I am trying to build a 12 volt circuit and am coming up against a few obstacles.

I am wanting to control a servo motor that has 5 different preset stops (actually off a car aircon unit and it was used to control the position of air flow flaps for face, feet demist etc). I will be using this servo motor to select gears on an automatic transmission but I am coming up against a few safety concerns and a few switch limitations.

The first thing to note is the servo motor has a permanent 12v power supply and earth and the different stop positions for the servo motor are activated by bridging the earth to various pins. eg by earthing pin 2 the servo moves to position 2. Earth pin 3 and the servo moves to position 3.

I first thought I'd use a rotary switch to select Park, Neutral , Reverse, Drive, Second, First, by providing an earth circuit via the switch to each of the relevant terminals. The first issue is: What if someone bumps the switch into reverse when you are driving along the road. The servo would try and select reverse. Ouch!!

I can overcome this by ensuring the switch isn't operational unless the brake is applied. There is still a problem here though because if the switch is bumped into Reverse and the driver doesn't notice it and sometime later the driver brakes for a corner.....again the servo would try and select reverse. Ouch!!

I then thought I could use momentary on push button switches and still wire it through the brake light circuit, if someone bumps the switch nothing will happen as long as the brakes aren't being applied and because the switch is a momentary on it won't stay on so there is no risk of the driver braking sometime later and the switch activating. There is still a risk here though (a smaller risk, but a risk all the same). What happens if the Reverse switch is bumped just as the driver is braking to go round a bend. Again the servo would try and select reverse. Ouch!!

The latest idea now is to have another momentary safety switch so that the driver has to push the safety switch and the Reverse switch while having his foot on the brake to get the servo to activate. This is sort of how a car works anyway you put your foot on the brake, push the button on the shifter and move the lever. The chances now of an accident must be negligible you are not likely to bump two switches in unison and have your foot on the brake.

What I am still struggling with now though are the following;

Could the safety switch be activated (and have a flashing LED to show its active) and hold on for say 5 seconds, so that the driver can press the safety and then push another button to select the gear while the safety switch is active?

Can I then have the gear selection momentary switch stay active until the gear is selected as it takes a few seconds for the lever on the servo motor to move from say Drive to Reverse. With a momentary on switch if it isn't held for long enough the servo motor will stop mid travel as soon as the earth is removed and may well stop mid way between gears or in the wrong gear. How can I keep the earth connected until the servo reaches its stop? Note the servo automatically turns itself off when it reaches the stop even if the earth is left applied. However you can't then select another position without first removing the previous earth connection.

Last thought is to have a self canceling row of push button switches. The button could stay down and the car would stay in that gear. Then when you push another button to select another gear it would cancel the previous one out. The problem here is I can't find a 5 button self canceling 12 v switch gang. If I did use this set up I would still use a safety switch and again I would want that to stay active for say 5 seconds.

Any help with any of the options to overcome the safety issues would be a big help.

Thanks.

 

Such a set of mods brings up many potential safety issues. If anything malfunctions in traffic, the result could be fatal. Out guidelines prevent us from discussing this further -

Guidelines for automotive electronics help

1. Modifications to the ECU or engine sensors are not topics for discussion. Too many safety and legal issues come up. Air quality and safety trump performance.

2. Any homemade wiring modifications to signal or head lighting, or penetrations through the fiewall are not for discussion. The lighting function is to too critical for non-approved wiring and connectors. Many safety issues are involved. Preventing chafing in wires passing through driver-made holes is impossible to guarantee.

3. Add-on items that exist in the passenger spaces is generally okay. Exceptions are lights flashing to music and similar driver distractions. Assurances that the color organ will only get operated while sitting still is not credible.

Each OP should keep in mind the very public nature of these forums. While he may be able to install requested changes in a safe manner, there are others that may be tempted to do the same with no prior experience for guidance or proper tools for the work.

One further constraint to advice/help exists. We are international, but mostly have an audience here in the US. That means that we have to insist that aftermarket equipment must conform with US federal guidelines.

We have to choose a path between withholding all advice for fear of any possible harm, and simple telling anybody everything they wish, as they will only get the information on some other forum anyway. Neither extreme is sensible.