Switching between 2 inputs to light and LED strip

Hey all, this is my first time posting here and i hope this is the right place to post something like this.

Im working on a project for a custom brake light that will have 2 functions: 1. Light up with a dim light (at around 8v) when the car is on/parking light on, and 2. Flash then go solid (at 12v) when the car brakes.

There are 3 inputs i can get from the car ( brake(12v), park(12v), and ground) and 2 outputs to the light (positive and negative).

In order to make each of these indivdual functions happen i have 2 extra modules.

To dim the light i have voltage converter set to convert the 12v signal to 8v.

To make the light flash i have flashing module that makes the light flash a couple of times and the stay solid.

Whist each of these work great separately, im having trouble connecting it all together and switching between them. Ive tried using a 5 pin relay and the flashing module doesnt work in this set up (im guessing its because of the shared ground but not entirely sure).

Any advice or help would be amazing!

Hi Thomas
I think you should check up on the legality of all this. There are rules regarding tail and stop lights. I think about the only thing you can easily and legally do is replace the incandescent globes with equivalent brightness LEDs. The LEDs do have the advantage of illuminating far quicker with the corresponding earlier warning for following motorists.

I do recall some time ago a project from Silicon Chip I think which probably came out in kit form to flash the third stop light (or maybe all of them) and it was illegal until the car was subject to a defined deceleration force which required an accelerometer and all sorts of problems like mounting this thing dead level for a start. I think I can get access to Silicon Chip index so will see if I can find it. Could have some useful info here.
Another thing, how old is this car. Mine is nearly 15yrs old and is modern enough to have the system check all the lighting circuits before shutting down. I have fitted LED stop lights and can see a faint flicker for some 30Sec after switch off when the car does its checks which would be invisible with incandescent lamps.
Cheers Bob.

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Hi Thomas
Found it. Titled “Rapid Brake” Silicon Chip July/August 2017.

There are apparently some rules and I quote.
The permissible flashing rate is de- fined by “Vehicle Standard (Australi- an Design Rule 13/00 – Installation of Lighting and Light Signalling Devices on other than L-Group Vehicles) 2005” Section of ADR 13/00 states that “all the lamps of the emergency stop signal shall flash in phase at a frequency of 4.0±1.0Hz.” However, section states that “if any of the lamps of the emergency stop signal to the rear of the vehicle are filament types, the frequency shall be 4.0+0.0/-1.0Hz.”

RapidBrake uses a 3.85Hz flash rate and that’s just under the 4Hz maxi- mum for filament lamps. We chose this frequency to suit both LED and filament lamps.

There are options for how rapid braking is detected. “ADR 31/02 – Brake Systems for Passenger Cars”. Section states that “emergen- cy stop signalling shall be activated by the application of the service braking system at a deceleration of or above 6m/s2 and de-activated at the latest when the deceleration has fallen be- low 2.5m/s2.”

Alternatively, section of ADR 13/00 says “the emergency stop

signalling may also be activated when brakes are applied at a speed above 50km/h and the anti-lock braking sys- tem (ABS) is fully cycling. It shall be deactivated when the ABS is no longer fully cycling.”

End of Quote

There is more info in the complete article available from Silicon Chip.
Cheers Bob


If they work separately, then switching between the two ought to require only a double-pole changeover relay.

When the parking switch is on then the relay cuts in, the parking input is connected to the voltage converter and the brake input is connected to nothing. When the parking switch is off then the relay switches off, the brake input is connected to the flashing module and the parking input is connected to nothing.

If that’s the setup you had and you couldn’t get it to work then show the devices and wiring that you used and describe exactly what the problem with it was.

Note that most DPDT relays are on/off, not changeover. So they have both poles connected or both poles not connected. That sounds like your 5-pin relay. That won’t work for this configuration. If DPDT changeover is difficult to find you could use two SPDT changeover. They might also be described as DPDT 2-way. For instance:


Hi Jeff
DPDT…Double Pole, Double Throw. Change over??? Yes. Like 2 double throw or change over switches ganged together.
Cheers Bob


You are describing a DPST relay. Double Pole Single Throw


Hi Thomas,

Welcome to the forum :slight_smile:

Was this for a road-legal registered vehicle or a track car? As Bob has pointed out the project gets a lot trickier compliance-wise if it needs to meet the laws surrounding registered vehicles. If this is for a go-kart on a farm things are different.

Do you have some particular lights or globes in mind? Since you mentioned controlling brightness by changing the voltage I’m assuming you were going to use old-school incandescent globes.
Modern LED lights will usually control the brightness either through current limiting or PWM dimming techniques.

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