HEADPHONE / MONITOR SELECT BOARD - PART 1

Hi—it's me from the future. This was my first attempt at designing the Headphone and Monitor Board and calling it a failure would be an understatement. As you’ll see at the end, I ended up scrapping the entire concept. In a later blog, you’ll discover how I eventually perfected the design. Enjoy this early, experimental version.

We have been diligently working on various parts of our project, shifting focus from one component to another. (I tend to have short attention span.)

After numerous interruptions and restarts, we are now working on the MONITOR AND HEADPHONE Controller Card.

This card communicates with the output board to specify the desired audio source for monitoring, whether on the studio speakers or the headphones.

This part of our project was particularly interesting. I set out to eliminate the push-on switches used by Radio Systems, which had a tendency to collect dust (and smoke back in the day) and become unreliable. It turned out to be the toughest challenge for me, but I learned and relearned a great deal about digital electronics along the way. I didn’t end up with what I initially envisioned, so check out this blog and the next one for the full story of how I achieved my goal.

First, let's take a look at the Radio Systems card. This is for the Monitor version of the card. We will NOT be mounting the monitor volume control to this card.  (YEAH RIGHT- WAIT TILL PART 2)

And here is the Headphone Monitor card.

The similarities are sufficient to allow the use of the same PCB board for both applications. Further Information will be provided later. The cards by Radio Systems were suitable for their needs, however my goal is to replace these switches with CMOS technology.

But how do we convert a chip into a switch?

We utilize the CD4066 chip 4-channel switch integrated circuit. While these switches are single-pole single-throw (SPST), we can effectively combine all four channels to create a double-pole double-throw (DPDT) which meet our requirements.

And here is how we do it.

The transistor (2N3906) and associated resistors receive the +5 logic from a CD4017 counter. This signal performs the toggle functions.

Let's redraw this diagram and show the Output Switches

Now, let's make it all function.

Here is the first part of the schematic:

J12 transmits the switch pulse to an LS18 switch debounce IC, which eliminates extra pulses to smooth out the signal. The output pulse is then routed to a CD4049 Hex Inverter, converting the GND pulse to a + Logic HIGH. Pin 4 of the 4049 exits through J12 and powers the internal LED within the switch. While not necessary, it adds an appealing visual effect.

The additional output from the 4049 feeds into the CD4017 counter. A pull-down resistor ensures stability for the 4017, a 0.01 µF capacitor further mitigates any residual switch bounce (although it may not be strictly necessary). 

Each activation of the switch advances the 4017 counters by one step. After nine steps, the counter resets to one, restarting the count.

I may have come up with an idea using flip flops. More on that later.
(WE WILL do that in Part 2)

Now we need a row of lights to tell us where the count is located and what function is active.

The output of the counter goes to a resistor/transistor circuit that will light up a panel mounted LED. Let's take a peek at the PROGRAM MONITOR circuit.

The output of the 4017 (pin3) goes thru a 1.2 k ohm resistor and turns on Q10 providing a ground pulse that will turn on Q18.

Q18 turns on allowing 5 V to flow to a connector that connects to an LED.

All the rest of the circuits works in the same manner. When the count goes to 9, ALL of the lights will be off. This stage is used on the HEADPHONE FOLLOW MONITOR select. 

Another count will reset the counter to step 1.

Now to the CD4066 chips.

Since all switches operate in the same manner, only the Program function will be discussed. The 4017 counter feeds a 5 V signal into a 4081 AND gate. When both pins 1 and 2 see 5 V, the chip sends a + signal to the resistor and transistor. The activates, delivering 5 V to the chip's control pin.

In other words, when the counter reaches 1, the 4081 chip supplies a positive logic signal control pins 13 and 5. When the count is not 1, a positive logic signal is supplied to control pins 6 and 12. This creates the 'toggle" motion of the switch.

The rest of the CD4066 chips work in the same manner.

We will address the outputs at a later point, now let us breadboard what has been designed so far.

Yeah, may be uglier than the folks on "The View", but it works. Here we show it in Position 1 (PROGRAM MONITOR)

We can also check and make sure it steps properly

Now we can show you the toggle function of the CD4066

When the 4017 is in position 1, look for a HIGH on Pin 13 & 5, and a LOW on Pins 12 & 6. You may have noticed a bit of switch bouncing. This is caused by the tactile switch not making good contact in the breadboard. That will not be an issue on the PCB. 

When you go to Positions 2-8 you should see the opposite conditions. LOW on Pins 13 & 5 and a HIGH on Pins 12 & 6.

Here is the new breadboard. It is wired up to the 4066 inputs. Using my logic probe, I stepped through all the functions making sure the 4066-input transistor was functioning.

Yes, it works!!!!

Now let's finish up the breadboard by wiring the CD4066 switches and the outputs.

Now that the breadboard functions, we can move onto designing a PCB board.

(Please note that the part designators may be different on the boards, but you get the idea.)

Here is the schematic that the boards designators were based upon.

The switch circuit drives the CD4017 Counter. The counter commands the indicator LEDS, as well as the CD4081 AND Gates.

16 Pin cable that runs to the Output board.

Some Power indicators and power bypass capacitors.

The next diagrams show the CD4066 chips switching output.

Here we show the Headphone Monitor follow circuit. When the Headphone board is in position 9, the Headphone Output will follow whatever position the Monitor card is in.

Now that we have a breadboard version that steps properly, let's see if it works.

We have wired the outputs of the 4066 chips. Now we can add the parts from the output board so we can generate the BCD codes needed for the 4051 chips on the output board.

We set that up on a separate breadboard. Let's take a look at the diagram for this interface.

Here is the interface board connected to the select board.

Now we can check for the proper BCD code. Here is the Truth Table.

Once everything checks out, we can order the boards.

Not so fast, my friend.

As I’ve done throughout this journey, I’m making some changes. I realized I don’t like having to push the switch eight times just to reach position 8. I’d rather press any button and have it activated while turning all the others off. 

We’ll work on that idea soon.

Yes, I am scraping this whole page and starting over.

     And Bob's Your Uncle.

God Speed, Mother Nature

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