Thread: Controlling back lights directly?

x2 on the PICs. Far better than and simpler circuit and calcs than SMPS controllers (why use them?).

Light brightness is proportional to the PWM duty cycle (for most typical lighting), hence why they are the common dimmer for mixes of tungsten, halogen and LED lights. Otherwise they are the common dimmer simply for cost, sizing, and heat reasons.
Hence to the use of PICs. Apart from simple applications where the 555 circuit or similar is fine, any time the ramping up or down is required, PICs are the go - eg, dimming of lights instead of turn-off.
And if BOTH rampings are required - eg, the brightening "ON" and then dimming OFF for dome or puddle lights - the PIC IMO is the only answer (the electronic circuitry for that gets quite complex).


PICs are essentially cheaper than the 555 anyway. Plus it's easy to add a full-on for 100% unlike 555 and similar circuits which have a max of 99% or 99.9% etc (without additional circuitry). (Their are various commercial chips that variable PWM with full-on, but they are hard to source if not to pricey and may have limits (eg, minimum 30% duty cycle). PICs overcome such limitations.


All the PICs have PWM outputs.
I finally crossed over (from logic etc circuits and CPUs) to PICs when the $4 8-pin 08M2 with its ~2,000 lines of program space was released. I have several - not that any are used yet (But hey, I finally soldered some SMD 08s to a 30 year old circuit board I had... Now for the voltage regulator, and then...)

Just been reading the thread and I was reminded about looking at my LED backlighting once again. I just now popped the monitor from the vehicle and was surprised to find that this monitor does indeed adjust LED backlighting. However it’s only a 3 step adjustment for Bright, Normal and Dark and is automatically controlled via a small Photo Diode. It’s a PWM control from a small separate PCB labelled “funnily enough” LED DRV.

This PCB has normal 2 wires running to the LED array and another small 4 pin connector. The 4 pins were earth, +5v, +5v and the other varied from 0.4 volts to 1.2 volts in response to the Light detector circuit on the Main PCB. Quick test and I found I could get the LED backlight to go way down in brightness with 1.8 volts (normally 1.2V at Dark setting) on that pin and brightness variation was linear with voltage from 0.4 to 1.8V.

I’ll see about either dropping the Vehicle PWM control line down to match this variation with a resistive divider or just play it completely safe and use a $1.50 Optocoupler for a totally isolated connection. In either case I’ll gate the existing 3 step Auto control (it’s handy for day use) and have the Vehicle PWM control signal switch in with Vehicle lights.

Total display consumption @ 12V is 6 Watts with bright and around 2 Watts dim backlight.

X3 on using PIC's especially as davekra mentioned, picaxe for those starting out.

Yeah It's looking like I'm going to check the PICAXE out. I tried getting into PICs a few years ago but then (at least with the tools I knew of) I had to code in assembly. I just didn't have the time to invest to learn assembly to an adequate level to get beyond much of anything so it ended there. I see these are programed in basic which is a huge bonus. It's been a LONG time since I used basic - it had line numbers back then - so I've got a bit to learn but I could kinda follow the code here so maybe there is some hope there. Even looking at the 8 pin chip, for what it has onboard for only a few bucks just seems ridiculous! Oh the possibilities.

Exactly, the PICAXE is just a Microchip PIC device with a boot-loader and Basic interpreter built in. As said, very low cost and great for small projects without the steep learning curve of ASM or expensive programming hardware, tons of code out there to copy or modify. I'm sure someone here who uses them can offer a few good links if needed.

G'dam high level languages! It's Assembler for me!


But maybe for a few simple mods to existing coding...

Well the plot thickens.

I have traced out the circuit for the LED Backlight Control. The IC appears to be a small (SMD) 6 pin Step-up convertor. The current through the LEDs is sensed and SET by parallel connected resistors 2R2, 4R7. With this layout I would assume overvoltage protection, open circuit protection, over current protection etc.

Interesting Note: This monitor also adjusts the Brightness and Contrast levels along with the Backlight level with the 3 Step Ambient light sense circuit located on the Main PCB.



My FE reads the PWM dash-light signal from the Vehicle and adjusts the Windows Brightness level, this works great in conjunction with the Auto Backlight on the Monitor – however it is STILL not quite enough for country night driving and this is where further reduction of the LED backlight will make a big difference. My initial interest in Backlight control was the hope it would maintain good contrast and color depth at low output levels, which is not always the case with trying to bring Brightness and Contrast way down with normal controls and the Windows API.

Test carried out last night appear to indicate this is a far superior method and I had really good color and contrast with the LED backlight wound way down.

I’m currently trawling the NET trying to find an IC/circuit similar to this. There are some very complicated LED backlight driving schemes out there but mainly for large screen devices, haven’t found anything much like this as yet.

BTW this circuit layout looks very similar to the layout in the PCB pictures posted on page 1.

Well, back to trawling the NET.

for a minute there, i thought you knew what IC they were currently using in the screens..

Hi Soundman - Well look what I just found.

I'm HAPPY - I may have found something dam close! That line I labelled as a possible Enable is more than that as it can be driven with a PWM signal for BL control. This circuit even has the DC control of BL intensity (as in my unit) and the possibility of PWM control via the FB pin as well.

And here is the PDF I managed to find which is close to this device and circuit. 7937v130.pdf


EDIT: My unit is in fact PWM not DC on the FB control line (may bad) see next post. But DC control worked perfectly, after the filter (C5 and input resistor) the PWM would likely be a DC voltage anyway.

Small “embarrassing” update After looking at the circuit I decided to check the control voltage coming from the Main display PCB which drives the Backlight control line. Man, I have got to stop using my DVM for this as it shows the value of any PWM signal as a nice DC voltage. After checking with a CRO (oscilloscope) it is of course PWM as per the filtered input in the PDF and my circuit. So this circuit is a piece of cake to connect to the vehicle dash light control line.


Updated my previous circuit