Part 5 Of A Digital VFO For The G6LBQ Irwell HF Transceiver

For part 1 of the Digital VFO click here.

For part 2 of the Digital VFO click here.

For part 3 of the Digital VFO click here.

For part 4 of the Digital VFO click here.

Digital VFO I/O Expander Module

"For The G6LBQ Irwell HF Transceiver"

Welcome to part 5 of my blog post series covering the Digital VFO developed for use with my “Irwell HF Transceiver project”.

In this concluding article I present the I/O expander module which interfaces to the STM32 micro-controllers I2C bus, it's job is to extends the number of digital outputs available and provide switched logic outputs to drive my BPF and LPF transceiver modules.

Illustration of the I/O expander

 

 

 

I looked at the PCF8574A I/O expander in part two of my VFO article so I wont go over the same ground again here but finish off from where I left. 

One thing I should of mentioned previously is that the 8 bits of the I/O port can be configured in software as either inputs or outputs but for my particular use all 8 bits are configured as outputs.

Having established that the PCF8574A only has a total sink capacity of 80mA I decided to add a darlington transistor array IC to my circuit, this provides a level of protection to the expander IC and acts as a secondary logic switch with a much higher current sink rating.

The darlington array IC that I chose is the popular ULN2803A and for those readers who are not familiar with this part it is basically a device consisting of eight NPN darlington pairs each with a collector current rating of 500ma.

Here is the schematic diagram for the I/O Expander.

Click to enlarge to full size!

 

Here's an image of the back (bottom layer) of the completed I/O expander showing three ULN2803A chips mounted in Turned Pin IC Sockets. I used through hole DIP devices as appose to SMD as they made laying the PCB out more convenient!

Actual PCB Size is 110mm by 51mm

The next image shows the front (top layer) of the PCB and on this layer you can see the three PCF8574A I/O expander IC's. Each of the PCF8574's has been configured with an individual HEX address.

Actual PCB Size is 110mm by 51mm

Finally here's the I/O expander PCB mounted on the back of the VFO Oscillator module forming a 3 PCB stack including the controller display module.

Since adding the I/O module to the VFO stack everything is working fine and the I/O board is now reliably controlling the BandPass Filter module. The LowPass Filter has yet to be developed and will be added at a later date.

All of the modules for the Digital VFO are now complete tested and working and all that remains to be done is software changes as the project develops. 

Project files are available via the Groups.io platform by joining my G6LBQ community group where you can discuss my projects, ask questions and help others.

Joining my group is free just click on the button below.

 

Until next time... 

  

73's From Andy G6LBQ

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