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G.G) Digital Tach

Having got an analogue tacho working on the boat it eventually gave up the ghost (when the mechanics became unreliable) and having the information digitally there was one quick way to get a tacho back.

The first step was to hack the back from the broken Tacho with a hacksaw, allowing the guts to be removed and the glass cleaned. A serial interface, 7 segment, four digit LED display module was connected with short wires for +, gnd and a serial connection (that works at 115,200 baud).

The 0.8" LED was bought from Embedded Adventures and fills the tachograph housing nicely. Its nice and bright and the size makes it readable. Interfacing is as simple as print "1234"

(c) Embedded Adventures

Having tested that it worked the display was temporarily fixed to the glass with tape, a small amount of black silicone sealant was added to the edges, ensuring no air was trapped (i.e., no voids created). Once this was set the tape was removed and more silicone added to a depth of approx 1 cm. The rear of the LED display was left open, entirely clear of silicone sealant.

Care was taken to minimize the number and size of any voids created; voids appear as 'bubbles' and affect the look of the gauge. The finished result (let down by the yet to be painted bezel) looked like this:

A superbright LED was also added. The purpose of this is to indicate that the speed measured by GPS is less than or equal to the speed limit on the local waterway; this logic was defined so that the LED will not be lit continuously when not using the waterway, i.e., only display things that are relevant. 

The LED display is driven by the Propeller chip within the project and measuring the RPM using pulses from the ignition leads. 

To make it more readable / usable the code prevents the the least significant digit showing every variation in rpm; it is restricted to being either 0 or 5, using a value of rpm averaged over five samples. It is updated at 4 Hz - a value chosen because it balances the need to have a reasonably steady display and the need for it to be responsive.

The values displayed are right-justified, padding the unused digits with space characters.

Because the display was being powered by an existing 5V supply that already under stress (with the voltage drifting upwards past 7V under load) a 5V voltage regulator was added.