All of our preamps are controllable via handheld remote. We use a 7 button Apple remote that uses infrared (IR) light to communicate with an IR receiver module connected to our V25 preamp controller. IR communications is fairly old-school tech these days. And mostly it just works. Except when it doesn't.
While recently working with one of our preamps on our development workbench I noticed that the preamp had suddenly stopped responding normally to remote input commands. It had become sluggish and at times flat out non-responsive when moments earlier it had been working flawlessly. I'd noticed similar behavior off and on before but it was short lived and would suddenly be ok. I'd wonder if this was a symptom of a design defect but if so I'm sure I'd be hearing from customers having similar issues. I hadn't.
I decided that this time I was going to get to the bottom of what was happening. I hooked the IR receiver module up to an oscilloscope and monitored the output of the IR receiver. Sure enough, it was firing away even when I was NOT using the remote. Time to eliminate all sources of IR light noise. First I closed the blinds. Then I turned off all 3 LED/LCD monitors on the bench and turned off both the LED bench lamp and fluorescent bench lamp. Finally I turned off the ceiling light with its high wattage compact fluorescent light bulbs. Looked at the oscilloscope and it was still firing away. But wait! It wasn't firing randomly, the signals were coming in exactly every 40 milliseconds like clockwork. I covered the IR receiver module with my hand and it stopped. Removed my hand and it started again. Something in the room was generating periodic IR signals but what was it?
My Google FI Pixel 2 smartphone had been sitting on the workbench getting in the way so I picked it up and put it in the side pocket of my cargo shorts which it where it usually lives. I looked up at the oscilloscope and noticed the pulses had stopped. Wait what? I slowly took the phone back out of my pocket while looking at the scope and suddenly the pulses were back. Phone back in pocket - no pulses. Phone out of pocket - pulses. Aha! It was my phone that was stepping on the IR remote commands. But how and why?
I then did what any man would do. I did a google search. And after some digging around I learned a lot about smartphone "proximity sensors". A proximity sensor is a clever little feature that allows a smartphone to turn off the screen while you're holding the phone up to your face during a phone call. How does it do that? It sends out a weak infrared pulse kind of like miniature infrared radar. When the IR echo off your face gets strong enough it decides to turn off the screen. When you put the phone face down on the table, it blocks this signal. Same thing happens when you put the phone in your pocket.
What to do. In the short term we'll be advising out customers to be aware of this potential interference problem and if problems arise keep your smartphones put away. I suspect this is only a near-field issue since I often have my smartphone next to me when using my Tortuga preamp in my living room and do not have the same interference issue with the preamp being 10 feet away. But smartphones aren't going away anytime soon nor are their IR proximity sensors. Fortunately, manufacturers of IR sensors like Vishay are upping their game with ever better IR receivers that are less susceptible to this type of smartphone noise. We will be switching to one of these newer IR sensors soon. Unfortunately the pinout of the new sensors are not compatible with our existing hardware so we'll need to change the adapter boards we use going forward.
Meanwhile, another mystery solved!