hk backlight mod

[Daryoon]

I keep seeing the mention of a 470ohm resistor being used on the HobbyKing backlight. Mine looks to have an extra zero. 4700ohm.

I use 2 li-Ion battery. So I get 8.4v. Looking at this thread, I think if I drop the resistor value to about 360ohm or so, I will get brighter backlighting.

*strikeout*Anybody know why mine came with 4700ohm instead of 470ohm? Mistake at the factory?*strikeout*

[pmackenzie]

Mine was the same. Check it with a meter and it will be 470 ohms.
They must be using 1% resistors, which have a 4 digit code

http://www.marsport.org.uk/smd/res.htm

4700 = 470 followed by 0 zeros = 470 ohms.

Pat MacKenzie

[Daryoon]

I was waiting for that post to come out of moderation so I can edit it. But you beat me to it.

I checked with my multimeter shortly after posting and it was indeed 470ohm. I replaced the resistor with a 320 ohm one I stole from and old circuit board I have that I use for donated smd components.

Honestly, with the white backlight...it was fine with the 470 ohm resistor in place when using two cell Li-Ion. No complaints there. However, on a buddy's TX, he chose a blue LCD one and I thought it was too dim and hard to read. I think reducing the resistance on his TX will likely have a more substantial impact.

[Westy]

So does reducing the Resistance increase the brightness much? I was thinking of ripping out the 4 LEDs that come on one of the many Xbox 360's i have stripped for parts and using them in there ..... run 3 in series rather than 2 and take the resistor right out? will they burn out do you think? or should I put 4 in to use up the excess voltage and use it for light rather than heat?
Cheers
Westy

[ShowMaster]

Led data sheets may give a current vs brightness graph? I've never look to see, just faked it.


Current flow through LEDs determine the brightness and the voltage is needed to start current flow. I find that 1.3 volts seems to be a good place to start but I'm sure that's debatable and others will more precise value. Once the led starts to conduct the current goes up and brightness goes up quickly but not necessary linearly. Most LEDs shouldn't be allowed to draw more than 20Ma or their life will be cut short.
In the case of the HK BL that runs off of the TX battery, let's use 9.6v, what isn't dropped across the LEDs must go somewhere and that's where the 470 ohm resistor comes in. It limits the current to around 14.25Ma (14Ma for this example) for the white LEDs (I never measured my blue) and drops about 7volts leaving 2.6 volts or 1.3 volts across each of the 3 LEDs. So if we use 1.3 volts and 14Ma as a safe voltage and current for our LEDs, we can add or subtract LEDs in series as well as calculate the required series resistor value needed. All this is a little fuzzy math but should be close enough to not burn your LEDs up and still get the brightness. If you add up the number of series LEDs voltage ( 1.3v/led) and subtract that from the max voltage you will use, and devide the voltage left by the current of .014 or .020, you'll get the resistor value you'll need.
So E(battery- #of 1.3V led drop)/.015 or .020= resistor needed.
Example for white HK BL with 2 LEDs and 9.6v pack.
9.6V-1.3v-1.3v=7v/ .014Ma=466 ohms and 470 would be used.
If you use 1.3v/per led drop and .014-.020(max) Ma, and simple ohms law you should be able to add or subtract LEDs, or change resistor values to keep from damaging the LEDs.
Other types and color LEDs do require more or less current and voltage to achieve the desired brightness without being damaged.
As always do your own math before powering your circuit and wait for additional corrections or additions to my post from others as mistakes in math and theory does happen but this should be a good basis to start with.
I posted an my actual white led HK BL current draw vs voltage earlier.
SM 

[Westy]

Thanks SM .... I will leave it for now I think.

NOT ...... I just cant resist (excuse the pun) .... got to make it the best it can be
Cheers,
Westy

[Westy]

I keep seeing the mention of a 470ohm resistor being used on the HobbyKing backlight. Mine looks to have an extra zero. 4700ohm.

I use 2 liion battery. So I get 8.4v. Looking at this thread, I think if I drop the resistor value to about 360ohm or so, I will get brighter backlighting.

Anybody know why mine came with 4700ohm instead of 470ohm? Mistake at the factory?

What size..... (Not Value) are the original 470 Ω .... I was thinking perhaps a 602 series? or is that too big physically to swap out?

0201 (0603 metric): 0.024 × 0.012 in (0.61 × 0.30 mm) Typical power rating for resistors 1/20 watt

Cheers
Westy

[Westy]

Ok ....

I measured it and the size we need if changing out is
1206 ... (3.2L x 1.6w) So i am getting some

250 of them for $3.50 shipped to NZ .... just can't beat it!

[pmackenzie]

FWIW, I am using 470 ohms and driving the backlight directly from the MCU pin so it is only getting 5 volts.
A FET is not required this way and the MCU can easilly handle this load. Pretty standard to drive LEDs from mcu pins with 470 ohm resistors.

Seems plenty bright to me this way, not sure I would want it any brighter. (HK blue backlight kit)

Pat MacKenzie

[Westy]

Awesome
can you show us how you did it PM?

So basically from the +5VCC line on the MCU that same as the USBASP progammer feed line? off that exact same pin?
Did you use the existing 480 Ohm Resistor that is already on the HK backlight PCB?

Cheers
Westy

[pmackenzie]

Took an image and noticed an OOPS

I used a 47 ohm resistor by mistake instead of 470
I checked and it is dropping 1.4 volts, so that means 30ma from the mcu pin. No wonder it is bright enough!
AFAIK that is safe enough for the pin (data sheet shows 40mA) , but I might change it out for something a bit larger to get it down to ~20 ma.

The negative lead from the LED goes to ground and the positive goes through a regular (non surface mount) 47 ohm resistor to the pad marked " Vdd" on the Smartieparts board.
This is is connected to the backlight pin on the mcu so all the normal ER9X backlight functions work. Make these solder joints quickly so you don't affect the pogo pin.

Pat MacKenzie

[Westy]

heh Heh

So I am really thinking the 360 ohm should be perfect LOL

I dont have a smartie parts ..... went for Vanilla DIY Style! ha ha.

Not that I am not i do not like/ respect or appreciate the Smartie parts technology ...... I love them ... but looking at the price .... i would rather find another way and buy another TX at the same cost.....
must be the KIWI in me ....... DIYer at the core and lovin it!

I think I am on the right track and will perhaps continue down this path and ride the gauntlet to the bottom of the hill and let you guys know what bumps and bruises I get at the end LOL

I Must pull this thing (9x) apart and measure the feed voltages coming off the HK board and see what is actually there
Cheers
Westy.....

Ps .... finally putting my summary of upgrades and mod into one vid ... just in case anyone may like to have a peek?

[ShowMaster]

FWIW, I am using 470 ohms and driving the backlight directly from the MCU pin so it is only getting 5 volts.
A FET is not required this way and the MCU can easilly handle this load. Pretty standard to drive LEDs from mcu pins with 470 ohm resistors.

Seems plenty bright to me this way, not sure I would want it any brighter. (HK blue backlight kit)

Pat MacKenzie

Pat
I too am looking for a simpler way to drive the led BL that seems to be popular these days. I do want to elemnate the 2 connectors and it's wires that comes with the HK BL but still use the BL. From my previous posted voltage vs current list I see that with the 470  ohm resistor still used  and the 2 series LEDs being driven with 5 volts only draws 5 Ma. That's almost 1/3 of the current draw at 9.6 volts and still you're OK with the brightness. Off hand what's the max current pin 17 can source? ( I think it's 20Ma max?)
I am thinking of just using the Fet and a resistor (470 ohms to start) powered off of the TX battery source from the switch board to down size the whole circuit footprint and get rid of the HK circuit. If I read the atmega64 specs right I see that the max source current can be 20Ma. If this is correct than a 270 ohm resistor would limit the current to 10M and should make the LEDs brighter and still be safe? 
I also looked up the 78L05 current max output and it's 100Ma. Is there any concern of the added led current stressing the regulator where it mat shut down or be damaged? I'm not sure how much it's delivering under normal use as well as the heat sink area?
I'm just not comfortable putting any unbuffered current draw on the CPU that controls my plane to save parts but I'm open to more discussion about it.
I posted this for discussion and in no way am I an expert, I'm just doing some research planning ahead of doing some changes.
SM

[Westy]

Why not run a full batt feed from the charge port, which I have utilised as my common port and then run the resistor off that as a completely separate circuit seperated from and other part of the system then you can draw what you want and do whatever you like with it.....you could run a diode to prevent any non wanted activity on any other linking circuit from the one we need to drive the LED circuit?

I am certainly not an expert in this field .... but am an Auto Engineer and Turbo and Fuel injections specialist by trade well used to be .... would this work?
Cheers
Westy

[ShowMaster]

Yes, basically that's the way the HK BL does it through the in Line connector assembly provided with the BL. I'm leaning towards just using he BL, a 470 ohm resistor, and a fet with the voltage coming from the battery. Your charge point is the right idea because it should be battery. I was thinking of the power switch area but it's all connected. I'm not sure you need the extra diode since the LEDs are already diodes but can't hurt except it will drop another .6 volts. The battery buss is somewhat isolated because all other circuits are fed after going through regulators and some filter caps.
SM

[Westy]

yeah,

I run My battery charge circuit through the Battery charge plug on the tx. I cut the old wires off the TX module board at the diode and went back the other way and then hard wired the original battery terminal/ plug back onto the Charge port as well as the battery connector plug which I added into the system.

However thinking about it, I might remove the Diode and replace with a jumper wire and then use that diode to protect a back surge from the LED feed I will be running ... If we can nut it out.

[ShowMaster]

Quote of post directly above removed by your friendly MOD
If you remove or jumper the diode consider using a fuse, Futaba uses a 2A fast blow I'm told and Spektrum uses a 3A fast blow. This will save the circuitry if you short the charge jack. Never put the fuse in the actual TX power circuit because you don't want it blowing while flying. Only put it in the charge circuit. I use 3a FB Pico fuses the size of a 1/4 watt resistor with solder leads. I jumper the diode so that if i do blow the fuse I still have a charge path through the diode again. Without the jumper the charge jack can't be shorted and do any damage but it limits charge current to 1A because of the diodes limit. Poco's aren't easy to find so use the smallest package you can find. In the past I used a small single wire strand as a jumper but it has to be thinner than the smallest conductor in the circuit to go first if there's a short.
If you're using a totally different power source to power the led circuit the diode or diodes could be used to isolate feed circuits. If only one source for power " back feeding" of power or surge is not an issue unless you're using some circuit with an inductive property or higher voltage or large value caps.
I'm a little confused about your actual charge circuit desiring but the main thing is not be able to power the TX on while charging. This can damage parts from higher voltage and voltage spikes and that why the power switch is wired to prevent that.
SM

[Westy]

Hi Sm,

Funny you say that about picos
I was looking at the replacing the DIODE for a Pico as it has muscle Memory...... let me ask you this...... does it matter about thee voltage rating? I would think not .... .as fuses are designed to blow on heat (current)

What about a Poly Switch (PTC) Fuse?

if the short is removed the circuit recovers almost instantaneously


I can pick one up from the local Electronics shop for about $5.00 I was thinking about a 1.8amp rating to allow a 150% over amp rating will allow for a gaurenteed blow at 2.7amps

Will that work instead of throwing in a jumper lead..... at least it has a rated shut down value.

Cheers
Westy

[ShowMaster]

Quote of post directly above removed by your friendly MOD

I have no experiance with PTC fuses but as long as their not in the tx power circuit give
It a try. We all have done some R&D and learned something new. I shy away from fuses in the actual tx power path but JR and Spektrum for sure use a 3A FB glass fuse that if blown kills the whole radio. I think it's to keep a major Internal short from burning up the TX, and I mean burn.
To be safe I guess we all should have a 3A fuse in the battery lead to protect the TX and maybe a 1.5-2a in the charge jack circuit if we use it to prevent a plug short from doing damage.
I'm thinking as I type, maybe a good idea?
Try your fuse ideas so we can learn something.
SM

[jhsa]

I don't think fuses on the radio circuit is a good idea.. on the charge circuit yes..
Just my 20c

[ShowMaster]

Quote of post directly above removed by your friendly MOD
I agree but found out that all thee big radio co's have had them in the power circuits for years, and years.
Many newbees at our field keep blowing the fuses charging backwards and their TX's goes dead!
Happens a lot these days it seems. One flyer said he just puts in in a lager fuse each time it blows until it stops blowing and tells others to do the same. I imagine he's close to a piece of wire instead of a fuse by now. Not all rc fliers are electronic technicians and not all electronic technicians can fly rc, and then there's the rest of us ha ha.
So for me, no fuses in the main power path for now. I believe the new ersky board uses a .1 ohm resistor as a failsafe device/fuse if I read the posts right?
I'm using the 9x circuit traces and small connector wire and pins as my emergency fuse for now but did add a 3a Pico fuse to the charge circuit.
SM

[Westy]

Well ... That did not work....

if did the Mosfet fix and changed out my 470 ohm SMT resistor on my HK BL kit and the 2N7000 Mosfet ..... and it all works but the light does not shut off.... what the????

[pmackenzie]

MOSFET connected backwards perhaps?
(Source and drain reversed)

[Westy]

Got it the same way as in the picture ...



Trace is cut below Resistor and I tested it ... there is no connection there between the left side of the resistor and the second pint from the right side of board.

Looking at the mosfet as it is.... the right pin is connected as shown. middle leg goes to pin 17 on ATMEGA64 at board and left most pin goes to neg side of LED Backlight

I did change the resistor for a 360 ohm ..... don't think that would make a differnce... would it?

[jhsa]

source" goes to ground and "Drain" should go to the negative leg of the LED (K).. then LED positive leg (A) goes to the resistor and the other side of the resistor to the +
The load (LED and resistor) must both be on the positive side of the fet (drain).
It looks like you have the resistor on the drain and the led connected to the source????
I hope this helps.. and as always if I'm wrong please someone correct me

[cre8tiveleo]

Can you post a pic of how yours looks?

Posting a pic of someone elses 'working' mod, when yours doesn't , is like bringing in a friends car when yours needs work done.

maybe you toasted your transistor.




---
I am here: http://maps.google.com/maps?ll=44.056166,-79.476086

[jhsa]

I have a EL backlight not LED.. and al so I used a smd fet that I took from an old broken servo.. if I remember is all under hot glue
Can't post a picture of a blob of hot glue.. I guess it wouldn't help
João

[jhsa]

according to the datasheed looking from the top from left to right it is "Source", "Gate", "Drain"..
If we look at his picture and if the led goes to the left leg, then is going to "Source"... Source should be connected to ground..

[Daryoon]

I used a BS170 transistor in the above. So the legs of your transistor may be different.

See the original link I used. Has a video as well and the schematic for the BS170 transistor.

http://ok-rimr.com/en/how-to-do-it/turn ... -hk-skiner

Changing out to the 360ohm resistor shouldn't be your issue. Going from the 470 to 360ohm allow my backlight to be brighter with 2s Li-Ion.

[Westy]

Yes that is the one I followed.

How do I know which let is which if it is reversed.?

Is there a test I can do on the mosfet itself .. I have 5 of them so can don one off the board and put it into a test bread board....