Debugging the digital speedo

Its Sunny !

The British summer turned up and the weather has been good for driving, so while there are over 800 miles on the clock now I've made no progress on the roof. 

Reminds me of an old cartoon in my favourite childhood book: Story Book Dictionary -now- Best Picture Dictionary Ever  by Richard Scarry. 

One of the Busy Town animals has a large hole in his roof:

Wow - just thought - theres my '80/20' rule or 'Good enough' lifestyle choice right there - learnt probably before I was 5 years old ha!

Digital Speedo Circuit Design

The circuit itself is cribbed from a number of forums existing projects etc & I found some great online tools for checking & drawing circuits:

  Circuit Lab allows full simulation over time & 30 day free trial

  Digikey Scheme It allows diagramming for free\

My current circuit intention is something like this:

S1,S2 - Front panel momentary select & reset buttons

R1,R2 - 5k6 pull-ups for the I2C bus

R3,R4 - 10k pull-downs on the switch inputs

R5,R6 - 200R current limiting on the 12v sensor inputs

D1,D2 - 5.1V voltage limiting Zener Diodes on the 12v sensor inputs

Mocked up currently looking like this:

+5v will be via a solid state regulator to drop me down from the car's 12v line

Inputs are 12v current and voltage protected with resistors & Zener diodes.

The illumination input will be to turn the brightness down rather than up - i.e. when instrument illumination is on its dark outside so I want to avoid glare from the LED display; not yet even mocked up.

The display is from ByVac and accepts an I2C/Serial input so I don't need a rats nest of wires/pull-ups to multiplex the 4x digits or continuously refresh it. In theory I2C bus can support multiple devices on its two wires so I could upgrade to a small LCD, Multiline or any I2C output device.

How long does debugging take...

Its been a while since I coded anything, its pretty good fun though;

I think I read somewhere (I could have the numbers completely wrong) you can write about 4 lines of bug free code a day - not through slow typing, just because debugging takes time. Arduino tools are pretty basic too, the debug cycle is to embed print statements in the code, download, run, monitor the serial output - no single stepping :/

Some progress though - the project has expanded & I now have basic menu code working for functions with two input buttons: select & reset. The functionality will be:

• Speed - to 10ths of MPH
• Selectable speed warnings at 30, 50 & 70 mph
• Trip 1 & 2 - to 100ths & 10ths of a mile respectively
• 0-60 timer - starts when the car starts moving or red button pushed
• 1/4 mile timer - starts when the car starts moving or red button pushed
• Calibration - on the fly tweak speed up/down & entry of pulses per mile - could even drive this via the USB/Serial input i.e. console base calibration.
• Sense instrument illumination to adjust brightness

I'm trying to tidy up the timing code for measuring speed based on the time between bolts passing the speedo sensor but its been hit & miss without a proper function generator to test it. I have this rudimentary pulse generating code creating an output on one pin which I feed back into the sensor pin - hence that flying wire on the picture above taking the output back to the sensor input.

Unfortunately - up to a point - I am a perfectionist with code, keep tinkering, re-arranging, checking until it both works and looks pretty.


Update - the digital speedo is on hold until the dash next comes off and I add circuits - which wont be while the weather is so good!