Project: Vehicle Control System

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Thread Starter


Joined Feb 9, 2011
Hey guys. I am new here, and to electronics. I come from a more software-based environment. About a month ago, I got an idea to build a vehicle control system. That is what I call it anyway. The concept was to be able to use rfid tags implanted in the skin to unlock the doors. Nothing implanted yet, but my MCU and tag reader recognize tags and can differentiate between an accepted vs unfamiliar tag.

The second part of the project was to be able to communicate with the vehicle mounted MCU via bluetooth where I could control various actions such as remote start, rolling the windows up/down, door locks, lights, and horn via a program on my Motorola Droid. I have the software written for the MCU (Parallax Propeller) and phone to have two way comms, and a simple system that executes commands as they come in seperate cogs (cores) to have a form of multitasking.

Sorry for the drawn out stuff above, just trying to establish what I want my system to do. Obviously I've gotten to the hardware part of the project. My thoughts were to use relays to switch existing circuits, and place those relays at the location of those circuits, so that means fairly long (5-10ft) runs of control lines from the MCU to the relays. I figured 12v relays might be best because of distance (just an assumption here).

I've also heard that Propellers as a rule, do not like vehicular environments because of noisy power, spikes, fluctuations, etc. The power part I can take care of, but I was thinking of using optocouplers to isolate the MCU from induced noise/protect it in case of an "oh crap" failure ($0.25 opto vs. $8.00 MCU + S&H, lol)

So here it is, the propeller runs on 3.3vDC and that is what its pins output. I was thinking of a circuit that goes...

1. Propeller 3.3vDC --> Optocoupler.
2. Optocoupler switches low current 12vDC to a larger transistor.
3. Transistor switches larger current (@500ma at 12vDC) to power a relay (don't forget a freewheel diode).

Does this sound like a good way of doing this, or are there better and simpler methods? The 500ma is going on the assumption that a lot of auto relays require up to 90-200ma for the coil, and that to save pins and circuit board space on line may control up to two relays. Not to mention it doesn't hurt to have overhead for voltage drop (again assumption).

I have TIP120s and 122s, and 4n25s and 4n33s to play around with, but most of my tinkering has the TIP120s getting super hot with anything over 100ma flowing through them at 9v. Then again, its hard to tell because at that current consumption, voltage falls steadily making current creep up. I'm just using a battery at the moment for quick circuit tinkering. I have a 35amp 12vDC supply but it isn't where I'm at this moment.

If you can help, I'd appreciate it, even if it is just pointing me in the right direction. I love to learn. Nothing above is set in stone.. and most of it is based on my assumptions based on limited knowledge rather than fact. Thanks for even just reading this monster long post.

- T.J. Davis


Joined Jul 17, 2007
Please have a look at the Automotive Guidelines "sticky" thread.

Your project brings up many safety issues; "control various actions such as remote start, rolling the windows up/down, door locks, lights, and horn via a program on my Motorola Droid."

Normally, a vehicles' engine can only be started when a human is in the drivers' seat, turning a key. If a remote start capability is added, the safety aspect of a driver being present is removed.

Rolling the windows down unattended is not likely a problem, but rolling the windows up, is.

Door locks are also a safety item. Should your circuit/programming/reception of interfering RF cause the locks to unlock while driving and a crash occurred, the likelihood of doors opening (reducing body structural integrity) would greatly increase the probability of injuries to driver and/or passengers.

Lights and horns are also safety items. A horn sounding at some random point may cause drivers to become distracted, leading to an accident. What happens if you're driving at night and your Droid turns your lights off?

There are just too many safety issues with your project.
I don't think we can help you with it.


Joined Apr 5, 2008

As SgtWookie told you, safety should be garanteed.
There are to many safety isues in your project.

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