How to sense when our TV is on? - Page 1
Forum section: electronics / projects

author: zapta
author_rank: Super Contributor
timestamp: September 18, 2016, 08:17:38 pm
content: I want to build an MCU based one-off circuit that will turn our audio system on/off via IR based on the TV on/off state. Any idea how to have the MCU sensing the TV state?Desired goals1. The MCU circuit will be near the location where the TV plugs to the mains. Having to pass additional cables from the TV (e.g. 5V from its USB connector) is inconvenient.2. If dealing with high voltages, I preferred an off the shelf product that will deal with the high voltage, isolation, and safety aspects.Any suggestion how to do that? The best I found so far is this master/slave switch that with a 5V charger can provide 0-5V signal.https://www.amazon.com/dp/B002K8S2J6

author: ovnr
author_rank: Frequent Contributor
timestamp: September 18, 2016, 08:48:45 pm
content: Honestly, the cleanest solution will be using the USB ports, assuming they turn off when the TV turns off.You can always sense the power consumption in a variety of ways, but it's substantially more awkward than an extra wire.

author: zapta
author_rank: Super Contributor
timestamp: September 18, 2016, 09:10:26 pm
content: Quote from: ovnr on September 18, 2016, 08:48:45 pmI plan to do so to sense the stereo system, to know when to issue on/off IR toggle commands. With two USBs I will probably have to isolate at least one of them with opto coupler or something like that. I would hate though to pass another USB cable in the TV cable duct.Can I detect the TV USB remotely? E.g. by plugging some RF transmitter?

author: TheBay
author_rank: Unknown
timestamp: September 18, 2016, 09:20:40 pm
content: Is there an optical audio out that's not being used? Mine goes off when I turn the TV off, that could be a way using an optical sensor and a simple bit of circuitry?Or is there an RS232 port on the back, a few have this and that might be something you could use?

author: promacjoe
author_rank: Regular Contributor
timestamp: September 18, 2016, 10:55:30 pm
content: maybe start with something like this.https://www.amazon.com/gp/product/B00WV7GMA2/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1The sensor voltage is 3.3 ~ 60V DC, 12 ~ 120V AC. You could use a Hall's effect current sensor and a comparator to since the current. But that means an extra 5 vote power source. It would be better to use the TVs USB port, if it turns off when the TV is off. if the USB port does not turn off, you could use it to power the halls effect sensor/comparator.Joe.

author: zapta
author_rank: Super Contributor
timestamp: September 19, 2016, 12:08:24 am
content: Quote from: wilfred on September 18, 2016, 11:48:14 pmI don't want to turn off the audio system by shutting power to it because it's slower to turn own, having to go through boot and such. There are a few other considerations, when the audio system gets power it enters the standby mode and require an IR code to turn on, also both the TV and the audio system on/off IR code are toggle, not direct code so I need to make sure that the states don't get out of sync.I thin I can make a reliable and fast reacting system with these IOs1. in1: Low latency sensing of TV on/off state (e.g. via current consumption, so I don't need to pass another cable in the TV cable duct).2. in2: Medium latency sensing of AV on/off state (e.g. via it's USB +5v output).3. out1: sending AV on/off toggle IR code.I see on the market many many clamp DMMs and inductive AC sensors but I wonder how people actually use them to measure current of appliances since to separate the wire to pass through the clamp. I would expect an off the shelf pass through adapter that include the AC transformer or provides an external loop of a clamp.

author: Buriedcode
author_rank: Super Contributor
timestamp: September 19, 2016, 12:27:43 am
content: TV's generally have unused IO, be it USB, DVI, HDMI (pin 18 I think carries 5V that I doubt will be available on standby). Any of those coupled with a micro for IR/RF will do.Unique and convoluted idea: for non-invasively detecting if an appliance is 'on' I thought perhaps a couple of hall sensors would work, because the magnetic fields in parallel conductors don't truly cancel. So I googled, and found someone beat me to it:https://moderndevice.com/new-products/current-sensor/Using that and a comparator (or one in a micro) means you can send out an IR command, or use RF when its turned on, or off. Or just check every 10 seconds in case things get out of sync. It would need is own power supply of course.

author: zapta
author_rank: Super Contributor
timestamp: September 19, 2016, 02:05:54 am
content: Quote from: Buriedcode on September 19, 2016, 12:27:43 amThat's a clever idea. I may give it a try.BTW, capacitive sensing will not work right? Since the voltage on the wires is independent on the on/off state of the TV.

author: cncjerry
author_rank: Unknown
timestamp: September 19, 2016, 02:36:52 am
content: I've found that TVs, referring most to flat screens, LEDs, etc, will activate a number of IR receivers for some reason. I've never looked into the reason since most IR receivers require some type of carrier like 40Khz or something. If you put a IR phototransistor or a boxed IR receiver in front of one or even in the vicinity, you could probably scope the output and determine the characteristics related to it being on.

author: grifftech
author_rank: Frequent Contributor
timestamp: September 20, 2016, 02:31:07 pm
content: my LCD TV has some white light come out through the back when on so just use a photocell and wire it in with some magnet wire.more light = less