So technically, I do have the ability to put the satellite into bind mode without the use of any specialty equipment from Spektrum. The data is openly available online for 3rd party developers that need to interface with Spektrum receivers. Using a micro-controller like Arduino you could put together a few lines of code to get the satellite into bind mode, which I have done in the past, but I hadn't worked on any electronics projects lately so I kind of wanted to see if I could do it without the use of a micro-controller.
Using the information found in the Spektrum receiver interface guide I came up with the initial plan for a bind module. In order to set the satellite into bind mode it needs to see nine falling pulses, which is the number of pulses recommended for use with the DSMX protocol being used. This shouldn't be too difficult to pull of with discrete components, but I have decided to use a 555 timer for the clock pulse so I can simply adjust a potentiometer to change the duration during testing. Once I figure out the ideal timing I'll switch to an entirely discrete component setup for the final revision. I'll also attempt to put this together on a single sided SMD board for easy reflow.
A snippet from the specification sheet for Spektrum receivers
I originally kicked around a couple different ideas for this circuit. At first I was just going to use a simple timing circuit to essentially wait long enough for the nine pulses to occur and I even developed the circuit, which was operational, but the more I thought about it the less I liked it. It felt like it would be too prone to errors and I did not want to risk pushing the wrong number of pulses onto the satellite and binding in the wrong mode. I decided instead to count the number of pulses and stop the clock from ticking once it hit nine.
To do this I wanted to use a JK flip-flop, but I was finding it a bit too complicated to recreate with discrete components. That's not to say it can't be done, I just didn't want this project to get away from me too much. After-all, this is supposed to be an easy step back into electronics after a pretty long break. Scouring the internet for a simpler solution I came across a much easier way to create a 4 bit counter here. Props to the original creator, it is a much simpler way to flip-flop on a clock. Reducing the component count is going to be a big deal here as I would like this to be as small or smaller than a credit card.
After developing the 4 bit counter all that would be required is to simply look for the number 9 to come up. This is a pretty simple task with the use of some logic gates. The number 9 in binary is 1001 and I decided to flip the 0's with a couple inverters so I could run all 1's into a 4 input AND gate. From there I would just need to latch it once it's detected and pull the clock pulse high for the duration of binding. I inverted the signal using a NAND gate instead to trigger the latch, then the latch output is fed into an OR gate so once it goes high, the output of the OR gate will remain high. This is a pretty simple setup and I don't have any reason to believe that it won't work, but I will be doing more testing after putting together the first prototype.
After developing a small test circuit I probed it with my Digilent Scope to verify that the output was as expected. I'm pretty happy with the results of the initial test. I've got a small dip on the first high pulse after startup, but it clears up after that and I don't believe it's going to cause any problems. One thing that I didn't like was that my turn of with the Q output has an unusual shape near the bottom, which I unfortunately didn't capture, but I will try to get a screenshot of it next time I'm testing. I believe that it's being caused by the capacitor at the base of the NPN transistor charging or discharging into the circuit at turn off. It just means that I will more than likely end up using the !Q output instead for a nicely shaped square wave.
Below you can watch a video of the circuit simulation designed using Paul Falstad's circuit simulator. It has a few glitches and for some reason starts at a binary count of 7 (I had the LED's oriented backwards during this test so count runs from left to right here), but after the first glitch it starts up properly. Later in the video you will also see the latch reset, but that's another glitch that causes the voltage to suddenly disappear, which triggers a reset of the latch. This should not happen in real life, but I will be testing for it.