Clock using analogue multimeters ( PIC16F628A )

Posted on May 17, 2012

The Multimeter Clock consists of three multimeters, the first meter displays hours, the second displays minutes and the last displays seconds. A 16F628A PIC microcontroller keeps track of time and outputs a calculated current to each meter to display the current time. The user enters the time by pressing three time adjust buttons. The first button increments the hours, the second button increments the minutes and the third button resets the seconds. Once the time has been entered the microcontroller will keep track of time from there. An interrupt fires every 10th of a second to increment a 10th second counter. Another routine checks to see if we have at least one full second of time, if we do the current time is incremented by a second.

Clock using analogue multimeters ( PIC16F628A )
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The microcontroller has a separate output for each of the three meters. The meters are all in 0.5 DCmA mode, the negative lead of each meter is grounded and the positive leads are connected to a microcontroller output via a current limiting resistor. The resistor in this case is a 4.7K however this can be adjusted depending on the meter current scale available. Keep in mind that the PIC can deliver a maximum of 25mA to each meter so a meter with a lowest setting above 25mA would not work without additional circuitry. You also want to make sure that you can easily drive the meter to full scale, if the meter is not very accurate it could take 10 or 20% more than the stated current to drive it to full scale. See the table 1 below for a few suggested resistor values. The controller uses PWM (pulse width modulation) to pulse the current to the 3 meters which allows for precise needle position. For example if the minute meter needs to display 30 then the PWM would be half of the pulses needed to show full scale. When the system is initially started the PWM value is 50% so that the meters aren’t damaged by being accidentally overdriven. Originally the current limiting resistors were going to be a resistor and a calibration pot so that the full scale current to be adjusted by turning a pot. To get rid of the requirement of 3 calibration pots a scale adjustment mode was created. This allows the user to set the PWM value needed to...

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