Wideband Voltmeter

The Auto-Ranging Voltage Meter was a project that I chose to design as part of a self-directed examination for a microsystems course. It was one of my favorite and most functional projects, so I have published the documentation of its construction. The basic premise is that a cheap microcontroller equipped with a low voltage ADC can be equipped to measure voltages well beyond its normal range, and can automatically tailor its resolution to fit the input waveform.

How it Works:

The design relies on a series of voltage dividers, each consecutive one tuned to reduce the input voltage by a factor of two. Each divider allows twice the input voltage of the previous, without overextending the capabilities of the ADC. A zener diode is placed in parallel with each divider to insure that the controller is not exposed to voltages above its tolerance. When one divider reaches this maximum voltage, the controller may select the analog channel corresponding to the next divider. This allows the system to cascade upward while maintaining full voltage resolution.

The controller used in this project is the PIC 16F887, an 8-bit microcontroller with a 10-bit ADC and a bank of up to eight analog channels. Because of the mismatch between ADC resolution and CPU word size, only the top or bottom 8-bits of any ADC result can normally be used. To overcome this task, and because felt I like pulling out my own hair, I decided to utilize all ten bits afforded by the ADC. The 1024 available values conveniently matches up well with the 1000 available values on the 7-segment bank selected for the project. However, because the microcontroller architecture only supports 8-bit processing, a special library had to be crafted to handle 16-bit operations. The selected hardware lacks the ability to perform multiplication or division natively, and the compiler does not supply routines to emulate said functionality. Creating a full 16-bit arithmetic library on an 8-bit architecture that only supports addition and subtraction was certainly the most challenging part of the project, but once completed was extremely helpful in crafting the binary-to-BCD converter necessary to run the display bank. The logic and hardware for measuring the voltage was the simplest part of the project, and presented few holdups. The photos provided present the design diagram for the hardware and a demonstration of the microcontroller running at ~7 Volts, more than two volts above its maximum allowed input. The meter shown here was able to measure inputs of up to 20V without exceeding the range of the device.

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