Modifying the WH3080/3081 for constant backlighting when USB is plugged in
The WH3080/1 base station console has a green backlight that normally comes on for 10 seconds whenever a button is pressed.
A number of people are now using the console as a bedside clock. The problem is, when one wants to check the time and/or weather conditions in the dark, a button must be pressed to activate the backlight. And often, the backlight is blaringly bright.
So, to turn the WH3081 console into a usable night clock, a modification can be done to keep the backlight constantly on at a dim level whenever the USB cable is plugged in. This mod takes advantage of the fact that many users will have the base station constantly connected to the computer for datalogging.
Background
The 3081 console can use two power sources - 3 x 1.5v AA cells (4.5v) or 5v from the USB socket.
Diodes at each power source prevent one from attempting to flow into the other. This is especially important for USB power, because we don't want it to force a charge into the batteries.
There are four green LEDs that illuminate the LCD. These are normally switched by a PNP transistor, by an active low signal from the microcontroller.
The LEDs anodes are connected (via their own current limiting resistors) to the collector of the switching transistor. The emitter is connected to the 5v supply rail.
When the transistor is conducting, the LEDs receive the full voltage and light up brightly.
The Mod
The mod takes advantage of the existing diodes. By tapping power from the USB source before the diode, we are able to get power only when USB is plugged in. If USB is unplugged, no voltage will be present. And most importantly, no power will be drawn from the batteries. If we were to make the LEDs light dimly regardless of whether the USB cable is plugged in or not, the batteries wouldn't last very long (perhaps a week or so).
The console needs to be stripped down. Start by removing the batteries and the stand. Then remove the 8 screws. Pull the back cover off, being careful of the antenna wire. Remove the screws that hold the circuit board to the front bezel. Lift the board off, and take the LCD panel off.
To get the LEDs to light up dimly, a resistor is needed. A 10k resistor will allow the display to be lit so it can be easily seen in the dark once the eyes have become accustomed to the dark. For a brigher display, use 5k, and if 10k is too bright, 15k can be used.
An additional diode is also added, so that if the USB cable happens to be plugged into an unpowered computer, the backflow from the normal lighting won't attempt to send power back to the USB bus. The diode also helps to drop the voltage further, so a high value resistor doesn't have to be used.
One end of the 10k resistor is soldered to an existing SMD resistor that is connected to the collector of the PNP switching transistor. The other end is soldered to the cathode of the diode. The diode's anode is soldered to the existing USB power diode's anode. If done neatly as per the photos, the added resistor and diode will sit nicely behind the LCD panel.
Tests can be done at this stage. Position the circuit board so that it won't short circuit on anything metallic. Plug the USB cable into a running computer. Observe that the backlight remains dimly lit when their normal brightness times out after being activated from a button press. If the LEDs don't remain dimly lit, the added diode could be in reverse polarity. If you want, you can sit the LCD back onto the circuit board to observe exatly how the backlight looks in dimmed mode. Now is the chance to fix any problems or to substitute other resistor values (to adjust desired brightness) before reassembly.
Once the tests are complete and the dim backlight is working properly, reassemble the console (in reverse to the disassembly steps).
Once the mod had been done, the console will be dimly lit whenever a powered USB cable is plugged in.
When pressing a button, the backlight will go into full brightness as it normally does. Once the timeout is reached, the backlight will become dim again.
This mod is very handy to let you know if the computer it is connected to is switched on. A number of users are using USB-to-Cat5 extender plugs, which allow the console to be located up to 20m away from the computer, which is especially handy when the console is to be used in a quiet area such as in a bedroom.
Because so little power is being used to light the LEDs dimly, the unit should not become warmer than it normally does. This is especially beneficial if the indoor temperature is being monitored for an important purpose.
If one does not want to keep a computer running just for the backlight, a USB wall power adapter with USB extension leads could also be used to keep the backlight lit, as well as preserving the batteries' shelf life. When the console is USB powered, the batteries are effectively disconnected.
ABitChilly, Tue, 11 Mar 2014 06:37 am: Reply