Giving voice to a Spektrum Dx8

[Nik Harrison]

As lockdown continues and boredom set in I found the below item on Youtube.

**LINK**

This reminded me of something I have wanted for some time, which was to give voice to my Spektrum Dx8. I even bought a Graupner MX-12 Pro set as one of the facilities it offered was voice on its switches. A lot cheaper than a Dx9. I didn't find it very satisfactory as the software to produce new voices left a great deal to be desired. So with nothing to lose I sent off for the Speech Synthesiser XFS5512CE from Banggood.

There were a few requirements on Tx performance that I decided were important to achieve.

1. Tx must not be compromised.

2. Voice Controller must be removable.

3. Must function the same even if the Speech Controller was switched off, flat battery or even removed.

[Nik Harrison]

The Dx8 I had was well out of warranty so the next thing was to open it up. I admit I had to look in the manual to see how to do it. The first thing to discover was how the switches were organised. The three position switches were centre pin 0V and the two outer pins were signals. Therefore I only need to monitor two signals for every three position switch. The two position switches were just 0v and signal. Most signal levels were 3.26v but some were 2.4v. So the Tx works on a 3.3v system. Adding up all the signals required to monitor came to 15. I ignored the rotating switch just sticking to the digital stuff.

My first thoughts were to use optical coupling devices to monitor the signals but decided in short order that there is not enough juice to drive them. I then tried unity gain opamps as they would have high impedance inputs so should not effect the Tx signals. This worked but the power source was a problem. I had planned to have an external power source drive the extra components required in the Tx but if the external power source was not there then the Tx went crazy. By crazy I mean unusable and chirping like mad. Obviously not a happy Tx.

The solution I came up with was to use two 8 to 1 CMOS (CD4051) multiplexers and as these are happy to operate at 3.3v so they could be powered by the Tx power supply. The current drawn would be minimal and there was enough room under the SD card reader to fit two DIL chips. The use of the multiplexers made the number of connections between Tx and Speech box come down to only seven. This was important as I did not want to have a large connector on the rear of the Tx.

[Nik Harrison]

The Speech box was also operated at 3.3v partly because that is the voltage that the Speech Syth. uses. Using an Arduino Nano 33 BLE as the processor it all seemed to work. I say "seemed to" because when I changed the model selection on the Tx it made odd chirps. When I selected a model that used the trainer link the Tx went crazy again. Why changing the model selection should have such a profound effect is very strange.

There was obviously too much current drain on the signals in the Tx so I fitted the unity gain opamps again, this time in the controller, to reduce the load. It worked out better this time as I only needed two opamps as the multiplexers supply two data streams. So instead of an amp for each signal it now only required two. This fixed that problem but now one of the lower voltage signals was not responding on the controller. Fix for this was to make the opamps amplify the signals.

I fitted a rotary switch so that I can have up to 10 different scripts for different models and a Status button that will announce which script is selected and then run through the position of all the selected switches. Therefore you can know the switch positions while flying and not having to look at the Tx.

One thing that you have to accept is the Chinese accent. It does make some words too similar. Example "Enable" and "Disable" do not stand out. I used "Engaged" instead of "Enabled". Just requires a bit of experimentation and imagination. The final bit I added was a nice loud alarm call if you left the Speech Controller on and were not using the Tx.