Voltmeter Clock w/ F°, C°, and K° temperature readout.

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The objective was to create a real time clock using three analog voltmeters controlled via Pulse Width Modulation (PWM) to display "hours", "minutes", and "seconds".  At the press of a button the three meters display temperature in degrees F, C, and K.

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Here is a short (time lapse) video of the rig in action:

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There are four major components to the build.

• PICAXE 14M2 microcontroller

• DS1307 Real Time Clock (RTC) module

• DS18B20 Temperature Sensor

• Three 0-2VDC analog voltmeters (refaced to display as "hours", "minutes", and "seconds")

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The meters were taken apart to install custom faces.  A free program called "MeterBasic" was used to create the custom faces for the three voltmeters (see pic below).  Note that the meters have labeling to display temperature in degrees F, C, and K.

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See the black button below the "seconds" meter?  Pressing this button causes the meters to display their respective temperature.  

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Here are a few shots of the build process.  The "red thing with the black tip" is the DS18B20 temperature sensor.  In front of the meter in the middle (what will be the minutes meter), you can see a small PCB with two buttons.  These are used to set the time on the DS1307 Real Time Clock.  Pressing both buttons will set the seconds to zero.

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The digital outputs on the PICAXE 14M2 put out about 5VDC maximum.  Our analog voltmeters only go to 2VDC maximum.  We set up a voltage divider circuit and a trimpot to allow the output from the PICAXE 14M2 to read full on our 2VDC meter.  The 100K trimpot (the little blue things in the pic below) allow for precise adjustment of the full scale reading.  Pushing the hour set, minute set, and read temperature button at the same time will force all meters to full scale.  This is to provide for precise tuning of the trimpots to calibrate the full scale reading of each meter.

After we have calibrated the meters to read "0" with no voltage and "full scale" from a high digital output on the PICAXE 14M2 we still have to be able to control the meters to display the time.  This is done by reading the time on DC1307 RTC via the I2C bus and the PICAXE 14M2.  Pulse Width Modulation control is used to convert the time (hours, minutes, and seconds) into a corresponding "average" voltage.  That PWM signal drives the three meters to display time.  The PICAXE 14M2 has four PWM output drivers that are well up to the job for this.

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Here is another look at the time set buttons.  They are located on the bottom side of the clock enclosure.

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The rig is powered by a rescued wall wart from an old Sony CD player.  The wall wart puts out 9VDC which is tamed to 5VDC with a LM7805 voltage regulator.

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If you want to build your own, the schematic looks like this.  I will send my (sloppy) source code on request.

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Another pic of the finished rig.

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