LED Light Panel Build, Part 2: The LiPo Battery

For Part 2, I’ll be going over the aspects of the lithium polymer (or LiPo for short) battery setup that I’m using for portable setups (such as the LED light panels), sans AC power. LiPo battery were used in this project, specifically for their long-life spans and price points.

DSC_0102The reason why LiPo batteries are so popular is that if you properly maintain them, they can be recharged several times without degrading the battery over time. I won’t go over the basics of LiPo batteries (a Google search will yield a lot more information). Instead, I’ll be focusing on the basics of the specific type of LiPo I used within these projects. I snagged a few ZIPPY Flightmax 8000mAh 4S1P 30C off HobbyKing. If you happen to find a comparable brand, that’s fine. As long as it’s a 4 cell LiPo battery, rated for at least 8000mAh. If you’re curious about learning more about the battery terminologies, such as 4S1P, you can go here and play around with the settings on the Script Asylum website.

A 4S1P battery is essentially for 4 battery cells in series (hence the 4S). A 4S2P battery would be 2 parallel (hence the 2P) connections of the 4 battery cells in series. I personally wasn’t interested in the 2P versions. Although they do yield slightly more current capacity (8400mAh vs 8000mAh), I prefer the 4S1P for its low profile and weight factors.

The LiPo battery protection.

One could hook up a LiPo positive and negative terminals directly to their equipment or circuit, but the problem with that is the stock LiPo batteries don’t have a battery protection circuit. A battery protection is a must-have to prevent over-discharging the battery below a nominal voltage level. Discharging a battery past its nominal voltage will damage the LiPo battery. A cheap LiPo battery alarm can be used, as a quick solution to alert users when one of the cells have dropped below a preset voltage. However, a better solution would be to create a circuit that automatically shuts off power from the LiPo battery when one of the cells have dropped below a voltage level. Matt from DIY Perks has a great, basic circuit layout that I used as a stepping stone to work off of. The image below is my version of circuit diagram that I’ve adapted from DIY Perks, to fit the 4S1P LiPo batteries.

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Figure Alpha – LiPo Battery Protection Circuit

The LiPo battery alarm.

IMG_1029The first step is to purchase a bunch of cheap LiPo battery alarms. The more, the better, because I have noticed that the voltage read outs on these may be off by 0.2-0.5 V, which may be a big deal if you’re discharging the battery down to as close to the recommended minimum level of power (around 3.2V/cell). To weed out the alarms with poor voltage read outs, I HIGHLY recommend measuring the voltages of each cell off the balance connector. I used a digital multimeter to check the voltage between the 1st (black) and 2nd wire of the balance connector. This is the 1st cell. Then I checked the 2nd and 3rd wire. This is the 2nd cell. So on and so forth. Record what the values are for each cell. For instance, the voltages may be 3.8V, 3.81V, 3.79V, and 3.82V for 1st, 2nd, 3rd, and 4th cells, respectively.

Next, attach the LiPo balance connector to the LiPo battery alarm, as shown below. The seven-segment displays will display the voltage read out. Make sure, as the display cycles through all the cells, that the values roughly match up to what was recorded. The image below shows how you should connector your balance connector to the LiPo battery alarm.

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Once you’ve weeded out the LiPo battery alarms that are way out of range from what you recorded, it’s time hack these good alarms. You can click here to check out the step-by-step that DIY Perks has out on his channel on dismantling the alarm speakers from the battery alarm testers. Once the two black alarms are removed, I attached a couple of pigtails to the top left and bottom right solder pads (see left image). These represent the +5V and ground to set off a dual latching relay.

DIY-Battery-Protection-Circuit-2On the schematic I provided above (see Figure Alpha), J1 and J2 are both connections made from the LiPo battery alarm to the battery protection circuit (see the zoomed in picture on the right). Since these LiPo battery alarms are so dirt cheap, it makes more sense to grab these off-the-shelf parts and modify them to our needs. As shown on the right, the yellow and blue wires off the top of the LiPo battery alarm will go to a relay. I made a small connector to connect between the 9-pins on the LiPo battery alarm to the rest of the circuit.

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The LiPo battery connections.

DIY-Battery-Protection-Circuit-3I used male pin connectors to interface the female connector on the LiPo battery. The LiPo battery connections are pretty straight forward (2 pins, one for positive and one for negative). You will probably notice that the color coding on the LiPo battery balance connector (see below) is different from the color coding I used over my schematics. The reason why I used a different color coding from the LiPo battery balance connector was because the connectors that I purchased off of eBay were a different color than the LiPo battery balance connector. So to avoid any confusion, the following table will hopefully clear this up:

Pins Schematic Colors LiPo Balance Connector
1 Black Black
2 Red Light Blue
3 White/Gray White
4 Yellow Yellow
5 Orange Red

DIY-Battery-Protection-Circuit-4I spliced the connections on the LiPo battery and balance connector, since the ones from the battery were going straight to the board. As explained in the DIY Perks video, this allows us charge the battery, without have to remove it from the circuit, every time when we charge the battery. It’s a bit extra work, but well worth the effort.

IMG_1071As you can see on the image on the left, the LiPo battery leads were shortened significantly to fit inside the battery case. To do this properly, without shorting the leads of the battery, make sure you work on one wire at a time. Whatever you do, DO NOT cut the positive and negative wires simultaneously! You will end up arcing whatever metallic cutting implement you are using, most likely damaging the tool and yourself.

Instead, work on one battery lead wire at a time. I suggest working on the negative lead first. Shorten it, and then solder a wire from the lead to the board. Use shrink wrap and hot glue to “cap off” the end of the exposed wire. Then move onto the positive wire.

The XT-60 connector I used, at the top of the left image, was the female connector. I decided to provide some protection to those terminals by using the end of a male connector, and putting shrink wrap and hot glue around the solderable end. This provides a protective “cap” on the exposed terminal.

The 6-pole switch.

The crazy 6 pole switch (S1 on the schematic), or the technical term, 6PDT latching switch. 6PDT stands for 6-pole, double throw, meaning there are 6 set of “switches” and 2 throws, or connections that the switch can make contact with. The latching switch is a switch that can stay continuously stay on when pressed, or completely off when pressed again. This is the basis of a light switch or power button. In this case, the 6-pole switch acts as the power-on button.

Below, I’ve shown what happens when the 6PDT latching switch is “on”. The circuit between balance connector and the 9-pin LiPo voltage indicator/low voltage alarm is completed, turning on the LiPo battery alarm. At this point, you should the seven segment displays cycling through each cell, displaying the voltage of each cell, as well as the overall voltage. Likewise, when the 6PDT latching switch is “off”, then you break the path between the balance connector and LiPo battery alarm, which effectively turns off the LiPo battery alarm.

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The relay circuit.

Without the relay circuit, the 6PDT latching switch only turns on the LiPo battery alarm. You could wire the LiPo battery’s positive terminal to a switch, and forget about adding the relay circuit. But you would still have to monitor the LiPo battery alarm, making sure that the battery doesn’t drop below 3.2V/cell. And you would have that switch on the LiPo battery itself, which is inconvenient, especially if you end up forgetting to turn off the battery. And if you end up leaving the battery hooked up to your circuit, draining the battery well past the 3.2V/cell minimum, then you’ve ruined your LiPo battery pack.

This is where the relay circuit shines. The work done previously in removing the alarm speakers on the LiPo battery alarm will also pay off in the relay circuit. As the 6PDT latching switch is turned “on”, the relay doesn’t get set, because of the momentary switch (S2 on the schematic). S2 is a normally-open (NO), momentary switch which only closes the circuit for as long as the switch is pressed. Once you let go of the switch, the circuit is broken, which opens the circuit again.

The specific relay chosen was a Panasonic DSP1A-L2-DC5V. It’s a dual coil, latching relay. A lot of relays require constant power applied to the coils on the relay, to either keep a circuit open or closed. This type of relay doesn’t require that, and that’s why we use a momentary switch to energize the coil. As shown below, when S2 momentarily closes the circuit, the reset coil on the relay is energized. D1 is a diode that drops the voltage from Cell 2, from approximately 7V to a manageable 6V (it takes 5V to energize the coils on either the reset or set coil). When the reset coil becomes energized, the open switch in the relay will close the circuit, which allows for the negative terminal on the output power to power your electronics. When you release your finger off the S2 switch, the relay will continue to latch (thus, keeping our electronics powered), unless the 6PDT latching switch shuts off power, or our LiPo battery alarm triggers the set coil on the relay.

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Manually shutting off power to battery pack.

This can be done by switching our 6PDT latching switch, to “off”. To release the contacts within the relay, voltage across the relay coil may be applied in reverse. This is where the cleverness of the S1 switch comes in (I have to say that it’s pretty genius of Matt, from DIY Perks, to come up with this!). The last set of “throws” on the 6PDT latching switch, has its pole wired to ground, off one end of the relay contacts. This should be the opposite relay contact of the LiPo battery ground, since we don’t want to continuously draw power off the battery.

DIY-Battery-Protection-Circuit-7So what happens here is that we use the line off the very last cell of the balance connector (in this case, cell 4) and the battery ground to reverse the voltage across the reset coil on the dual coil relay. Remember how when we turned on the LiPo battery alarm, via 6PDT latching switch, and pushed down on the momentary switch (S2) to allow current to flow from the bottom to the top of the reset coil of the relay? Now we are simply reversing the voltage across the coil, allowing the current to move in the opposite direction. However, we cannot just apply power from the last cell of the balance connector. A resistor is required to step the voltage down to 5V from nominal 14.8V off the 5th pin (last cell) of the balance connector. I’ve also added some basic math to help those determine how to size their R2 resistors in other configurations, below.

DIY-Battery-Protection-Circuit-8

In the above equation, there are a few known values. If you’re using the Panasonic DSP1A-L2-DC5V, the datasheet will provide RCOIL (the coil’s resistance) and VCOILMAX (the maximum voltage that can be applied to the coil), which are 83Ω (ohms) and 6.5V, respectively. The maximum voltage for the LiPo (VLIPOMAX), in this case, is approximately 16.8V. If another dual-coil latching relay is used instead of the Panasonic one, refer to the datasheet for that component and look for the resistance of the coil and maximum voltage that can be applied to the coil.

The other great thing about the relay circuit is that once the reverse voltage is applied, as the power to the battery pack is shut off, when the contacts release, it creates an open circuit. Although it may seem crude, to have two switches that allow you to turn on power, you don’t risk a constant low current draw that will ultimately drain your LiPo battery past its minimum voltage.

Lastly, the auto shut-off circuit.

The auto shut-off circuit takes advantage of the LiPo battery alarm’s low voltage indicator. Typically, if we kept the speaker alarms on the the LiPo battery alarm, a loud noise would sound off when the battery drops past the threshold. The threshold can be set by the small pushbutton switch on top of the LiPo battery alarm.

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If the circuit is powered up, and the reset coil is energized to close the contact on the relay, then when the LiPo battery alarm is set off (due to the voltage of our LiPo battery dropping below the minimum allowed voltage), the set coil is energized on the relay. When the set coil is energized, the contact on the relay opens. One important thing to note is the orientation of how voltage is being applied to the reset and set coils. Make sure the reset coil has the positive voltage and ground connections aligned similarly to that of the LiPo battery alarm voltage pads. If the coils are not both positive on one side and negative on the other (don’t worry about the manual shut-off circuit with R2, because we want that specific one to be reverse voltage applied to the reset coil), the relay won’t work correctly.DIY-Battery-Protection-Circuit-9

And that’s pretty much it. This was a really long article that I put up. And these types of articles require a lot of work, especially when providing colored diagrams to illustrate what is happening within the circuit. If you like this, let me know. If the work is worth it for everyone to see how something works (especially with electronics), then I’ll try to put up more detailed content like this in the future. If you’re confused with anything, let me know and I’ll try to clean up the descriptions a bit more.


LED Light Panel Build, Part 1

Ok, ok. This is finally the first post that even remotely relates to what the website is all about. However, I confess. This isn’t exactly a step-by-step build. More of a build that’s about 75% complete. But I’ll go through some of the steps on this build.

The main purpose of why I wanted to build 3 of these LED light panels was that it would be relatively cheap and provide the necessary lighting that I wanted to use for either photography or shooting video.  There’s a whopping 468 daylight white LEDs on each light panel! A brand new unit would cost around $500-$800, whereas the cost of each panel built is about 25%-30% of that!

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The LED array and wood box frame.

I drew inspiration from DIY Perks build (you can check out the Youtube video here). I picked up a few reels of daylight white LEDs (I’ll have links to the stuff I’ve implemented under a budget breakdown table in either Part 3 or Part 4). Since there are 468 LEDs, I had to use two reels to do a single panel (each reel comes with 300 LEDs on the strip). I decided to go with the cheapest jelly roll pan as a heatsink surface to apply the LEDs to, instead of applying it to a non-metallic backer. The jelly roll pans were unfortunately covered with a non-stick coating, so I had to rough up the surface with a cheap chisel to allow the LED strips and any adhesives to adhere better to the surface. If I had to do this over again, I would probably use an aluminum baking sheet that didn’t have a nonstick coating on it.

DSC_0087 (3)I epoxied blocks of wood onto the jelly roll pan to raise my flat tinned copper braids, off the surface, for my positive and negative terminal power bars. A bit crude, but it does the job. Before applying the LED strips down, I also made sure the ends of the LED strips would lay over some electrical tape, since I would be soldering the ends to my terminal bars. I didn’t want to risk a short by accidentally solder directly to the metal surface. That’s why you see the black electrical tape. Once the LED strips adhered in place onto the jelly roll pan, then came time to solder the respective points on the LED strips to the terminal bars. As you can see, the left side of the panel had all the positive connections, and the right side of the panel had the negative connections.

DSC_0085I used some 24AWG magnet wire to bridge the terminal bars to the LED strips. These are coated in enamel which burns/melts off quite easily with a soldering iron. Once this was done, I decided to make a wooden pine box frame to hold the jelly roll pan. I made a dado cut within the box to make space for the jelly roll pan to sit in. Nothing too fancy. I didn’t want to waste my time and effort hacking the jelly roll pan to fit in the pine box frame. I also made a second dado cut to allow a 1/8″ piece of plywood to sit right behind the jelly pan, just so it would be easier to attach the electronic components onto the surface easier.
I made some 45 degree mitered cuts on the plywood frame, to create the box. Instead of using splines (since I don’t have a real spline jig yet), I used some screws to hold the box together. For those that aren’t aware, mitered joints are weak, due to the joint being end-grain to end-grain. Woodworkers will use a wood-type joinery, instead of metal fasteners, as they’re more elegant and they don’t look awful.

The build quality of the wooden box frame doesn’t scream “fine woodworking.” I know. If I took the extra time to build a decent spline jig, this wouldn’t look so horrid with the screws showing. But the screws will be covered up with some wood putty and this will be painted, eventually.

I wanted to make the LED panel versatile. I wanted to add legs onto back of the panel so I can prop the light panel at an angle on the floor, to light up a background or whatnot. This is composed of a single block of wood with a carriage bolt through the top. I drilled a hole through each side of the back portion of the light panel, to accommodate the bolt through the panel. This would be held on tight with a 5-star plastic knob.

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DSC_0092-(2)I also wanted the ability to use this on a light stand, so I snagged a few brass Manfrotto threaded studs that would thread onto some t-nuts. I only needed to attach a stud to either the bottom (for the panel to sit horizontally) or one side (for the panel to sit vertically). So only two sides had the t-nuts installed. Again, nothing too crazy. Unfortunately, the Manfrotto threaded stud was longer than the thickness of the wood box frame, so I had to notch out one of the legs to make room for the threaded stud clearance. I could’ve ground down the Manfrotto stud, but I didn’t want to waste time grinding down several brass studs. Notching out the leg (as shown below) is way easier! Also note in the below picture, that I made sure the t-nuts would stay secure with some screws on both sides of it.

Oh, the reason why I had to recess the t-nut from the inside panel was because the dado that I cut in the pine wood, for the 1/8″ plywood, had clearance issues. So I decided to recess the t-nut with a Forstner bit on the good ol’ drill press.

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That was the bulk of the main panel build. A lot of planning was involved, trying to figure out how to make the cuts that I wanted, as well as the features that I wanted to add. This was definitely not a lightweight build by any means. The box alone was weighing in at under 10lbs. I would probably try my hand in some ultralight MDF next time. But the features that I had were well worth it. The ability to prop the light panel up on itself was a huge plus for me, as well as propping it onto a regular old lightstand with a tilt attachment (see picture below).

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The dimmer circuit.

DSC_0101 (2)To simplify the build, I went with some cheap electronic parts off of eBay and AliExpress. There are some people that complain about the Chinese DC-to-DC power converters, but I haven’t had any issues with them. There are very limited instructions/manuals out there on how to use these, so I can see the challenge for most folks trying to figure out how they work. I may consider doing a write-up to breakdown these power converters so that folks out there can spend less time tinkering and more time applying these to their projects.

The first part of the electronics that I wanted to get out of the way was the 12V dimmer circuit. Dimmers out on the market typically consist of a pulse-width modulating circuit, or PWM. The basics of a PWM is that there’s a duty cycle of an on and off “pulse”. Depending on how long the pulse-width of “on” time is, compared to the “off” time, will ultimately dictate your brightness level. There’s nothing wrong with a PWM dimming circuit, except they tend to show up on video footage as a pulsating/strobing lighting effect that can be undesirable.

Instead, I wanted to build a voltage-controlled dimming circuit, which starts off with a DC-to-DC power converter. The one I used was a DROK DC Buck Converter, which typically steps down voltages. This type of converter is great for LED strips in boats or cars, because they can protect the over-voltage when the engine’s on. Buck converters will typically step-down voltages, and step-up currents. This means an input of 1A @ 15V, can be converted to 1.25A @ 12V. DIY Perks does a great job of how to setup the DC buck converter for the dimming circuit.

As you can see in the picture above, I had to create some custom enclosures out of 1/8″ baltic birch plywood. Instead of waiting for your typical wood glue to dry, I sped up the glue process of the small custom plywood enclosure boxes with some cyanoacrylate glue, commonly called CA glue. There’s a ton of different brands out there, but I prefer the 2P-10 brand, as it was more convenient for me. There’s usually an activator spray and several “grades” of CA glue, ranging from a thin liquid to a gel-type glue. I prefer using the gel-type glue because you can reinforce the interior of these boxes with a thick gel bead, spray some activator over it, and it’ll yield a strong wood box enclosure. The cure time for CA glues is usually a minute; however, spraying a mating surface with the activator will yield a strong bond instantly.

Below, I’ve created a simplified PCB layout of the modifications that were made to the DROK DC-to-DC power converter. On the bottom of the power converter, I soldered a 6.8K resistor in series with a 22K potentiometer, parallel to the onboard trimmer potentiometer pins. This provided the dimmer circuit, to dim the lighting. On the top, I had to add an extra line from the input terminal block to wire the fan controller…I’ll get to why I had to use a fan on this circuit.

DROK-DC-to-DC-Converter-Graphic

Once I modified the buck converter, I plopped it into my small custom wood enclosures. I went with custom wood enclosures, instead of a readily available plastic enclosures, because of the clearance of the potentiometer knob. I wanted everything on the rear of the light panels to be relatively flush to the brim of the back side of light panels. This would allow me to stack these light panels on top of each other rather easily, and nothing would be sticking out and off the back side.

DSC_0098 (2)After I finished this circuit, I did a quick test with the LEDs and my 12V bench power supply. As noted by DIY Perks, the aluminum heat sink on one side would tend to heat up. This would worsen when you applied 15V to the input of the buck converter, and stepping down to 12V. So, to mitigate the issue, I decided to go with a centrifugal-type fan. They’re a bit pricier than your typical axial PC fan, but they can provide a ton of CFMs. They do tend to be noisy as well, so I had to use a fan controller to reduce the voltage being supplied to the fan.

I soldered the input of the fan controller to the input of the buck converter. But before connecting the output of the fan controller to the fan, I made sure that the small potentiometer, on the fan controller board, was turned down so that I didn’t supply the fan with a voltage greater than 12V. This is the potentiometer to control the fan speed, show in the image below. Since the fan itself was pretty powerful at 12V, but overbearingly loud, I stepped down the voltage to the fan to around 6-7V. I didn’t want to noise level of the fan to be too loud, especially if the light panels are being used in videography. This keeps the fan quiet, but still pushing a decent amount of air onto the one heatsink on the buck converter that tends to heat up.

Fan-Controller

A cheap plastic enclosure was used on the fan controller, just so I could stay consistent with keeping most of the electronic components covered and out of the way. Also, I used a sheet of automotive gasket material for the fan to sit on, which can be purchased at any automotive store. I figured that it would reduce the amount of vibrations being transmitted from the fan to the light panel.

If I were to do this part of the build all over again, I would prefer using a laser cutter or 3D printer to build a custom enclosure to house the buck converter, fan controller, and centrifugal fan in. There was some trial-and-error involved, but on future builds, I would definitely consider automating the build of the custom enclosures. Cutting each piece of plywood to the correct shape with a coping saw and hand saw is tedious and time-consuming.

So, is it worth it? Well, I did a quick test shot with two of the LED light panels. One light source was at mid-brightness, aiming down from above the subject. The second light panel was at 75% brightness and it was placed standing up, towards the front left, aiming at the subject. My faux table top is too narrow for this particular shot, but you get the idea. It’s amazing what good lighting can do to your photos!

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In Part 2, I’ll cover the battery pack system when using the light panels in remote locations where you don’t have access to AC power. One other thing I want to add to this build are some barn doors, so I can focus the light on a specific portion of a subject, and not flood an entire scene with light. Still contemplating a decent design for that!