Wednesday, July 22, 2015

Voice Display Board - Based on Kitt from Knight Rider

1. Overview

While waiting for the mobility platform parts to arrive from the US, I decided to start a side project. I liked the idea of having some sort of visual indication for when AVA was communicating and coincidently came across a voice display board advertised by Think Geek (which says it all really). This display is a simulation of that used by Kitt the robot car in the TV show Knight Rider.

Following a quick internet search it became apparent that there were two main approaches to developing a display like this, analogue or digital. Given that the rest of AVA was largely digital I thought I would go old school and try the analogue path.

The Kitt display is basically a VU (Audio Power) meter reflected horizontally. To make things simple, Texas Instruments produce a chip called the LM3915 Dot/Bar Display Driver which does most of the hard work and is designed for exactly this application. The LM3915 senses analogue voltage levels and drives ten LEDs to produce a logarithmic 3 dB/step display. The LM3915 even provides a regulated LED current supply, eliminating the need for current limiting resistors.

The logarithmic output is suited to signal inputs with wide dynamic range (like audio levels, power, light intensity and vibration). There is also a linear version of the chip which could be used for things like battery level and I may incorporate one of these in a later project.

2. Prototype Design

To ensure that this design works as advertised and to identify any design issues, I built a quick prototype and hooked it up to AVA's logging Arduino (which is the only thing which makes a noise currently). Eventually the robot will have a voice synthesizer, but one step at a time.

Based on the audio power meter design from the LM3915 Data Sheet, I built a version on a breadboard. Note that the LED bar graph doesn't have it's polarity marked (that I could see), a quick check with a multimeter indicated that the anode is the side with the writing.

This works ok, but highlighted a few issues:
  1. The PWM digital output on the Arduino which drives the piezo to make the robot sounding beeps, has sufficient residual voltage when making no noise to light a couple of the LEDs.
  2. The voltage output from the Arduino is low enough that it doesn't need the voltage divider formed by R1 (18k) and R2 (10k). Consequently the audio signal can plug straight into pin 5 of the LM3915. This pin can handle +/- 35V without damage so we are pretty save with our 5V Arduino.
  3. The LM3915 can only sink a maximum of 13mA of LED current (see data sheet electrical characteristics table). To produce the Kitt display we will need 6 of the LED bar graph displays, so in the worst case we need to be able to provide enough current to drive 6 LEDs. Checking out the data sheet for the LED bar graph display, we find that each LED typically draws 20mA (forward current). In practise, as little as 1 mA will light the LEDs, but they are fairly dim. 
  4. As can be seen from the breadboard photo, even one 10 LED bar graph needs a heap of interconnecting wire. Six times this would be a mess, which suggests that a Veroboard solution is not the go. We will need to layout a PCB or two.

3.0 Final Design

Addressing the issues raised by the prototype, I created a design which utilises two PCB's on top of each other (display on top). The display board schematic looks like this:

The associated double sided PCB layout is below. I used Fritzing to do the schematic and PCB layout. It is FREE and I highly recommend it for simple to medium level designs. They have an associated PCB fabrication service (in Berlin) which is fairly expensive, but for a double sided board like this with a number of via's, it was a good option.

We will cover the display driver board in the next article. Finally, a quick video of the prototype connected to AVA.

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