I've thought up another project that i probably ought to do before I do the gauges in my camry - for practice, as it were. I'd like to make a very simple pair of indicators which will take 2 lm3914's to make. For my miata (mx5), I want a simple bar graph to indicate throttle position and brake position.

Before anyone else says it... Yes, just like a video game. I'm a Gran Turismo fanatic.

The practical purpose for this is actually just like it is in the video games - to easily be able to see what I was doing with my feet... and it will be visible in in-car video shot at the track. I would mount the twin bargraph high on the dash so it would be visible to the camera mounted on the roll bar, and the further i press each pedal, the more LEDs light up.

the car has a factory throttle position sensor that i could probably borrow a signal from, but there's no such thing on the brake pedal. Anyone have any suggestions on what I could use for the pedal position sensors? Obviously some sort of variable resistor or severe duty trim-pot... hey, what about a household dimmer knob/slider?

am i overcomplicating this by trying to use a pair of lm3914's or do you think that's probably the best solution?

 

If your car has drive by wire system, perhaps you have a something you can hook into at least on the throttle. BMW used a drive by wire system already in 1989, and I think most modern cars have it now. For the brakes it is perhaps more easy to measure pressure in someplace in the break system.

 

the mazda mx-5 miata from 1990-2005 isn't drive by wire. it's got a conventional throttle cable, though there IS a factory throttle position sensor on the throttle body, and i could easily tap into this signal in the cabin at the engine computer. dunno how regulated that signal is, but if it's good enough for the engine computer, it's probably good enough for my little bar graph.

the only electrical on the brake pedal is the brake light switch - obviously this isn't enough to run a bar graph, though i could use it to turn the bar graph completely off when my foot is off the brake. i hesitate to modify the hydraulics of the brake system - i really don't want to add another possible point of failure that might go south on me at the race track. i'd much rather do something like zip-tie or glue/clamp a potentiometer to the brake pedal arm and/or bracket. the question is, what can i use to accomplish this? all the trim pots i can think of are way too small and fragile for this application.

 

Perhaps it is more correct to measure the force used on the break pedal, not position. As it is for the throttle.
And also I have to say that BMW did not use drive by wire on most models in 1989. They used it only on the very expensive 750 model. And in 1989 a drive by wire solution was very exotic. That is why I remember it

 

i think that force and pedal position will be directly related to each other when considering the brakes. as you push the pedal farther in, the brakes clamp down harder and with more force, and as you lift a little, it clamps with less force.

i also do not expect the bar graph to be at 100% while at the threshhold of max braking. my car does not have ABS. If i simply stomp the brakes to the floor, it locks up the tires and i can no longer steer. if i brake to about 75% or so (all by feel of course, it could be even less), i can brake more effectively. obviously i won't be looking at the bar graph while on course, but when reviewing video later i can see how hard i hit the brakes and whether it was too much, not enough, too early/late, and how consistent i am from lap to lap. this works for gas too - did i get on the gas aggressively enough exiting that turn? i could even put a position sensor and bar graph on the clutch to see how consistent my shifting is or isn't.

i agree - drive by wire was pretty exotic in '89... but now it's standard on even cheap cars like a civic. thankfully none of my cars have it. it's easier for me to work on... kinda hard to open the throttle when underhood if there's nothing to move by hand!

 

Better not to tap into microcontroller sensor lines. If you do, isolate them. You can achieve it it in many ways of which a variable resistor would be the easiest. A pot with a gear transfer would be easy, but you know, the throttle is the most used device in an automobile and it would wear that pot very often. You can use a fragile pot with a gear ripped from something else to drive it linearly. The best way is to use a pressure sensor/transducer fitted parallel with your pedals to the Lm3914. Other methods include IR proximity detection. There are lots of circuits online. You ca nuse the comparators they include and supply the levels to 3914 as the proximity changes.

I am attaching a ckt. You can tap into the line after D4 and supply it to LM3914. Adjust the Vref suitably.

 

interesting. the right side of that ckt appears to be about 1/3 of a 3914. nice. infrared means no moving parts to wear out, but is suceptible (sp?) to dust/dirt buildup.

that said, it looks like this might be simpler: http://www.imagesco.com/sensors/flex-sensor.html - mount the connector end to the pedal bracket and zip tie the free end to the pedal arm. has anyone used these things before? what kind of resolution do they provide?

oh, even better: http://www.imagesco.com/sensors/stretch-sensor.html - i could mount one end to the floorboard, both providing a ground and a mount point, then zip tie it to the pedal arm to keep it electrically isolated. I would make it so that when the pedal is fully depressed, the stretch sensor is completely relaxed, and when the pedal is fully released, it stretches the cable to increase resistance.

does anyone know how durable these stretch sensors are? do you think it would be an issue to leave constant tension on the stretch sensor?

 

I dunno why i didn't think of this before. I have a set of video game racing pedals (missing steering wheel - lost during a move) sitting in the garage. I could cannibalize that for the pots in the brake and gas pedals, EASY. since it's a video game controller specifically made to emulate vehicle pedals, they ought to be fairly durable. i just hope i can retrofit the pots onto actual pedals. this should be a piece of cake!