## Sunday, November 18, 2012

### Analog Clock turned DC Voltmeter

Why create, what could be, the most impractical way to measure a DC voltage?  Well, because "Measurement Matters".

Actually, the idea was given to me in the comment section of my Voltmeter Clock project which was featured by the awesome team at "Hack A Day".  That is where rue_mohr wrote the comment, "who can be the first to turn a clock into a volt meter?"  Well, that is (and remains) the stupidest ideal I ever heard.  Now, read on....
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The concept is easy.  Use the "time" on an analog clock to display measured voltage values between 0-12VDC.  1 o'clock means 1VDC, 2 o'clock means 2VDC; if the time reads 10:30 that means 10.5VDC, etc.   To do this a stepper motor is connected to the adjustment knob of the analog clock.  This short video of the rig being tested should help make things clear:
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One of the ADC inputs on a PICAXE 18M2 microcontroller is used to read/digitize the voltage to be measured.  After the PICAXE measures the voltage, it gets translated into the number of steps the stepper motor needs to move.  This stepper motor movement adjusts the clock to display the voltage that was measured.  The PICAXE doesn't have enough output current to directly drive a stepper motor so the PICAXE feeds a L293D Motor Driver IC.  The outputs of the L293D are then used drive the stepper motor.

Since the clock can display a maximum value of "12 o'clock", the rig was designed to measure a maximum 12VDC input signal.  However, the max input voltage for the ADC on the PICAXE 18M2 is about 5VDC.  To tame the input signal for the PICAXE a simple 3:1 voltage divider (10K, 10K, 10K) was implemented.  This limited the input voltage into the PICAXE to 4VDC.  A trim pot was also used to adjust for the input impedance of the PICAXE and to help calibrate the measurement results.
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Here is a video of the rig in action side by side with a DVM.
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All in all the rig works as designed but has terrible lag and is no way what anybody would consider NIST traceable.  The accuracy is estimated at ~100mV.  The two control buttons pictured below help by allowing manual calibration of the rig to 0VDC (12 o'clock position).
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I can't imagine that this project will ever be duplicated, but as always I will send schematics and source code to those that request it.  If you're still with us, thanks for checking out our site.
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