Big LED Temperature Display


My wife and I get up early and walk most mornings and we wanted a way to know the outside temperature before we ventured out of the bed.  We looked at some of the wireless models but the bed was too far from the display to see the temperature.  Since my Homevision home control system, already has the outside temperature based on a direct connect analog temperature sensor, I decided I would see if I could build a display that was large enough to see from across the room.  Here's a picture of the finished display, read on to see the hardware and software details.

Finished display


Of course the most important piece of hardware was large LEDs that would be easily visible from across the room.  I ended up finding exactly what I wanted on Ebay.  I purchased 5 - 2.3" LED displays for about $9.00.  Here's the only data I was able to find, but it had all of the data I needed.

led specs

Once I had the displays it was time to specify the rest of the hardware.  I chose to use an ATmega controller from Atmel to control the display.  It was too far from the controller to send the temperature via a serial link so I decided to use RS485 as a communication protocol.  I needed to drive the displays from a voltage greater than 7.5V as you can see from the display data and I had 13.3V available from the main controller, so I chose to use that voltage.  Due to the fact that the ATmega168 and the LTC485 were limited to 5V, I needed to use a high side driver to control the 13.2V for the display.  Finally I added an LDR (light dependent resistor) to detect ambient light levels so that I could dim the display at night.  Once I had the schematic nailed down, I designed a custom PCB to make the job easier and more professional.  Here are the schematic and PCB, you can click on the thumbnails to see a full size image.

And here's the front and back side of the finished PCB.


The software was written in BascomAVR and is pretty straightforward.  The main loop measures the light level, checks for an update from the home controller, and then services the LEDs.  The light level is read from an analog input and the program does a 10 point moving average to get the best data.  The home controller sends a new temperature once per minute in the form of @XX where the @ sign indicates the beginning of a new reading, and the XX is the current temperature.  In this part of the country, we really don't have to worry about negative temperatures or temperatures greater than 99 degrees.  Finally the software services each digit for 3 milliseconds, which produces a 6 millisecond update loop.  Each loop the LED is on from about .250 to 3.0 milliseconds based on the ambient light level and then off for the remainder of the time.

Finished Project

This was my first project using LEDs in this fashion and I'm very happy with the way it turned out.  Here are a few pictures of the final result.  First the back side of the assembled PCB.

Here's the final project installed on the bedroom wall.  The LDR is located just above the green LED which indicates the status of the security system.

Below is a link to download the BascomAVR source file.  The schematic in TinyCAD format and the PCB in FreePCB format.

Download Files