LED Bunker Light Teardown

I recently picked up this LED bunker light for $33 and I thought it'd make an interesting tear down.

AT5700 bunker light

When you remove the diffuser from the front the first thing you see is the PCB that holds the 32 LEDs.  Ideally you're meant to be able to loosen the screws and turn the PCB slightly and then lift it out, the only problem with that is that there isn't enough room to get your fingers down the side of the circuit board.  As it took me a couple of minutes to remove the PCB I'd suggest that in their next design iteration they route some indentations into the edge to allow you to hold onto the board.

Surface mount LEDs

The light is just a low power bunker light for non task illumination and is rated for outdoor and indoor use.

Box specifications

The LED driver is mounted to the back of the PCB and seems to be of the constant current type.  It's hard to tell but I think the voltage range of 12-25 volts indicates that it will regulate the output to 350mA and vary the voltage accordingly.  When measured the LEDs were drawing 350mA at 12V for a power of 4.2W.  The box indicates a power draw of 6 watts so the difference is likely to be what gets wasted in the driver.

LED driver specifications

The light was chosen because of its small size, almost too small in fact.  As we'll see later, the 70mm thickness doesn't leave much space for cabling.

Dimensions

The connector that supplies power to the board has a cover over its soldered terminals on the front side of the panel.  I assume that this is to prevent shorting occurring and not a safety thing as the voltages on the board are low and still accessible on the sides of the surface mount LEDs.  It's a good idea but I wish they'd put the same amount of thought into the installation process as it's quite hard.  Ripping a surface mount component off a board isn't that hard to do.

Insulator over connector solder points

The main problem I have is that the driver is mounted to the back of the LED PCB.  During installation you have to first connect the cabling to the terminal block.  This means that you need to leave about 8 inches of extra wire so you can get a screw driver in there before putting the board into the base and if you can't shove that extra cable into a wall or wherever the light is going, it has to be curled up inside the fitting and that's hard as there is only about 30mm of space behind the board.  I think a better solution would have been to mount the driver to the base of the fixture so that the driver can be wired in first and then connect the LED PCB via the header connector to the driver.  The connector on the PCB could be also moved to the front side to allow connection after mounting the LED board.  There is plenty of room, you just bring a small cable up the gap beside the PCB and connect it on the front side.

Rear of the PCB as it sits upside down in the base

Curious about how the LEDs were arranged I mapped out the layout of the board.  In the image below each different section of copper track has its own colour to clearly identify them.

Electrical layout of PCB

This shows that the 32 LEDs are arranged in 4 serially connected groups of 8 parallel lights.  I would usually have an issue with this because there isn't any form of load balancing, but I think the lights are under-driven enough that it isn't a concern.  I'll take a minute to explain that better.  If the LED driver is generating 12 volts with a constant current of 350 mA, you would assume that each component is getting 43.75mA at 3V, but due to manufacturing tolerances it's unlikely that the 350mA will get evenly split between the 8 LEDs in each group.  You then have a situation where one will be drawing more current than the others and if there isn't enough of a safety factor in the design it will eventually fail.  When this happens, the 350 mA will now be divided between the 7 remaining lights and another will fail this will continue until an entire group fails.  The reason that I'm not too concerned about this is that I think the packages are 2835 LEDs and they can usually handle more than the (0.04375 x 3) 131.25 mW per device.  The forward voltage of a white LED is usually considered to be above 3V.  This is what makes me think that to get the reliability they wanted, all they had to do was add extra LEDs to split the current and under drive each chip.  You would probably find that if there were only 7 LEDs in each group the higher current would cause the forward voltage to increase and instead of regulating at 12V, the driver may go to 13V.

LED schematic

It's not too bad for $33 but I think the design could be improved with only some minor adjustments.

Using a Solder Paste Stencil for a Prototype

If you've seen any of my previous post you'll know I'm in the process of making a PCB that contains a configurable LED Grid.  Why?  Something to do I guess.  Anyway I finally got around to assembling the board I had made by BreadboardKiller.  I've assembled small surface mount PCBs in the past and have always manually applied the solder paste by hand with a syringe.  It's hard to get right, you might not get paste in the right place, you can use to much or too little, but when it's a only a few parts it's not too hard to fix.  This board has 96 surface mount components on it, which means that doing it by hand was going to be a close to impossible.

I decided to get a laser cut Kapton stencil from OSH Stencils.  At 40 dollars it looks expensive for a piece of plastic, but that includes a one off cost for set of board holders the exchange rate wasn't kind either.  These are the black acrylic L shaped pieces in the image below.  The stencil was only 20 dollars US which was well worth it.

There are demonstrations on how to use the stencils online but I'll show my setup.  The board holders are taped down and the stencil is aligned and taped down on on side to act as a hinge.  This allows a board to be put in the holder, have the stencil flipped over it, the paste applied, the board removed, and the process repeated.  Application of the paste is easy.  Squirt some out of the syringe where it needs to go, and use the paste spreader (basically a credit card) to swipe the paste across the stencil.

PCB Assembly Set-Up

The stencils are easy to use and make sure you apply the right amount to each pad.  For this project I chose to use lead free solder as I assumed that I'd get it everywhere when using the stencils.  I was right.  I wasn't sure how to clean it but mild soapy water did the job.

Solder Paste Stencil

Part of Stencil for LEDs

Part of Stencil for Resistors

Stencil label

Once again I used my toaster oven to solder the boards.  It has no automatic controls, I stand there and watch the board and time the steps by counting aloud to myself.  The temperature is also set manually by turning the dial.  The process is described in a previous blog post.  It helps to put a little bit of solder paste on a fiducial mark so that I can see the moment the solder liquefies through the oven window.

PCB in Oven After Reflow

It's important to shield the board from direct IR radiation from the heating elements, that's why there are trays above and below the board.

PCB in Oven after Reflow

After the surface mount parts were soldered the through hole parts were added by hand.

Assembled Board with LEDs

When I designed the board I screwed up and made the holes in the footprint for the terminal blocks too small.  This means they had to be enlarged by hand, and because of the way the tracks were laid out, the positive terminal block has to go on the other side of the board.  No biggie.

Assembled Board

Assembled Board

Surface Mount Resistors

The parts seem to have been soldered nicely.  There isn't an excess of solder or too little, and there's that nice little fillet you expect to see as well.

0.25 Watt 1206 Resistor

The LEDs are harder to judge as the pads are under the board, but they all work.

Osram GW JCLMS1.EC-GUHQ-5L7N-1 LED

Just to prove they all work.

LED Grid

To show the reconfigurable nature of the board I disconnected the jumpers that power the middle section of the LED grid.

LED Grid with Sections Turned Off

Over all I'm very happy with the results.  Obviously boards produced this way aren't going to be of the same standard as professionally made ones, but these are rather sturdy and fine for prototypes.  I'd gladly use OSHstencils again just for the time that it saves me.

Breadboard Killer Review

I recently did a post about a small PCB to test out some surface mount LEDs.  Apart from the main objective of seeing how bright LEDs appears under certain lighting conditions, there were several other reasons for designing the board.  I hadn't designed a PCB in a long time and had an itch to make one, there was also curiosity about how Kicad had progressed since I'd last used it.  For this board I'm also going to use a stencil for the solder paste for the first time.  It never hurts to learn new skills.

The other reason I wanted to do this was to test out a small volume PCB prototyping service called Breadboard Killer.  It's an Australian company and I really like the name.  The use of breadboards has basically become unsustainable.  It seems ridiculous that I still see them in use at universities.  If you're training people to design contemporary electronics that run at high frequencies and use surface mount components, starting with breadboards is a set back.  Sure, give them a one to wire up a 555 timer with a blinking LED to get them started, then throw an EDA tool at them.  If you know your stuff, and have faith that the schematic you design works, just go straight to PCB.  They're are cheap, and if you screw up, who cares.  That's why PCBs have Revision numbers and Kynar wire was invented.

So how did the boards turn out?  Really well.  For $56 Australian I got 5 copies of a board that's 10.5cm x 6cm.  The boards have nice clean routed edges and don't have rough bits where break away tabs are left behind.  The board house does add a serial number, (seen in the footprint for the terminal connector in the image below) but that's standard.

Top Side of PCB

The silkscreen is high quality and very sharp, whereas the silkscreen on some other boards I've seen for other manufacturers look pixelated and blocky.

Bottom Side of PCB

The image below shows the registration between layers.  The drill holes seem to line up well with the copper layer, but there is a small offset in the solder mask.  That's nothing to worry about and is completely within specification.  That's why you have solder mask expansion after all.

Header Footprint

It's hard to see in the image below but the footprint of the LED I'm using has pads that are slightly T-shaped.  The right angles in the corners are slightly tapered, but that's to be expected.

LED Footprint

The ordering process is easy too.  I had a slight problem with the files names I used, but that was quickly resolved.  The site also generates a nice render of the uploaded gerber files as a final verification step to make sure that all the layers are correctly identified.  I intended to make my project open source and share it on the site, but I think I needed to include a license in the zip file I uploaded and I didn't realise this until I was part way through the order process.  I'll look into that for next time.

UPDATE:  Matt from Breadboard Killer let me know that you can make designs open source after submitting the project by going into your account and editing the project.  So I've gone ahead and done that.  Here is my project page https://breadboardkiller.com.au/designs/55-ledgrid.

Order Confirmation

The order took 19 days to arrive.  I picked the cheap slower option that has a range of 8 to 20 days, so everything is OK there.  The price is pretty reasonable as well.  It's hard to compare this to services like OSH Park as they have different capabilities and multiples of boards.   I'll have a go though.

As of 27 - September - 2015

Breadboard Killer - 5 boards shipped for 56.18 AUD
OSH Park - 3 boards shipped for 48.82 USD ≈ 69.50 AUD

To be fair the OSH park boards have an ENIG coating on the pads, but most of the other specs are the same.  OSH park could also look expensive due to recent changes in the Australian dollar.  Not sure, but I think the breadboard killer PCBs are made in Singapore.  This is based on the Singapore Post shipping option.

All up I'm really happy with the quality and services Breadboard killer provides.  I've recommended them to a couple of people and I'll use them again.  Now I just need to wait for my solder paste stencil before I can have fun.  :-)

LED Array Test PCB

A quick post today.  I was discussing the idea of an illuminated display recently and was unsure of how bright LEDs would appear when driven with different currents.  After looking around, I found some nice 100mA white LED devices and thought I'd make a PCB to do some tests. My original plan was to make a 7x7 grid of 4 LED clusters for a total of 196 lights, but the price of the PCB was too much.  As I'm using a prototyping service that gives me 5 copies of the board I decided to break it up into smaller boards that could be tiled.

As this was for testing, I wanted the lights to be configurable, so each cluster of 4 LED's goes to a standard pin header.  To power a cluster, you just add a jumper to the header.  Simple.  The board also includes some current limiting resistors to help current sharing, but the board isn't designed to be a standalone device, and needs external control to be driven properly.  This is why the header pins are in between the LEDs and the resistors.  If needed an external connector and be plugged onto the header pins to drive the LEDs, removing the resistors from the circuit.

After weighing up my objectives I decided to use 20 ohm resistors to limit the current, but at 100 mA the dissipated power would be 0.2 Watt.  It turned out to be more economical and flexible to use 2 x 10 ohm resistors instead of 1 x 20 ohm.

Top Layer

Breaking the board up into smaller portions is probably a blessing.  I'm not even sure I can solder these let alone a board that had something like 300 components, but I'll give it a shot.

When designing boards, if possible, write some notes on them so you'll know how to use them in the future.

Bottom Layer

I'll release the boards after a little more work (pressed for time at the moment)

Tiled PCBs

RF LED Dimmer Teardown

I was recently contacted by someone that had read my post about MeanWell LED driver units with dimming capabilities.  They needed advice on how to connect a driver unit to a wireless dimming controller they'd purchased.  After a couple of attempts at connecting the two it became clear there wasn't a simple way to it because of the way they're designed.  After buying one for myself for 20 bucks I thought I'd do a teardown and analysis of how the wireless dimming controller works and explain why they can't easily be connected.

RF LED Dimmer

The wireless dimming control unit is designed to operate from a 12 - 24 volt input and vary the brightness of a regulated load up to 8 Amps via PWM switching, whereas the MeanWell LDH-45 driver is a DC-DC converter that has an input terminal to switch an unregulated load on and off in a PWM manner.  The PWM input has a maximum input voltage of 8 volts, I think it can do more but the documentation is unclear.  So in theory we can put a resistor divider across the 12 V output of the dimming controller to reduce the voltage to a safe level and connect it to the PWM control terminal.  See if you can spot the mistake I've made in my logic.  I'll explain it later on but needless to say the connections in the diagram below wont work.  I'll give you some clues, disconnecting the positive output of the dimmer causes the light to go out, but disconnecting the PWM DIM terminal causes the light to stay on.

Initial Incorrect Connection

A 3 button remote control key fob unit is used to increase and decrease the brightness of the lights as well as turning them off.

Remote control key fob

For the low price I paid I expected the remote to be a low quality PCB with cheap components in a flimsy case.  Surprisingly the remote control is well built, with a water resistant rubber seal.

Rubber waterproof seal

There isn't anything too interesting in the remote.  It contains a user replaceable battery, some surface mount tactile buttons, and a status LED.  The marking on the crystal indicates that unit operates at 433.92 MHz.

Remote control PCB

The main dimming unit is also reasonably well constructed for such a cheap item.  Although screw terminals would've been nicer, it comes with spring loaded connectors like you'd see on the back of a set of speakers.  These are directly soldered to a medium quality single sided board of low complexity.

Bottom of LED Dimmer PCB

After the board is removed the operation of the device becomes clearer.  You can see that it contains a radio module (with a coil of wire for an antenna) that communicates with the key fob, some memory, an unknown microcontroller, a switching FET, a pull up resistor for the I2C line, and 3 extra components that make up the power supply.  The board is named "RF-DIMMER-MBK-V7"

Top of LED Dimmer PCB

The memory IC is an ATMEL 24C02N 256 byte serial EEPROM device.  I assume this is used to store the  device state as it is able to return to its previous on/off and dimming state if power is removed and reapplied later.

ATMEL 24C02N - 2048 bit serial EEPROM

The RF control module is connected to the board via a 4 pin header.  Two of these are connected together and I assume send data to the microcontroller.  The other two are 5 volt and ground lines.

RF Module PCB

Although it's incredibly hard to see, the switching MOSFET is a 50N03-10 CP device.  It has a 30 volt maximum VDS and a maximum drain current of about 50 Amps (Not at the same time of course).

Switching MOSFET

To explain why the device didn't operate as I first expected, it helps to colour code the different nodes on the circuit board below.  Once again the input is on the right.

Black - The negative input terminal and ground point for the rest of the circuit.
Red - The positive input terminal.  It's directly connected to the output positive terminal and supplies power to the rest of the control circuit.
Purple - This is the input voltage after is has passed through a reverse protection diode
Yellow - This is a 5 V line supplied by 7805 voltage regulator
Dark Blue - This is the data line that is returning from the RF module
Light Blue - The Mosfet gate drive
Green & White - SDA and SCL lines of the I2C bus
Orange - Negative output terminal

Voltage Node on PCB

To help make it even clearer why my first connection attempts didn't work I redrew the schematic in a more conventional fashion.

RF LED Dimmer Schematic

As you can now see, the circuit is relatively simple. It takes the input voltage through a reverse polarity protection diode and feeds it into a linear regulator to provide a 5 Volt line.  This is used to power the memory IC, RF module, and an unknown microcontroller.  The microcontroller reads data from the RF module and stores this configuration data in the EEPROM memory.  The microcontroller then adjusts the PWM waveform driving the gate of the MOSFET turning the load on and off rapidly and changing the brightness of the light.  It does this by effectively disconnecting the negative terminal of the load.

Why my first idea didn't work is becoming clear.  I assumed that the switching element would be on the high side of the load, with the load connected to ground and the switch between the load and the 12 volt line as in figure b below.  Connecting a voltage divider as a load would have produced a PWM signal between 0 and 6 volts, but this wasn't the case.

The switching mosfet is on the low side of the load as in figure a below, and for a good reason too.  This allows the gate drive voltage of the mosfet to be independent of the supply voltage.  A high side switch would require a p-channel mosfet and a gate voltage of 5 volts less than the input to drive it, but because the input voltage can vary from 12 to 24 volts the gate drive voltage would also have to vary.  This can be done, but it adds extra parts.  It's much easier to use a low side switch and drive it with a 5 volt signal that doesn't change as the input voltage does.  As this device is designed to control LEDs that aren't connected to anything else, it doesn't really matter.  The problem only arises when you are connecting multiple devices.  Placing a resistor divider across an output like this will give a PWM signal that changes between 6 and 12 volts.  This means that the light will come on and stay on.  Disconnecting the positive output of the dimming unit will cause the PWM DIM and the -DIM terminals to be connected with a resistor, placing 0 volts on the PWM DIM turning the light off.  If you disconnect the PWM DIM line the light will turn on as mentioned in its data sheet (leave unused wires unconnected).

The MeanWell LED driver really requires an output configuration similar to figure c, where the output terminal is forced to high or low voltage, the other configurations can force the output to a high or low voltage but not both.

Load switching configurations. Assume 12 volt input on the left of each figure, load on the right.

To look at the waveforms of the dimming device.  I placed a dummy load resistor on the output and measured the voltage across it.  The image below shows the device at two of its 32 dimming settings.

Voltage across load at dimmest setting

Voltage across load at approximately quarter-brightness

After thinking about it, there is a way to connect the unit to the MeanWell controller.  The voltage used to drive the FET is exactly what we need to drive the PWM DIM pin.  So if you were to open the box and solder on a wire to the light blue trace in the above diagram everything should work.  Bringing the output of a microcontroller out to the real world without protection isn't ideal, and the current the microcontroller can supply is unknown.  You could give it a go, it should work.

Voltage on the MOSFET gate

As an aside my initial tests were with a long strand of LEDs and I got some strange results.  What was happening here was when the low side switch was turned off the cabling in my setup was coupling in noise from the mains voltage.  Testing with a resistor fixed that though.  Doh! 

Initial incorrect experiment

Controlling a USB Relay with a Raspberry Pi

I love the Raspberry Pi.  It allows me to prototype ideas quickly and without too much effort, but one thing I despise is the GPIO.  Maybe despise is too harsh, but it's a clunky way to interface electronics to what is for all intents and purposes a striped down personal computer.  Don't get me wrong, the GPIO needs to be there and I've made use of it before, but if I want to quickly prototype something I have to spend time figuring out how to use it and then I've got jumper wires all over the place.  I much prefer to use devices that are connected to the USB port.  They can be easily reused on other computers and the final solution is much neater.

Recently I've been toying around with a project that needs to switch a load on and off.  It's a low voltage LED light so nothing too complicated, and I have no problem figuring out how to switch the load via a transistor connected to one of the GPIO pins, but everything gets messy and then I have to do up a small prototype board to mount the parts on.  You know what would come in handy?  A USB controlled relay.  Unsurprisingly someone has already thought of that, they're all over eBay and Aliexpress, some are controlled as USB HID devices and some use the good ol' USB to UART method.  I bought a couple of the USB to UART style devices from www.lctech-inc.com to play around with, and at $10 AUD each it's not worth my time to come up with a custom solution.

Top Side of PCB

There's not much to the module.  As you can see above it's what you'd expect.  A USB connector, a relay, a set of terminal, and a few electronic components.  When plugged in, you send commands to it over a virtual com port to turn the relay on and off.  There are two status LEDs, one to indicate the device has power and another to indicate the relay is active.  Couldn't be easier.

PCB Mounted Relay

The Songle relay used seems to be the de-facto standard in this area of the market.  It's a standard NC/NO relay that can be switched by applying 5 Volts to the coil.  The data sheet isn't too clear but requires 90 mA or less to actuate the relay.  The contact resistance of 100 milliOhm isn't the worst I've seen either.

Between coil & contact - 1500VAC 50/60HZ (1 minute)

Between contacts - 1000VAC 50/60HZ (1 minute)

The relay is sufficiently rated for mains voltage use and has adequate isolation, but I wouldn't use it.  I don't like running mains voltage through random crap I bought off eBay.  Besides that, even if the relay is rated for 10 Amps, I'd be surprised if the tracks on the PCB (hidden under the relay) are up to the job.  The connector definitely looks like it's not rated for that much current.  Based on that, this device will only be used for low voltage medium current applications.

CH340T USB to Serial Controller

As mentioned before, the relay is controlled by sending commands over a USB to serial converter.  In this case a CH340T IC is used.  The only data sheet I could find for this chip was in Chinese and wasn't very helpful.  The Raspberry Pi has the drivers by default and I've read that the latest versions of Windows do as well.  Disappointingly, the serial number of the devices aren't set or are unable to be read in Linux.  This has the side effect that you can't connect two of these relays to a computer and deterministically know which relay is which.  In Linux, if two devices are plugged in at once they enumerate at /dev/ttyUSB0 and /dev/ttyUSB1, but you can't be certain that they enumerate in the same order if you reboot.  I'd hoped that I'd be able to tell them apart by reading the serial number of the USB devices, but as they aren't set, no luck.  It doesn't really matter, I only need to use one of them, but if I did need to use 2 devices I could buy a two relay board.

Edit - Bingo.  I think I've found a way to do it by using the physical mapping of the ports.

pi@raspberrypi ~ $ ls -l /dev/ttyUSB0
crw-rw---T 1 root dialout 188, 0 Jul 22 19:46 /dev/ttyUSB0

pi@raspberrypi ~ $ ls -l /sys/dev/char/188:0
lrwxrwxrwx 1 root root 0 Jul 22 20:01 /sys/dev/char/188:0 -> ../../devices/platform/bcm2708_usb/usb1/1-1/1-1.3/1-1.3.1/1-1.3.1.3/1-1.3.1.3:1.0/ttyUSB0/tty/ttyUSB0

pi@raspberrypi ~ $ lsusb -t
/:  Bus 01.Port 1: Dev 1, Class=root_hub, Driver=dwc_otg/1p, 480M
    |__ Port 1: Dev 2, If 0, Class=hub, Driver=hub/5p, 480M
        |__ Port 1: Dev 3, If 0, Class=vend., Driver=smsc95xx, 480M
        |__ Port 3: Dev 5, If 0, Class=hub, Driver=hub/4p, 480M
            |__ Port 1: Dev 7, If 0, Class=hub, Driver=hub/4p, 480M
                |__ Port 3: Dev 20, If 0, Class=vend., Driver=ch341, 12M
                |__ Port 4: Dev 10, If 0, Class=stor., Driver=usb-storage, 480M
            |__ Port 3: Dev 8, If 0, Class=vend., Driver=r8712u, 480M
        |__ Port 5: Dev 9, If 0, Class=HID, Driver=usbhid, 1.5M

The 1-1.3.1.3:1.0 string describes the physical layout of the USB device.  The device is plugged into port three of the first hub which is plugged into port one of the second hub which is plugged into the Raspberry Pi on port 3.  I've highlighted the important bits to make it a bit clearer.  BTW The reason I use two hubs is because I have a USB 3.0 hub connected to the Pi and the USB 1.0 USB to serial converter needs to be connected to a USB 2.0 hub for the pi to see it.  It's a bug with the Pi.

Bottom Side of PCB

The quality of the board isn't too bad.  All the SMT parts that are automatically placed have nice clean solder joints, but the hand soldered through hole parts leave a lot to be desired.  There's a lot of flux residue on the board and the pads are too small causing the solder to ball up on the leads resulting in weak joints.  I have to give them points for routing out an isolation slot in the board though.

Operation is pretty self evident.  The SOT-23 transistor Q1 is used to apply current to the coil of the relay.  If you look at the top of the board there's also a reverse biased diode placed across the relay coil to stop back EMF spikes.

IC with Markings Scratch Off

The IC on the top handles the USB to serial conversion, but the serial commands still need to be interpreted and used to control the relay.  I assume the IC on the bottom side of the board with its markings removed is some sort of micro-controller to do this.

Controlling the relay is dead simple

You first need to configure the serial port at 9600 baud with a character size of 8 bits.

stty -F /dev/ttyUSB0 speed 9600 cs8

Then you send the command (in hex of course) A0 01 00 A1 to close the contacts

echo -n -e '\xA0\x01\x00\xA1' > /dev/ttyUSB0

Then you send the command A0 01 01 A2 to open the contacts.

echo -n -e '\xA0\x01\x01\xA2' > /dev/ttyUSB0

Testing the Relay

I think these are great for prototyping.  Not only can I use this on a Raspberry Pi, I can use it on a desktop computer or possibly a phone or tablet (not tested).  It's also great for people that are coming from the software world who may not be comfortable interfacing with the GPIO port of the Pi.  They can plug it in and deal with what they know best (software), and within a few minutes control devices like fans, pumps, lights or whatever else you could imagine.