Gear indicator circuit wiring

[boothybunch]

Hello out there,

I am new to the forum and have a problem my head can't get round.

I have just dropped a K75 bmw motorcycle engine into a reliant fox car. Double the horsepower, double the fun

anyway, The bike engine has a gear position switch on the gearbox which uses 4 wires.
yellow / black, Yellow / blue, Yellow / white and a common black earth. Combination of the wires shows which gear your in. I am looking to install a standard numerical display on the dashboard to show the gear, but I can't get my head round (probably down to still wiring the engine electrics)

The numerical display has numerous wires that when earthed show the number or N for neutral.

this is the list I have got for the wires. Bike wires on the left, indicator on the right

Yellow/Black and Yellow/White grounded = 1st gear - Brown & Yellow: 1st Gear

Yellow/Blue and Yellow/White grounded = 2nd gear - Black & Green: 2nd Gear

Yellow/White grounded = 3rd gear - Green & Blue: 3rd Gear

Yellow/Blue and Yellow/Black grounded = 4th gear - Yellow & Blue: 4th Gear

Yellow/Black grounded = 5th gear - Brown & Red: 5th Gear

Yellow/Blue, Yellow/Black and Yellow/White all grounded neutral - Green


Any ideas how to do this and give my head a rest??? Do you think I need to make a printed circuit to sort this or can it be accomplished with relays?

Thanks in advance

 

[crutschow]

One way would be a simple digital circuit using six 3-input OR gates plus 3 inverter gates. Each gear position has a unique combination of outputs which, after going through the proper gates, goes to one wire on the display.

Connect one inverter gate input to each gear position wire output. Also connect a separate 1k ohm pull-up resistor from the battery voltage to each of the gear indicator outputs.

Then, for example, the gear position 1 OR gate wiring is:
YBk to 1st OR input
YW to 2nd OR input
YBl inverter output to 3rd OR input
OR gate output to BrY of indicator
Thus when YBk and YW are grounded, and YBl isn't, then the OR gate will have three low inputs, giving a low output to the BrY wire and energizing the "1" indicator.

Repeat this logic pattern for all the other gear positions, with the gear indicator grounded inputs going to the corresponding OR gate inputs, and the remaining OR gate inputs connected to the inverter outputs from the gear indicator wires that are not grounded for that gear position.

You could use two CD4075 NOR gates and one CD4069U inverter for the circuit. It could be built on a small proto board. (Note: Be sure a ground all unused inverter inputs (not outputs) on the CD4069U.)

You could also do it with three 6PDT relays, but that would be somewhat of a kludge.

 

[boothybunch]

well,

if I can do it with relay's to get me away while I do the test running, it would help me out as I may end up redoing the whole dash later on and integrate the gear position into the gauge cluster.

let me know,

Cheers

 

[crutschow]

If you want to do it with relays then buy three 6PDT relays with coil voltage rating equal to the battery voltage.

Connect a relay coil between the battery and each of the gear selector outputs respectively. Black wire goes to battery common. (Note: Remember to put a diode across each coil to suppress contact arcing, diode cathode to positive side of coil).

Connect all wiper contacts of the YBk relay to ground. Connect one wiper contact of the YW relay to each of the indicator wires respectively.

Now you will need to draw a connection diagram for each relay position so that the proper indicator light is lit for each gear position.

An example for 1st gear is shown below (only one of the six sets of contacts is shown for each relay). Thus for 1st gear YBk and YW are grounded, energizing relays R1 and R3, but not R2. Thus, with the wiring shown, there is a path from BrY, 1st gear indicator, to ground, lighting the 1st gear display.

Do this for the other 5 sets of relay contacts to get the proper indication for the other five gear positions (and relay positions).

 

[OlPhart]

It's your lucky day, sport. I built my first custom BMW out of a K75 back in '99 (BMWMOA.org forum/Kbikes/pix/#58). I've built ~10 K bikes, mostly K75s (a favorite).

You have the answer in your notes: it's a binary coding pattern.
Bit 0 = yellow / blue
Bit 1 = yellow / black
Bit 2 = yellow / white
ground = brown

bit: 2 1 0 display
L L L 0 (N)
L L H 1
L H L 2
L H H 3
H L L 4
H L H 5

Use 10k pullups & a non-inverting buffer to feed a 7447 to a common anode 7 segment display (or equ). Neutral will show as "0" (fair enough). Probably don't need the buffer, either. I'd appreciate pix of your project, as well. Good Hunting... <<<)))

P.S you wouldn't have a spare gear indicating switch? I built the instrument cluster for my latest & goofed the switch by misplugging that side by 1 pin (DOH).