# The higher of two voltage signals "wins"?

I'm not an electrical engineer - far from it. I own a driving school. Essentially I have the electronics knowledge a 10 year old might gain from an educational "electronics lab kit".

I am trying to create a passenger-side accelerator pedal [Edit: I meant I'll use an additional off-the-shelf pedal] to work in conjunction with the driver's accelerator pedal. Each will produce a different voltage signal, and I would like the higher of the two voltages to go to the car's ECU.

I THINK I can use diodes from each of the two signals, and then join the output side of each diode, then connect that junction to the input of the ECU. I think the signal seen by the ECU will always be the higher of two voltages, is this correct?

I have searched, and I have watched videos about power supplies of different voltages in parallel, and I've learned this is "a bad thing", but it seems most issues can be avoided with diodes. (Of course these are competing voltage signals, not power supplies but same idea.) What I haven't learned is the resulting voltage output at said junction of diodes.

I could do it with a microcontroller with ADC's and a DAC. But if it's a simple matter of two diodes, that's FAR simpler and eliminates a point of failure.

Any help much appreciated. Experimentation under a real-world dashboard is difficult for my 56-year-old lower back, and potentially it could be very costly if I mess it up, so I'd rather have a bit of confidence that the approach might work before I go down this rabbit hole.

In an effort to avoid going on irrelevant tangents, I'll answer unrelated questions before they're asked:

• Yes, the brake is the most important thing but it's handled mechanically, using wire ropes and pulleys which pull down the driver-side pedal, so it all uses the vehicle's existing brake system.
• Yes, it's helpful to have a passenger-side accelerator, not just a brake. Not every driving school does this, but we do. (Up until now, our accelerator pedals have worked similarly to the brakes -- Mechanically with wire rope and pulleys, but the better approach is clearly to do it electronically.)
• No, it's not a concern if the student wants to go faster than the instructor, because the instructor's brake can easily be used to disable accelerator pedals in modern vehicles -- If your car is recent model automatic, you'll notice when the brake lights come on, the throttle does nothing. Even in older vehicles, well-maintained brakes can overcome power from an automatic transmission at full throttle.
• Keep in mind that diodes have some voltage drop. If you can tolerate this (i.e. 200mV of drop with a good Schottky diode), this will work. Commented Oct 30, 2022 at 21:09
• Do you have a wiring diagram of the cars accelerator pedal? Do you know what range of voltages it is meant to produce? Commented Oct 30, 2022 at 21:09
• Maybe off-topic but, what will your insurance company say if they find out that the car's controls have been hacked by somebody with "the electronics knowledge a 10 year old might gain from an educational electronics lab kit?" Commented Oct 30, 2022 at 21:10
• Jonathan, thank you, but the voltage drop could be a problem. I think I've seen ways around this. Andy, Yes. 6 wires which go to two hall sensors. For each there is 5v, gnd, and the signal. I believe they work in parallel for redundancy. The new pedal itself will be off-the-shelf. Solomon: If it works, they'll have nothing to say. They're aware that accelerator pedals are added, and I was the installer with no particular qualifications (using pulleys). Electronic versions exist, but the off-the-shelf kits which do this do not work for me for other reasons. (Physical layout mostly.) Commented Oct 30, 2022 at 21:42

This is extremely dangerous, especially since you admit you don't understand much about electronics.

A short circuit (or even an open wire) could cause the ECU think the accelerometer is fully depressed and therefore the vehicle would accelerate uncontrollably.

Secondly, the accelerometer pedal doesn't generally produce a simple 'voltage signal'. Usually (for safety reasons), the ECU tries to perform diagnostics on the wire and its inputs to validate the signal. So, either this won't actually work, or if it would, it is a bad idea because your vehicle isn't safe anyway.

• A simple foot on a brake pedal, by either person, stops all modern vehicles regardless of accelerator pedal position. I already use accelerator pedals which work with cables (wire rope) in housings, over pulleys... Now THEY are troublesome! Cables fall off pulleys, or bind in the housing. If (when) they bind up I simply stop the car, end the lesson, if needed I cut the cable and if the driver's side isn't stuck I take the student home and fix it. You seem to be assuming I'm building my own gas pedal... Nope, using an off-the-shelf unit with pinouts identical to stock. Commented Oct 30, 2022 at 21:48
• To be clear it's an accelerator pedal, not an accelerometer. And yes, it actually produces two analog voltages from 0-5v, with two wires from two hall sensors which exist for redundancy. For each sensor there is reference 5v, ground, and the signal, so six wires total. Commented Oct 30, 2022 at 21:54
• Could you post the part number or datasheet of the acccelerator? Commented Oct 30, 2022 at 22:00
• jonathanjo, there are many throttle position sensors, (TPS) and I haven't selected a specific model. The majority of them are similar with 6 wires including two wires showing analog 0-5v and the other two 5v and ground for each of two sensors. Inside is usually hall sensors, not potentiometers (which it seems to me would be a bad idea for wear and tear). Commented Oct 30, 2022 at 22:09

I don't know the setup exactly because there isn't a schematic applied to this question. But in case the pedal is just a potentiometer, you create a variable voltage divider with two pedals of the same type and with the same power source. I don't think is a good idea.

It is also possible that the pedal outputs a PWM signal. When you using two pedals at the same time it can produce mixed frequencies which is not a good idea.

\$ 1 0.000005 10.20027730826997 50 5 50 5e-11
g 112 384 112 448 0 0
174 112 112 400 96 1 1000 0.9653 Resistance
174 112 336 400 336 1 1000 0.401 Resistance
d 400 112 400 192 2 default
d 400 336 400 256 2 default
w 400 192 400 224 0
w 400 256 400 224 0
w 400 224 496 224 0
w 112 384 496 384 0
w 496 384 496 272 0
R 112 224 32 224 0 0 40 5 0 0 0.5
w 112 224 112 112 0
w 112 224 112 336 0
w 496 224 560 224 0
w 496 272 560 272 0
181 560 224 560 272 0 304.8680427916879 100 120 0.4 0.4
w 256 320 256 288 0
w 256 288 192 288 0
w 192 288 112 336 0
w 256 96 256 64 0
w 256 64 192 64 0
w 192 64 112 112 0
o 7 64 0 4099 2.5 0.0125 0 1

• Thank you. I will not build my own pedal. I will buy one similar to stock. It is not a PWM signal. Inside are two hall sensors, not potentiometers. (My original post didn't mention there are two for redundancy, simply because it's not relevant that I'd do the same thing twice.) Commented Oct 30, 2022 at 22:17
• "Inside are two hall sensors"- @PaulOTron2000 that complicates the situation significantly. You have 2 different signals fro each sensor and they don't both increase or decrease together, so "The higher of two voltage signals" is not going to work for you here. Commented Oct 31, 2022 at 0:17
• It's my understanding that the two voltages (from within one pedal) are offset by around 1.5 volts and that they are generally in sync otherwise, BUT... You're right, I can't just take the highest "signal 1" from the two pedals, and also us the highest of two "signal 2's" because I can't trust that the two highest voltages will always be from the same pedal. In conclusion: "Aw, crap." Commented Oct 31, 2022 at 4:39
• ...although, it may still be an option to use a microcontroller. Commented Oct 31, 2022 at 4:50