I decided to do this build because I have wanted to make a large 10 segment LED bar display for a while. Having come across a 5050 SMD LED light strip, I wondered if I could cut a length of it up to use for such a display. I already had a couple of LM3914 LED bar/dot driver IC chips on hand, so I figured I'd give it a shot.
Step 1: Tools and Parts
- Exacto/Carpenter knife
- Jigsaw (with wood blade and finer tooth blade)
- Tape Measure
- Soldering Iron/Solder
- Wire Stripper
- Potentiometer (For Testing)
- Hot Glue/Glue
- 1/4" Plywood
- 1/8" or 1/16" Plexi-glass
- Wire (rated for your amperage)
- (1) LM3914 IC Chip(LM3914N-1/NOPB)
- Power Supply (Mine was a 120VAC to 12VDC, 500mA)
- Translucent colored plastic (I used plastic dividers)
- Posterboard or scrap food boxes (white for reflectors, black for front panel)
- LED light rope (mine had 5050SMD LEDs)
- Breadboard or Circuit Board
- (1) Connector to connect power supply to board (I used "PJ-102A")
- (3) Jumper/Shunt (969102-0000-DA)
- (3) 2 position header (961102-6404-AR)
- (1) 5 position header to connect to control board (I used "PPTC051LFBN-RC")
- (1) 2.2μF tantalum or 10 μF aluminum electrolytic capacitor (I used a 2.2uF ceramic I had on hand)
- (1) Resistor for limiting current sunk by LM3914 (I used a 850 Ohm 805 SMD resistor)
Step 2: Dismantle the LED Strip
I ended up needing 30 of the 5050 SMD LEDs (ten strips of three) for this build. The strip I was using had the LEDs laid out in series. If you are wanting to do something similar, make sure your strip is the same way. It also had places to cut every 2 foot or so without ruining the rest of the roll. One two foot chunk was exactly what I needed. Using a knife, carefully separate the outer plastic without ripping the flexible strip on the inside. After this, you can cut it into the individual strips you need (3 LEDs per strip for me). As you can see in the pictures, the manufacturer has placed resistors every so often to limit the current. I wanted to make it so the LM3914 IC did the current limiting and needed to jumper out the resistors as seen in the last picture. If I didn't do this, the segments with the resistor would be dimmer than the rest. I could have removed the resistor entirely and soldered a wire in its place, but I didn't want to risk heating up the flexible circuit any more than I had to.
Step 3: Wiring Up the LEDs
Now it is time to wire up the LED strips. Prepare strips of wire, keeping in mind that you need enough to reach wherever your control board is going. I soldered my wire to each side of the strip, attaching it to the surface mount leads of the LED. Use a multimeter on the "diode" setting to find out which is the anode (+) and cathode(-) side (going across one of the LEDs). Get the forward voltage (voltage required to get the diode to conduct) from the meter because we will need it later. If you are unsure how to do this, there are plenty of tutorials out there. If possible you can also look at the LED markings and look at the spec sheet to get the forward voltage as well. My 5050 SMD LEDs have a forward voltage of 3.2V and a forward current of 60mA.
Step 4: Reflector Fabrication
Once your LEDs are wired, you need to build a reflector for each segment
to reflect the light and shield the light from other segments. I made mine from old food box with a white interior and it works pretty well. Measure everything out so each segment will be uniform. Notice the two holes poked in the sides for the LED leads. Fold it into a box and tape/hot glue the flaps. Place the wired LED strip into the reflector and hot glue it down as shown. MAKE SURE THE STRIPS ARE ORIENTED THE SAME WAY! All anodes to the right or to the left, doesn't really matter which, but keeping it uniform will save a big mess later. Also place some hot glue where the leads exit the reflector to help take the tension off of the solder joint.
Step 5: The LM3914 Circuit
I chose to use the LM3914 for the control circuit on this build. If you are making strictly a VU meter, you might want to choose a LM3916 or maybe a LM3915 if you want a logarithimic scale. Simply put, the 3914 takes a reference voltage and uses a voltage divider and comparators to look at the input in order to tell which segments to light up. If you are inputting a signal that is 1/10th or the reference voltage, it lights up segment one.
Now it's time to think about what voltage we will need to drive this thing. Here is where the LED forward voltage comes in. Mine was 3.2V. I have 3 of these in series, so the total voltage drop across the strip will be 9.6V. That means that I needed a power supply larger that 9.6V. I had a 12V supply with enough current supply to do the trick. My LED's are rated for 60mA, but after testing brightness by lighting them up (using a resistor to limit the current, do not exceed rated current!), I decided that 15mA was plenty bright for my application. This means that with all segments lit up, I will be drawing (10*15mA) 150mA plus a little extra for the LM3914. Look at a datasheet for the LM3914, it discusses how to use the chip to set the current draw through the segments.
If you take a look at my schematic, you can see I use three jumpers. I wanted to make this design flexible in case I wanted to change my reference voltage or the mode of the light bar. If JP2 and JP1 are jumped, this sets the reference voltage to the 1.25V generated by the LM3914 and the segment current is controlled by R1. If a different reference voltage is desired, the jumpers are left off and the voltage can be set with an external circuit by wiring it to the REFOUT, REFADJ, and GND terminals (consult the datasheet to see how that works). JP3 jumpered puts the controller in graph mode, no jumper sets it to dot mode. I have RLo (Reference Low) hardwired to gnd, but now I am thinking that I should have made that an option too. I suggest breadboarding it out first to make sure everything looks good before you continue. KEEP IN MIND, the LM3914 is sinking the current, not supplying the V+ to the LEDs. The cathode side goes to the controller and the V+ is wired to the anode side. To check operation, you can use a potentiometer. With JP1,2,3 jumpered, you can connect one side of the pot to GND, the other to REFOUT, and the wiper of the pot to SIGIN. As you turn the pot, it should adjust the voltage on the input pin and light up the segments accordingly.
I make my boards with the "toner transfer and etch" method. I had to add a few vias and get creative with my soldering as I do not have a through hole plating process, but I was able to get everything connected. Check the circuit with a meter after components are placed to be sure everything is as it should be.
Step 6: Wire Up the LEDs
The next step is to lay the pieces out like you want them. I chose to bundle the segment reflectors and the wiring in three groups. Make sure they are in the right order, or you'll kick yourself. Cut the LED wires to the proper length and solder them in. Again, keep in mind that the cathode side goes to the controller and the V+ is wired to the anode side. Give it a test. IMPORTANT: LEDs can get pretty hot, when you are running your test, light up all the segments and leave it on for a bit. Make sure your reflectors aren't getting too hot. Catching your project on fire = BAD. We are trying to make a LED bar graph, not a fire starter. We will want to do this check again after it is in an enclosure and all closed up.
Step 7: Enclosure
For an enclosure, I used a small sheet of 1/4 plywood. Making sure to leave room to route wires, measure everything up and cut the sides for the box, all but one for the front. You will want the edge of the enclosure to be flush with the tops of the reflectors once they are placed inside. You'll have to place the board in the enclosure and mark where the power and control cables are going to enter. I nailed mine together and gave it a quick shot of black spray paint. Once this is done, you can arrange your control board and glue the reflectors in place.
Step 8: Front Panel
For the front panel, I wanted to use some thin plexi-glass I had on hand. But I wanted to give the segments color and hide the wiring, controller, and the reflector edges. For the segment color, I cut up translucent colored folder dividers. I had to double layer them to get the right effect. I had to measure up and cut a piece of black posterboard to hide segment edges and wiring. You can then tape the colored plastic to the back of this piece. Last step was to cut my 1/8 inch plexi-glass to the right size. You can score and snap it, but I always end up messing it up. I have good luck using a jigsaw with a small tooth blade and going real slow.
Step 9: Finish and Test
Drill the plexi-glass, lay the segment cover on, and then nail/screw the plexi-glass down. Test it again. As I mentioned earlier:
IMPORTANT: LEDs can get pretty hot, when you are running your test, light up all the segments and leave it on for a bit. Make sure your reflectors and enclosure aren't getting too hot. You don't want it to overheat and catch fire.
For the video, I ended up wiring the display up to another circuit to get the sound input amplified to the right level. For info on this piece, check out the SparkFun site. I used a modified version of it.