QuickieLab Application Note #1
an Audio Visual Voltmeter

Configure your QuickieLab to serve as a voltmeter with Morse, Tone and LCD output

Joe Everhart, N2CX   email:  n2cx@voicenet.com

QuickieLab AN #1 illustrates how easy it is to make a unique test instrument with little more than the QuickieLab itself.  This instrument combines the familiar digital voltmeter function with an unusual audio “readout.”  It uses a number of the built-in QuickieLab functions such as analog- to-digital conversion (ADC), a liquid crystal display (LCD), on board pushbuttons and the built-in loudspeaker.  It also uses the experimenter’s plugboard to provide for connection of a potentiometer and an optional op-amp buffer.

AAV Specs at a Glance

• Digital Voltmeter

• 3-digit Liquid Crystal Display

• Selectable Variable Pitch Tone or Morse Numeric Output

• 0-2VDC measurement range

• 0.01V (0.5%) Resolution

What is the AVV?
The AVV is a digital voltmeter.  It features the same 10 megohm input impedance we expect with commercial test equipment to minimize circuit loading.   As described here, it reads 0 to 2 VDC with a resolution of 0.5% (0.01V) and features a familiar 3-digit LCD readout.  (More about ranges and resolution later). However it also offers a couple of unusual features in the form of audible outputs. 

A single pushbutton depression keeps the visual readout and adds a variable tone output.  The tone varies in pitch from about 300 Hz to just under 3000 Hz and is proportional to the measured DC input.  This variable tone can serve the same function as an analog bargraph display for easy observation of peaks or dips in measured voltages – with the advantage that you don’t have to even look at the meter.  A second press of the button changes the variable tone output to an audible three-digit Morse code output.  It reads exactly the same value as the LCD, but as with the bar graph display you can take readings without looking at the LCD.

Naturally if you don’t want or need the audio outputs you don’t have to use them.  In the same vein if you have a visual impairment or have to do measurements where looking at the display is awkward or impossible, you can “read” the meter with your ears.

AVV Operating Description

The AVV uses several of the built-in Quickielab functions.  Naturally the BS2 chip is the heart of the project, acting as a controller to coordinate overall operation.  Using the IOX chip it reads DC input voltages, performs calculations and outputs readings via the QuickieLab LCD screen or the loudspeaker.  QuickieLab pushbutton switch PB1 is used to select the AVV operating mode.  The experimenter’s plugboard section is used for connecting up an input buffering operational amplifier and ADC reference voltage adjustment potentiometer.

Figure 1 shows a schematic/block diagram of the AVV, showing how the added circuitry interconnects with the QuickieLab.  All connections are made via jumpers from the ADC connection jack J2 too the Experimenter’s Plugboard.   5-volt operating power and ground are supplied via the connector and the buffer amplifier output and a voltage reference from potentiometer R6 sets the ADC operating level.  

The input amplifier provides a buffered input to the ADC with a 10 megohm input impedance as is common with digital voltmeters.  It provides unity gain so that its input voltage is presented at the same level to the ADC input pin.  R6 is adjusted for a DC voltage of exactly 2 volts on J2 pin 2.  Thus full scale on the ADC is 2 volts.

Figure 2 shows the AVV software flow chart.  Most details can be seen from the figure, but the highlights are that the program begins by reading the ADC then converting the 8-bit result to a decimal number and outputting it.  The output is always displayed on the LCD while the audio mode flips between a Morse output, a variable tone output or no audio with successive depressions of the QuickieLab pushbutton.  Not that operation of the ADC and LCD are ‘hidden” at a low level by use of the IOX controller IC on the QuickieLab board.

A complete program listing for the AVV can be viewed.  (If you wish to save this to your computer, right click on this link and use "Save As ..." to select the destination on your local hard drive.  Then change the extension from txt to bs2 and your Stamp editor will properly recognize the software file.)

Other Possibilities

The Audio Visual Voltmeter as described here is really only a starting point to demonstrate the ease of applying the QuickieLab’s built-in functions.    As is the AVV is fully functional and can be used as a 0-2 VDC digital voltmeter.  However by simply scaling gain of the input buffer amplifier it can easily be set up for almost any DC input voltage.   The readout scaling in the BS2 program will also need tailoring to get correct numeric readouts.

The voltmeter function is a basic building block for a wide variety of measurement possibilities.  DC currents can be read by use of common resistive shunts as with other voltmeters.  However the 0-2 volt range of the AVV as implemented here represents an impractically high unwanted voltage drop.  Details on changing scale factors to 0-200 mV will soon be provided here on these pages.

AC voltage can be measured in a number of ways.  The simplest is a common diode detector that converts AC or RF voltages to a DC signal.  An op-amp with feedback can implement a very accurate detector for low AC voltages with a DC output fed to the AVV.

Sophisticated RF detector chips can also be employed in conjunction with the AVV to make very low RF voltage and power readings.  And for a familiar ham application, diode detected forward and reverse voltages from a directional coupler can be fed to the AVV for forward and reverse power readings.  They can also be compared using modifications to the BS2 program to give automatic digital SWR readings.

The QuickieLab incorporates an 8-bit ADC as part of its standard features.  However if added measurement resolution is required, 10-or 12-bit ADC chips can easily be integrated with the QL either internally on the Experimenter’s Plugboard or externally.  Chips using a simple 2-wire serial interface are particularly well suited to use with the BS2 processor.

The above are only a few of the many measurement possibilities for the AVV.  Future articles by N2CX in his Joe’s Quickies and Test Topics and More columns in the ARCI QRP Quarterly and in the QHB homebrewer will amplify on them.  And additional projects will appear on the NJQRP web site as referenced earlier.