Recreating wired remote control signal to control audio recorder

[reallyquiteconfused]

Hello all, I am building a control system to automatically synchronize video recording on my Canon 7D and audio recording on my Tascam DR100 MkII. The electronics will be mounted to my camera rig, which is set up tp provide 16.8-14.4 volts DC from the main battery pack (a pair of Sony NP-970's in series) as well as a regulated 5v to power the audio recorder and any microcontrollers that will be used here and in future upgrades.

Controlling the 7D was easy (After tweaking the camera's record trigger settings to half-pressed shutter with Magic Lantern, just connect the half-shutter remote pin to ground and it signals the camera to record) but the DR100 is proving much more complicated, since it is a digital signal rather than simply grounding a wire.

The remote control is an interesting solution. The DR100 ships with an IR remote (nothing special here) but also includes a little case that the remote snaps into, which contains some electronics and a 2.5mm TRS jack at the top. A cable connects this to a matching jack on the recorder. The remote itself has a watch style battery, and the case is powered by the recorder via that TRS cable. The electronics in the case "translate" the IR pulses from the remote into the digital signal I am trying to replicate.

The TRS connector uses the tip for the signal, ring for 3.3v in (supplied from the audio recorder), and sleeve for ground. To record the signals on the oscilloscope, I connected the ring to 3.3v from my Uno, sleeve to Uno ground and oscilloscope ground, and top to oscilloscope probe.

I've borrowed an oscilloscope to record the pulses sent by the remote control. The two which I need to replicate are Record and Stop (see first photo) and while I am confident in my ability to program each sequence of pulses, I have no idea how to get the voltage low enough (looks like -75 millivolts, see second photo) to avoid damaging the audio recorder.

With that out of the way, here are my questions.

1a) Does 75 (or -75?) millivolts sound reasonable for this type of remote control application? I was expecting 3.3 volts and am concerned that I may have measured incorrectly. To ensure my method was correct, I did also measure the 3.3v and 5v pins from an Uno and those showed up as expected.

1b) Why is this showing up negative? Dows this imply that the positive 3.3v connection used to power the electronics is also acting as a positive for the signal, and the signal is being grounded for each pulse? If so, How would I replicate this? If not, I would appreciate an explanation of why this is negative.

2. If yes to #1a, How can I achieve such low voltage? I was thinking of using a voltage regulator or PWM output with a low pass filter to achieve a constant 75 millivolts and then use a transistor to turn it on and off, generating the signal, but I know there is probably a much better solution out there.

3. Is there an alternative that may work better? Maybe another microcontroller that is better suited to super low voltage communication?

Thanks in advance for any help! I hope this was plenty of information to answer some questions, but I would be happy to supply anything else that could be useful for tackling this project.

[MorganS]

If it really is 75mV (which is unlikely) then a simple voltage divider can reduce the Arduino's 5V down to that level.

Double-check that your probe isn't overloading the circuit. Does it still function while you're probing it?

Have you heard of LANC controllers? That's an industry standard for controlling video recorders. The Tascam may be using that. Your description of the plug matches LANC.

[reallyquiteconfused]

MorganS:
Double-check that your probe isn't overloading the circuit. Does it still function while you're probing it?

Thanks for the suggestion! I changed my setup to allow the controller and DR100 to be connected while probing and sure enough, I'm getting a nice strong 3 volt signal out of the tip of the cable! The recorder functioned as expected while connected to the remote and scope. (This of course makes much more sense than -75mV when I had the tip just going into the probe on the scope)

Regarding LANC, I do have 2 LANC equipped cameras and while dapters are available to use a LANC controller with Tascam products, but they are not directly compatible. I know nothing about how the communication is handled, but LANC runs on 5-8 volts. Thanks for the suggestion though, I had not considered that.

Given that I now have a totally sensible and expected signal coming into the oscope thanks to your suggestion, I'll try recreating this signal at 3.3V and report back with the results.

Here are the new oscope screenshots in case you're curious.

[reallyquiteconfused]

I'm very happy to report that it is working flawlessly!

I got an accurate scope reading by connecting the probe to my signal wire (the tip of the TRS connector) and grounding it to the sleeve while the recorder and remote were connected. A simple Arduino code using multiple digitalWrite's and delayMicrosecond's allowed me to recreate the signal exactly, and a voltage divider got it down to 2.9 volts which has so far been perfectly reliable. Thanks for your help MorganS! I have to return my borrowed oscilloscope tomorrow and I'm so glad you were able to help me find the best way to use it before then.