Separate Brake Light From Turn Signal

[ThomsCircuit]

Resolving this makes a
solution more complicated. Detecting the state of static switches, like a
turn signal switch, or brake switch, is much simpler, but isn't useful for
the solution you want.

Awe man.
I did learn a bunch and for that i am thankful. Do you feel that the circuit member crutschow designed is viable? It is not that I doubt the design but there was a bit of confusion in the beginning as to how the circuit was to function. He indicated in post #42 the following which seems to be a problem that his circuit solves.

Crutschow said:
"It's not clear to me that any of the other circuits posted here operate properly for the combined signals of one side blinking and the other side steady-on, indicating a combined turn and braking situation.
Below is the LTspice sim of one that does:"

Your thoughts.

 

[eTech]

Only way to know is if all states are checked.
Another state to look out for is when removing brake, the third light delays a half cycle.
It should also handle the "emergency" state where both lights are flashing.

Just a heads up, the simulation can be misleading..

 

[eTech]

Awe man.
I did learn a bunch and for that i am thankful. Do you feel that the circuit member crutschow designed is viable? It is not that I doubt the design but there was a bit of confusion in the beginning as to how the circuit was to function. He indicated in post #42 the following which seems to be a problem that his circuit solves.

Crutschow said:
"It's not clear to me that any of the other circuits posted here operate properly for the combined signals of one side blinking and the other side steady-on, indicating a combined turn and braking situation.
Below is the LTspice sim of one that does:"

Your thoughts.

Crutschow's circuit uses a latch, which may or may not be ok. I thought about using a latch of some type in my circuit, but was concerned with a circuit failure causing the 3rd brake light to suddenly latch "on". Another thing that concerns me with this whole idea is the possibility of a circuit failure causing the brake light to suddenly come on, and, in turn, causing an emergency application of the brakes by a following vehicle (potentially causing an accident). Please advise.

 

[eTech]

Here's the circuit powered from the turn-signal lights:
(The delay circuit causes an occasional extra brake pulse when the braking ends, such as at the 14s time in the sim, but that shouldn't be a problem to those following the vehicle.)

Do you know how much current the brake light takes?
That will determine the required current rating of the diodes.
(The diodes shown are 3A).

View attachment 145878

Hi Crutschow,

I'd like to play with this circuit. I can redraw it in LTspice, but I'd rather just use a copy of your .asc file. Can you post the .asc please?

 

[eTech]

Hello TC,
Sorry for the delay....I've been getting over the flu..

I found a problem with the circuit. When both brake lights are on (no turn signals) the third brake light flashes once, but should be steady on. I found this during a bench test.

The way to fix is to make some wiring changes, and use AND (1 chip) and NOR gates (1 chip), instead of the CD4048B. If there is another way to fix, I'm open to suggestions.

See below:

View attachment 147475



Notice that every time the brake is applied (either, or both, inputs steady), the 3rd brake light is steady on. The 3rd brake should only flash during emergency mode (both inputs flashing).

I still need to bench test this new circuit configuration.

Hi TC

I've gone back and rechecked this circuit and it turns out I had a bad CD4025B chip. I must have zapped it. Anyway, the circuit works as intended. I did make some minor mods though.
I removed D10 and R4, not needed. Changed Rx1/Rx2 to 150k each to match the flash period of 705m Hz.

 

[ThomsCircuit]

I've gone back and rechecked this circuit and it turns out I had a bad CD4025B chip. I must have zapped it

What awesome news. Thank you.

 

[shokjok]

The unidentified trailer design may not need a third brake light ( aka collision avoidance light in Canada). 1986 model year vehicles sold in Canada were the first to get this additional taillight. Tall trailers got it in the 1990's, platform trailers and tow dollies don't need it.

 

[ThomsCircuit]

The unidentified trailer design may not need a third brake light

Correct. It does not need one. It's a simple 4x8 single axle. I just liked the idea of adapting the trailer with a third light.

 

[ThomsCircuit]

I put alot of time making this look good. Placing components where they should be with just one via and wide 25pt tracks and 40 on the output. I'm grateful for the assistance. Please, let me know what you think.

 

[eTech]

You could make GND a ground pour on the bottom side.

Assuming you left space on the right side of mosfet Q4 to optionally mount it flat on the PCB, you might want to double check the orientation of its pins. A copper pour, with a mounting hole for the mosfet, and sized to accommodate the mosfet w/wo a heat sink, is usually used as a mounting surface. Be aware that the mosfet drain pin is usually internally connected to the mounting surface of the mosfet (for heat transfer).

Other than that, looks good to me.

 

[ThomsCircuit]

A copper pour, with a mounting hole for the mosfet, and sized to accommodate the mosfet w/wo a heat sink, is usually used as a mounting surface.

Ok thanks. I added the heat sink to pin 1 and added a pour of the same net.

 

[Diver300]

It's the centre pin of the MOSFET that's electrically connected to the tab. Your heat sink area will need electrically insulating from the tap of the MOSFET.

 

[ThomsCircuit]

It's the centre pin of the MOSFET that's electrically connected to the tab

Oh boy. Thank you. So the copper pour is the drain but I also have to isolate the mosfet from the pour/pad? I found this example. This is what I thought I was going to be doing. Soldering the mosfet to the pour but not I'm a bit lost. The pours I have done are masked. Will this pour need to be un-masked?

 

[Diver300]

This is the most common layout for MOSFETs. The tab and the centre pin are both connected to the drain.

If you have a pad behind the tab that is not connected to the drain, you must isolate it with a thermal pad, and use a screw with an insulating collar. It does not matter whether there is solder resist or not.

If the pad behind the tab is connected to the drain, (or is connected to nothing at all ) you can leave off the solder resist and even solder down the whole tab for best heat transfer.

 

[ThomsCircuit]

If the pad behind the tab is connected to the drain, (or is connected to nothing at all ) you can leave off the solder resist and even solder down the whole tab for best heat transfer.

OK then. I created a copper pour without a mask. Placed a pad with a mask ring and a 4.5mm hole in the proper location. Connected both to the drain pin of the mosfet. It should look like the image in #193

 

[Diver300]

That seems fine. I don't know how hot the MOSFET would get, but you could increase the size of the pour to give it as much copper as possible.

 

[ThomsCircuit]

I am about to send this off for printing but I noticed a few issues. I forgot to remove the two components (D10 and R4) that were on the revised schematic posted on #185. I also did not think using the heat sink as the output was good practice. My thoughts were the heat sink is there for a purpose so I ran a track on the bottom plane to the output pin. If you fellas have any opinions i welcome them.

Note: The board will have a ground pour. That short track from the center pin to the heat sink is DS wanting a connection on the top plane to the heat sink. That heat sink is assigned to the center pins NET of the mosfet. I don't think I need one. DS just does not know or I don't know to tell DS one already exists. I could however remove the assignment from the heat sink. The board will still be properly printed.