Yet another cool use for the Rookey ...

Build the 'PPM'

PICAXE Panel Meter

Joe Everhart, NCCX

Here’s a simple project to highlight the versatility of the PICAXE 08M microcontroller chip.  It makes use of its internal analog to digital converter and provides a usable output interface conserving its limiting pinouts.


The idea is to simulate an analog panel meter rather than the more familiar linear bar graph displays.  To do the the PICAXE chip reads an input voltage between 0 and 5VDC and lights up one of a series of LEDs arranged in a semicircle as shown in the breadboard picture.  When 0V is measured only the green LED at the bottom left is lighted.  The red LEDs are lighted by higher voltages, the one next to the green LED with aobut 0.5V in, the second with about 1V in and the one farthest around the semicircle with 5V input.  This transforms the boring linear bargraph display into a more user-friendly analog meter display.

The circuit is extremely simple, as seen in the schematic diagram.  Both the PICAXE 08M (U1) and 74HC4017 (U2) are powered by a 5V source.  The PICAXE ADC input converts its analog input into a number between 0 and 255 with 0V producing the lowest number and 5V producing the highest.  This number is then scaled to 0-9.  A string of up to 9 pulses is then fed from U1 to U2 where they are counted.  U2 counts the input pulses and grounds one of its output pins (1 thru 9) to show how many pulses were received.  The LED on the active output pin shows this count.  The counting action occurs very quickly from U1 which then remains quiescent for about 100 ms to allow the display to be read by its user.  After this 100 ms delay time, U1 then resets U2 and begins another measurement cycle.  The green LED is lighted continuously to show the bottom end of the “meter”.

So all you need to do to get this project going is to build up the circuit as shown here on the perf board, or even just take two of the outputs from the Rookey board and drive the counter and LEDs.  Load the software into the PICAXE using the same straightforward 3-wire interface used for the Rookey card and you'll be in business!



Last Modified:  May 7, 2007