when I noticed that the circuit diagram I linked to (above) had an error on the circuit for the LM3914 LED driver, so I looked into the technical info for the chip on the RS Components website to find out what was wrong.

Reading through the details, it says that this chip also has a "dot mode" which means that a single LED is lit to indicate the scale rather than the default "line mode" where all previous LEDs are also lit. To do this, it's simply a case of adding a switch to the LM3914 circuit which I am going to do as I think this will be quite a cool feature and also one of the features of the DataTool LED rev light that originally kicked off this project.

The other thing that I wanted to do was add a brightness adjuster to the circuit to ensure that the LEDs weren't too distracting (bright) at night and still visible during the day in bright sunlight. Again, the technical info on the LM3914 circuit helped here as it shows how to set the output voltage for the LEDs. One variable resistor later and you've got yourself a brightness adjusting dial for the LEDs.

With both the mode switch and the brightness adjuster, I want to take into account that you may want to change these while you're on the bike and obviously with gloves on too. I've got a really nice push switch that I may use for the mode switch, although it is slightly bulky so I'm in too minds whether I should use a much smaller switch to keep size to a minimum. I have the same feelings with the brightness adjuster as it's more important that this is easily accessible on the bike, unlike the mode switch or the reference voltage adjuster (which will be a very small adjuster) which won't be used very often.

So, I've got a little bit more thinking to do before I get the soldering iron out, but the next step will be building the circuit and then testing it!

The only thing that was really missing from the circuit above was an 8v regulated power supply, taken from the 12v output from the bike. A quick Google and there is a nice little (read: cheap) circuit over here which uses a 7808 with a couple of capacitors to provide a good 8v supply from 10-25v. I'll also put a 1N4007 on the supply line (12v side) to protect all those expensive ICs from wiring it up round the wrong way!

28th March 2003
Bugger. Still haven't finished this thing yet and it's been nearly 4 weeks since I looked at it last time. I've got all the components now and I've even soldered most of them to the board, but it was at the point of putting the LM2917 into the board that I realised that RS had sent me the 8 pin version, but the circuit (above) is for the 14 pin version! So A quick redesign later and it should be sorted ... fingers crossed!

I'm thinking about what to do with the LEDs as obviously they need to be located somewhere where you can see them while sitting on the bike. So do I ensure that the board is small enough and solder the LEDs to the board (easiest) or do I need a load of trailing wires so the LEDs can go anywhere and you can mount the board somehwere else? I need to have a look around and see if I can maybe mount the LEDs onto some flexible plastic or similar as this will also make it easier to mount onto the bike. I have the same dilema with the switches and trimmer pots for the board - do I make these easy to use with gloves on, but they'll be big, or small and out of the way? Decisions, decisions.

1st August 2003
Ok, so I haven't done much with it lately as I've been busy with other projects (mainly baby #2) and this one has been put into the drawer. I've also been very naughty and managed to get a DataTool RevLight for 30 quid (about $45) off ebay, which I have actually fitted to the bike. I haven't got round to programming it yet, but it does look cool so far! Thanks to Rocket-Ron for the tip-off!

For all of those that have emailed me, don't worry - I will be completing this little project at some point and I will write it all up with a component list and a pcb layout so you can make your own. I appreciate all your comments (both the good and bad!) so feel free to drop me a line.