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VCF controller for function generator

DSC 13432 [320x356px] shadow

What interest will you tell me?
What can it be used for?

The answer is very simple, well ... maybe not that much in the end ...
If your generator is equipped with an input[ VCF 1 ], then you will be able to control it very finely, for example using a microcontroller system equipped with a DAC converter ( D igtal A nalogic C onverter), clearly a digital / analog converter.
It goes without saying that if your GBF does not have this VCF input, then this accessory will be of no use to you!

In fact the principle is to adjust the base frequency of its generator normally, with the buttons associated with it, then to finely adjust this frequency with the help of our VCF interface.

Usually, some micro-controllers already have such a converter internally ... yes, but on 10 bits, which gives us a resolution of 1024 different steps for example to change a voltage from 0V to + 5V. If we divide 5000 (mV) by 1024, we get the value of 4.8828125 E 0 which gives us a voltage of 4.88mV / step. It is little and not very precise for our generator.

We are going to give him a present! an external converter which converts on 12 bits, that is to say 4096 steps of resolution. This changes everything, because to control the generator with its entry VCF must precisely him a voltage between 0V and 5V, and then we'll do it on 4096 ( o u levels ... ).
I'm not telling you comfort!

During the tests on my generator , I noticed that for a given frequency, as soon as a VCF voltage of + 5V was applied, this frequency practically doubled! We can thus go from single to double ( or almost, it still depends on the characteristics of your GBF ... minegoes from 2000MHz to ~ 2935MHz max ... ) of our base frequency in steps of 1 thousandth of a volt! Do you see its interest a little better?
Finally, this is not quite true, in fact if we divide 5000 (mV) by 4096, we get the value of 1.220703125 E 0 which gives us a voltage of 1.22mV / step. The difference is therefore relatively small between this value and 1mV!

executable  PROJECT INFO
  Component Types: Through
  Level: ◆◆ ◇◇◇
  Construction time: 2 to 3 hours
  Cost: approx. € 57

Programming

The processor is delivered pre-programmed with a bootloader from mikroElektronika, which allows me all the fantasies to modify simply and very quickly ( 115200 baud / s ) my program via the USB port. I used to do this the mikroC software suite including all the necessary tools to carry out my project.

Be careful, however, as my software uses 4680 bytes of the microcontroller ROM , you cannot modify it as you wish without purchasing the mikroC PRO license . The free version is limited to 2K bytes.

From project to design

DSC 12788 [320x250px]I designed mine on a base from MikroElektronika, the "Ready for PIC" equipped with a PIC18F25K22 (DIP28) microcontroller operating at a clock frequency of 32MHz, as much to tell you that it reacts quickly!
It is obviously possible to use other processors ( in DIP28 or DIP40 and on condition of reviewing the option bits ... ), but it is with the PIC18F25K22 that I get the best operation.
The 12-bit converter is an MCP4921 from Microchip. I added a backlit 2x16 character LCD display ( whose brightness varies according to the ambient light thanks to an LDR cell driven via a PWM port), 5 sub-miniature pushbuttons that allow you to adjust the output voltage of the DAC as well as the scrolling speed ( increment / decrement ) between each step, and as a bonus, as I was tired of adjusting every morning my VCF voltage when designing one of my personal applications, I planned to save the last VCF value in the internal memory of the micro-controller. Each time you change the VCF value, it is immediately stored in memory. In the event of a power cut and even after reprogramming, the last value thus stored reappears immediately. Super comfortable!

The completed "proto" version

DSC 0017 [350x197px]This is indeed the end of my project using a development board as the main board.
I am in the process of studying and DAO under Eagle v7.0 of a "minimalist" main board, ie with the bare essentials. This board would only consist of SMD components, I am in the process of researching components at affordable prices. However, it is necessary to integrate the manufacturing costs of the new card ( double sided, metallized holes and screen printing ... ), as well as shipping costs. I do not know today ( September 11, 2014 ) which solution would be the most profitable ... subject to follow ...

→ As of today ( December 7, 2015 ) and after much research, it turns out that the final cost would certainly not be lower compared to this " Ready for PIC " card ( $ 29 at mikroElektronika excluding shipping costs ), unless of course we had several dozen made ...
but I don't run a business!
This remains personal development, which of course everyone can use according to their ideas and their technical and financial means.

 

 

 

Food

The VCF Controller can be powered either with a voltage between 9-32V DC or a voltage between 7-23V AC.
Powered by 12V DC, the consumption in working mode does not exceed 40mA.

The boxing and decoration of the front panel

For the boxing of this new tool, I used the box distributed by mikroElektronika ( Reference: MIKROE-1350 ), and performed the cutting ( sometimes hazardous ... ) using my trusty Dremel.
As for the front side, it received a silkscreen print on self-adhesive aluminum paper from 3M ( Reference: 3031 ) that I got from Selectonic ( now no longer store ) , which I printed with my laser printer.
This gives it a more "professional" presentation ...

Some other photos of my prototype

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DSC 13450 [140x145px] DSC 13451 [140x146px]      

Note (August 17, 2014): In order to try to lower the relatively high cost price ( > 90 € currently ... ) of my prototype based on a very well equipped development board ( too much perhaps? ), I am in the process of redrawing the diagram as well as the PCB ( under Eagle v7.0 ... ), by using SMD components as much as possible in order to reduce the dimensions of the printed circuits as well, and by putting only what is strictly necessary. The USB interface is therefore very likely to disappear, the programming of the microcontroller then only being done in ICSP mode. 

Note (January 20, 2015): I have finally finished the routing of the main board comprising CMS components except for the PIC18F25K22 microcontroller and the MCP4921 converter which I kept in DIP series. Note that I designed this new card under Eagle v7.2 with license ( Hobbyist and Education ), its dimensions being 113.3 x 86.4 mm. I would like to remind you that the Light version of Eagle only allows you to route cards not exceeding 100 x 80mm. I drew the card for a Hammond type 1591GGY box ( GRAY ABS ENCLOSURE 121X94X34 ) 


Package comprena nt : Software written in language mikroC ( source and executable ) Schematic diagram ( without PCB ) in Eagle v6.5.0 Schematic with PCB and images of different layers in Eagle v7.2.0save f2


 


  1 . VCFV oltage  C ontrol F requency ( frequency control by a voltage)

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