LocalGadgetGarage project journal
Have you ever looked at your (presumably) empty tow hitch and wondered how you can make it more appealing? Towing something would definitely solve that problem. But what about 99% of the time you more-than-likely aren't towing? An LED Tow Hitch Cover can help spice up your vehicle's rear end.
Have you ever looked at your (presumably) empty tow hitch and wondered how you can make it more appealing? Towing something would definitely solve that problem. But what about 99% of the time you more-than-likely aren't towing? An LED Tow Hitch Cover can help spice up your vehicle's rear end.
This project consists of 3 main components:
LED PCB: We will need a compact board that can fit within the tow hitch housing
3D Printed, Modular, Enclosure: We will need a durable tow hitch cover enclosure that can protect the PCB and easily swap designs
4-pin Trailer Connector Compatibility: We will need to plug into most vehicle's towing lighting equipment
Before designing the board, it is important to define the requirements. The board must:
Be powered by vehicle's 12V architecture
Comply with local, state, and federal laws
Be bright enough to see both in the day and night
Stay cool within the enclosure (this is really important!)
Before designing the board, it would be prudent to have an idea about how the board will be getting power. A few options could be:
Wiring from the 12V battery to the rear (requires harnessing)
Tapping a taillight harness (requires a splitter)
Powering it externally (requires additional circuitry)
Plugging into trailer 4-pin plug (requires 4-pin connector).
Many of these options require either additional work or additional components, where plugging into a trailer 4-pin plug means you only need the male connector; these tend to be anywhere from $1-$3 on Amazon, making it a good way to power the board. A typical 4-pin trailer connector diagram looks something like this:
We can ignore the green and yellow wires since we will only want the LEDs on when the vehicle's DRLs are on. This means we will only need two power pads on our board. One for the white wire (GND) and one for the brown wire (DRLs/12V).
Ultimately, the brightness of the LED will be determined by how you 'drive it'. In other words, how much current will flow through it? In this LED's case, the forward current (If) is 25 mA. According to the manufacturer's chart, driving the LED at this current will give us the highest relative luminous intensity. So, for Version 1 of the board, I will want to drive it at that.
There is another important curve to pay attention to. That is the Forward Current Derating Curve. This graph is essentially telling us that as the ambient temperature increases, you will want to drive your LEDs at a lower current to avoid overheating your LEDs. At 25 mA, we are assuming that our LEDs will be operating at about 25 °C, which will not be the case. We know this board will be in a fully sealed enclosure (which means no airflow), so the best approach for us will be driving at 25 mA. Afterwards, we can do real-world testing at different conditions to see how our LEDs fair. From there, we can determine the optimal driving current for our needs.
There is one more thing we will have to handle before creating our schematic. How many LEDs can we drive? LEDs have something called a forward voltage. This is the amount of voltage needed to get current to flow across an LED. It differs between LEDs and colors, but for RED, we can say it is a Vf of 2 (according to the manufacturer). This means if we have 12V from our vehicle, we can put 6 LEDs in series. This is because each LED 'consumes' 2V from our total 12. Six LEDs, however, are not enough to brighten our entire tow hook cover. What we can do is actually put more of the 6 series LEDs in parallel with our initial 6. This goes back to basic electronic fundamentals. In parallel, each branch receives the same voltage from the power supply. This means that we can add as many series of 6 LEDs in parallel until we reach the required amount (assuming your power supply can support the total current).
Great! We are almost done. The final piece of the puzzle is our current limiting resistor. Remember when we said you could use 6 LEDs? That is not entirely true. Our forward voltage (Vf) can change due to manufacturing defects, ambient temperature, etc. So, to allow for that variability, we can use 4 or 5 LEDs with a current-limiting resistor at the end. Due to our very wide operating temperature, we will use 4 LEDs in series. We can calculate the current-limiting resistor using this equation: (12V - 8V)/25mA = 160 Ohms. This means at the end of each of our 4 series LEDs we will need a 160 ohm resistor. This resistor will, at a minimum, have to be able to safely dissipate 0.1 W of power, or 100 mW. I want to re-iterate that we are driving our LEDs a little hard at 25 mA, but with real-world testing we can find the range we actually want to run at. You may be asking, Why did you use a 150 Ohm and 10 Ohm in series? This is not ideal, but the simple reason is cost. JLCPCB, sadly, does not offer 160 Ohm resistors at their 'basic' tier, meaning that if we use them, it will quickly drive up the price. So, using 150 and 10 is a reasonable compromise without any destructive drawbacks.
The final part of our PCB design is making sure it keeps cool. Driving the LEDs at a smaller amount of current is a reasonable way to do this, but we haven't done a real-world brightness test. So, we can try some things in our PCB. Some basic things we can try are:
Make a big copper pour for our GND plane. This can act as a sort of thermal spreader.
Make wider power traces. Wider traces have lower electrical resistance, which means less power is lost as heat due to resistive losses. A useful equation is R = (p * L)/A
Make mounting holes that can act for thermal relief if needed.
The board arrived from China!
Remember I said we would have to do some testing? What exactly does that entail? We received 5 boards from JLCPCB. We can run them within our enclosure and measure the heat generated after a few hours.
Sadly, I didn't capture many pictures of this process, but what I can tell you from real-world testing is that 25 mA in a full enclosure resulted in melted plastic! Given this, with some trial and error, we can add a current-limiting resistor at the input (for testing) until we get a brightness and heat compromise that works well.
I want to keep the enclosure's CAD design private to avoid the design being stolen, but some requirements it needed to satisfy were:
Use PETG Carbon Fiber to avoid sun fading and sustain high heat.
Use the PCB mounting holes to air gap the board to avoid melted plastic (a second time).
Use magnets to allow for modular and hot-swappable designs.
Use a cap design that will be water proof, dust proof, etc.
The 3D printed design is very solid, but there are a few improvements that can be made on the PCB side:
Wider power traces!
Better component placement; some spots on the board are very cool and some portions have hotspots
Larger current-limiting resistor value. Currently, some resistors are wired in-line with the 4-pin harness to lower the 25 mA forward current. This isn't ideal, and we can easily change our current limiting resistor equation to 15-20 mA after our real-world testing. This means increasing our 160-ohm resistance to anywhere from 200-260 ohms.
Ambient Derating! This still isn't exactly where I would like it to be, especially for an automotive application in California. We get up to 110F days in the summer. However, I want to note that because our 4-pin connector only relies on DRLs (auto DRLs are usually not on during the day), chances are the board will be on mostly at night (meaning it is cooler).
After all is said and done, we get a cool, robust, tow hook cover that lights up!
Features:
Lower current driving the LEDs
Consolidated & larger (value, not package) resistors. Down from 24 resistors to 12.
Wider traces! Better utilization and heat distribution throughout the board.
