Home > Advanced Projects > Giant LED Meter
GIANT LED METER

Modified: 22:34, 10 July 2014


I have always thought the look of LED VU displays were quite hypnotic and wondered what a giant one would look like....so I built one.

This one is modular and can be expanded a panel at a time. So far it has 7 panels totalling 140 segments each with 9 LEDS per segment which equals 1260 LEDs in total....so far!

There are also details of an amplifier module and PSU so the meter can respond to sound - in particular, music.

If you want to know what it looks like in the workshop when connected to music, click on the link below:

Giant Light Meter Video

Guess the 80's tune...


DESCRIPTION
PANELS

The whole design is modular and made from a chain of panels. Each panel has connections to connect to more panels as required. These can be inserted anywhere in the chain.

Each panel has 5 PCBs. Each PCB can control 4 segments of the display and is linked in a chain with other PCBs. A bus runs along the length of the display to provide power and resistor chain connections. In the meter so far are 7 panels - that's 35 PCBs, with more being planned. The last panel in the meter chain should have the RHI and RMX connected. RMX is connected to the maximum reference voltage Vmax which dictates the range of the meter - ie. the voltage at which Vin will illuminate the final output on the meter.

PANEL CONNECTIONS

  • +V = The supply to all the IC comparators. Uses the same power connection as the Audio amplifer driver (see below) +VA
  • Vin = The signal voltage to be displayed
  • 0v = Common ground connection - should be heavy duty wire if lots of modules used as it is also the LED return supply.
  • RLO = Low end of reisitor ladder, connects either to RHI on the previous panel or to lowest reference voltage if first panel.
  • VLED = Supply for LEDs, should be around 11-13v, needs to be heavy duty cable if lots of panels
  • RMX = The return route for the highest reference voltage - should only be connected to RHI on last panel.
  • RHI = Connects to either RMX if last panel or to RLO on next panel
CIRCUIT DIAGRAM & PCB
Each PCB contains the following circuit - the LEDs are mounted of the board. Only 3 are shown on each output but in fact there are 2 more rows of 3 in parallel on each output.

The circuit is very similar to the internal wiring of the common LM3914 series of bar graph drivers. The only problem is that they are expensive. The LM339 comparators are a few pence and are fast ans precise.

Unlike the LM3914, there is not dot/bar selection - it is just a bar.

Each output drives 30mA, thats 10mA per LED trio. Since there are 140 segments that totals 4.2 Amps when all on - I had to build a power supply just to run it!

The output from each comparator is an open-collector type which needs a pull-up resistor. The output can only sink so much current so it is recommended the IC is not run at more than 12v. without increasing the pull-up values.

The +LED voltage can be what ever you like since the outputs are driven with a standard NPN transistor.

R1-R4 form part of the resistor chain that the comparator uses as a reference. As soon as Vin rises above any voltage output in the chain, the corresponding comparator switches on.

So far, there are 35 of these connected together in the light bar.

The PCB measures 75mmx30mm. Connections are below:

+VA (8-12v)

0v

RHI

VIn

RLO

OP4

OP3

OP2

OP1

The connections are very similar to the LM3914.
  • RLO = bottom of resistor chain
  • RHI = top of resistor chain
  • VIn = Input voltage
  • +VA = Circuit supply
  • 0v = Common ground supply

Note the supply for the LEDs is different but both 0v terminals must be connected.

PANEL WIRING
The panels were laser cut with 5mm holes at 10mm intervals. The LEDs are wired 3 in series then 3 of those wired in parallel to make a square pixel of 9 LEDs. Wiring on the back was done using terminal blocks. Take care not to tighten too much and cut through the wires. Connections to the PCBs is through a hole to the front of the panel. Panels were joined with strips of aluminium. Also aluminium hooks were made to suspend it from a ledge on the suspended ceiling frame,

AUDIO AMPLIFIER DRIVER CIRCUIT DIAGRAM & PCB
The amplifier module allows any sound in the room to be amplified and smoothed to drive the LED meter. Itcan be broken into the following blocks:

The microphone input is about 5-10mV and is boosted by the first audio amplifier by about 45 times to around 450mV.

A switch selectes select betweern the mic input and an alternative audio source of around the same voltage - I think most audio pre-amplifiers have an output around this level.

The selected input is then fed via a level (volume) control to the second audio amplifier which has a gain of x10. This brings the signal to about 5v.

Finally the signal is rectified and filtered (de-modulated) to leave only the slowish changing amplitude of the signal rather than its frequency. The amount of filtering can be adjusted and has the effect of altering the speed of change of amplitude - which on the light bar affects the speed at which the bar moves. Vout connects to Vin on the display modules.

The circuit is below - all made from circuit bricks:

The PCB is shown right, size 78mm x 45mm

"see text"
There is an option to add extra gain control by inserting a variable resistor of about 100K in the feedback of IC2 increasing the gain up to x20. I added this in case the output was not enough for the meter, but it was not needed as there is plenty of gain already. At the bottom of the PCB are 3 spare holes for a VR if required. If not, just connect the left 2 holes directly together.

  • MIC = Microphone input.
  • TB1 = Amp1 output and Amp2 input.
  • TB2 = Power supply, 8-12v
  • TB3 = Signal output for the meter - top terminal.
POWER SUPPLY UNIT AND CONNECTING IT ALL UP!
The power supply unit was a bit of an after thought but desperately needed as there were different supplies to consider and also as the size of the meter grew, it had become very sensitive to poor connections and dodgy supplies.
The circuit for the PSU is below:

The circuit is basically 2 voltage regulators.

  • +VA voltage for the amplifier and the Comparator PCBs
  • VHI for connection to the RMX line used for the final RHI. This is adjustable from 1.2v up to the supply you use and is varied by VR3 coonected to TB7.

There is also a VLO supply for the low end of the reference chain connected to RLO. This removes any low voltage noise from displaying on the meter.

PCB size = 51mm x 31mm.


0V

+V


+VA

VLO

VHI

VMax

Below is a diagram showing how to connect the PCBs together and the connections to the panel wiring bus.
Terminal blocks were used to connect to the display cables. Wiring colours almost match the ones in the photo.
The case shown here was only meant to be a temporary one and was made from a base of 9mm MDF with 1mm blue HIPS stuck on using contact adhesive. the control panel was made from 3mm acrylic with a 90 degree bend using a strip heater. There is an addition of a 470uF capacitor wired directly across the power supply connection to the amplifier board. When the gain was up full, there was some low frequency feedback which was corrected by the capacitor.

DOWNLOADS:
by Phil Townshend 2012

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