V2FHardware

Voltage to frequency converter Hardware

Warning

The described hardware is intended for connection to a soundcard. Keep in mind that your soundcard (and everything else connected to it, including your PC) may be destroyed if something goes wrong. The developers of this project are not responsible for any damage caused by the described hardware or instructions.

Schematic

The schematic and the KiCad project are available in the downloads section of the project in the hardware-folder.

Brief description

Before reading this section, you should get the hardware documentation from the source mentioned above to know what we are talking about.

The approach is to use a voltage-to-frequency (V2F) converter which generates a AC signal with a frequency proportional to the DC input. The schematic consists of a "standard" 4046 of which the VCO part is used as V2F, a 7805 voltage regulator and some passive components. The device runs off a 9V battery.

The input and biasing circuit R1, R2, R3 provides approximately 1MOhm input resistance and a nominal input voltage range of 0...15V. Please note that the 4046 is a 74HC4046. The standard CMOS (e.g HEF4046) series does not work with the given values of R1, R2, R3. The HC-type seems to have better temperature stability, too. R4 and C1 set an operating frequency in the audio range and R5 through R7 reduce the 4046's output voltage to line-level for the soundcard. The OUT signal (and ground, of course) should be connected to the sound card line-in. C1 should have a low temperature coefficient. (Ceramic capacitors used for supply bypassing are usually not a good choice.)

Please note: A prototype was built and it worked well but the results may depend on the exact type and manufacturer of the 4046.

Calibration and Accuracy

The predefined calibration points are 0V, 5V and 14.246V. The reason for this values is the following:

• 0V is easy: Just short the input to ground.
• 5V is easy, too: Connect the output of the 5V regulator to the input.
• 14.246V: Leave U_IN floating (unconnected) and close the "Calibration jumper" P3. This results in an input voltage at pin 9 of the 4046 which is as big as if 14.246V are connected to U_IN while P3 is open.

Hint: The calibration voltages can be changed (along with lots of other settings) in the configuration file of the software. Please refer to the documentation available in the Help => About menu of the software for extensive explanations of all the available options.

If we assume that the sound card sample clock is stable and accurate, only the deviations caused by analog circuit are relevant.

If you want to use the calibration voltages supplied by the device itself, you should use 1% resistors for R1, R2, R3 and Rcal1 if possible, because otherwise the simulated 14.246V may be off too much. Additionally, the accuracy of typical 7805 regulators is usually somewhere in the range of 2% to 4% and causes additional errors. You could use a 5V reference like REF02 as supply to avoid this (but this wasn't tried).

Of course, you don't have to use P3 and the internal 5V supply. You can supply the three calibration voltages externally to U_IN if you want. Actually, this approach is better, because the deviations of R1, R2, R3 and the voltage regulator are then calibrated away and 5% resistors are sufficient.

The protoype uses a 74HC4046 from Fairchild. Using the "internal" calibration method and with P3 and 5% resistors, the accuracy is about 200mV over the input voltage range of 0...15V. With accurate external calibration voltages, the accuracy increases to 50mV or better over the full range. Not bad for such a simple circuit.

Stability

Temperature stability is an issue with this circuit. Here the proverb "You get what you pay for" strikes back. My first tests with a HEF4046 and 9V supply showed catastrophic thermal stability. The 74HC4046 is much better. If I put a finger on it for some seconds the voltage drifts about 150mV or so, so the temperature coefficient may be in the 100mV/K range, but this is a very rough guess. Anyway, you may need to recalibrate if the temperature changes.

But why don't you ...

... use a microncontroller and RS232, USB, etc. Of course, you're right. A small microcontroller with internal ADC and voltage reference can make a better, faster and more stable voltmeter than this one, but the goal of this project is to get a beginner started as fast as possible. Everyone who wants to build this voltmeter just needs buy the components, solder them and it is (hopefully) ready. If there's some kind of microcontroller involved, you need a programmer. This may be an additional obstacle, especially for the beginner.