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Thermometer II completion

A project log for Heroineclock II

Giant clock from a PIC18F6585, 135 cheap LEDs, foam core posterboard, hot glue, lots of hot glue

lion mclionheadlion mclionhead 08/14/2018 at 08:340 Comments

Be sure to label your GPIOs.

2 layers of wax paper, just like Heroineclock II were applied.  The ancient LEDs were maxed out & it was still dimmer than Heroineclock II's minimum  brightness.  If you don't have any money, it says 81.  If you have money, it says 72.

An accurate temperature reading is very very very hard because of the logarithmic resistance of the thermister combined with the logarithmic voltage from the voltage divider.  Didn't realize how far off the circuit was until comparing it to the Fluke.  Heroineclock 1 was calibrated only by testing ice & boiling water.  Fluke measurements from ice & boiling water are way off of 32 & 212.  Better results would be achieved by multiple GPIOs feeding variable current to the thermister.

The basic equation for a voltage divider gives reasonable ADC values near habitable 

temperatures.  A simple C program https://cdn.hackaday.io/files/284921219001728/temp_tester.c converted resistances from the data sheet to ADC values from the voltage divider.  Though it had some experimental test values, the ADC values were predicted purely from the data sheet.  This gave exactly the same temperatures as the Fluke, from 45F to 185F, accepting differences in thermal inertia.

To make a better video, the thermometer got a completely worthless test mode.  The test mode was manely to measure its maximum current.  It only used 140mAh with all the LEDs on.  The PIC was clocked at only 10Mhz to reduce the power.

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