An antenna and RF circuitry in a device that has NOT radio functions? - Page 1
Forum section: electronics / projects

author: mcinque
author_rank: Unknown
timestamp: May 17, 2015, 10:35:18 am
content: I'm about to troubleshoot a home gas detector that don't detect anything anymore, and on the main PCB I found this (antenna?) connected on one side and floating on the other.By its shape (and since the trace is coming under something shielded, it seems RF circuitry at the output), to me it seems a tuned antenna, but this detector is NOT wireless or radio controlled.What else can be?For what reason a manufacturer would put RF circuitry in a product that has NOT radio/wireless functions?p.s. Don't know what is and how to identify that 6pin gold K5(L?), what is that?

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 17, 2015, 10:46:35 am
content: The crystal * 32 is 434.2 Mhz = ISM Band. Everything looks and smells like a transmitter to me, maybe wireless was an option and they didn't bother to leave it out on the wired model? And where did you get 'buffered delay line'? Seems a bit odd.Google TKD5100

author: mcinque
author_rank: Unknown
timestamp: May 17, 2015, 12:11:57 pm
content: Quote from: PA0PBZ on May 17, 2015, 10:46:35 amI AM a bit oddbad googling...I've updated the image...

author: AndyC_772
author_rank: Super Contributor
timestamp: May 17, 2015, 12:24:35 pm
content: http://media.digikey.com/pdf/Data%20Sheets/Infineon%20PDFs/TDA5100.pdf

author: mcinque
author_rank: Unknown
timestamp: May 17, 2015, 01:06:39 pm
content: Thank you, so it's definitely a transmitter. The IC isvery cheapbut if you assembly the PCB in thousands without using them you will spend a lot of money, don't you? Or maybe it's more expensive to do 2 runs on the production?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 17, 2015, 01:20:56 pm
content: Quote from: mcinque on May 17, 2015, 01:06:39 pmYou'd be surprised. The overhead of creating two part numbers, two assemblies, two test procedures, etc... Who knows? Some companies are so poorly organized it takes every ounce of energy to just get one design out the door, there's nothing left over for structural niceties like automated documentation control.I'm continually impressed, in a bad way, how many hardware companies refuse to use documentation control, versionning, automation, etc

author: Fraser
author_rank: Super Contributor
timestamp: May 17, 2015, 01:21:36 pm
content: Many such devices are sold as either stand-alone or 'monitored as a part of a building alarm system. The manufacturer makes just one detector (like yours) and configures it for stand-alone or 'monitored'.House alarm systems often have an additional capability to monitor both fire detectors and carbon monoxide detectors via wireless connectivity. These are options that are enabled as and when required.Not that unusual but something I had to be aware of when authorising such fire and carbon monoxide detectors in sensitive areas that prohibited radio transmission equipment. Manufacturers have an unfortunate habit of providing 'added value' in the form of Bluetooth or other wireless ISM band technology when you do not want it ! Some actually still transmit as though they are part of a monitored system, when in fact they are stand-aloneThe 'dead traces' are the antenna system in your unit. Its a pretty standard telemetry transmitter.As to why the unit has stopped working. The power supplies are often the first point of failure as they run 24/7 and degrade with time. The other issue is that the detectors in gas detectors can be organic material based and have a finite life. Once they have degraded they fail and the detector assembly needs to be replaced with a new one. Life span was IIRC around 5 or 10 years depending upon the model. One British Gas Carbon monoxide detector I worked on had a self destruct sequence built in..... Once the long life Lithium battery died (after 5 years) the detector was considered expended and should be replaced for safety reasons. Trying to open the case invoked a self destruct that snapped a fine wire and isolated the battery supply rail. This was designed to prevent the fitting of a new battery into an old detector that was potentially degraded to the point of not being safe.There is the possibility that your gas detector unit has deliberately stopped working as it has 'timed out' or has detected deterioration in the gas detector cell.Please consider this when trying to repair such devices that will be placed back into service. A life is always worth more than a detector unit, no matter how expensive it is to replaceThere is also a serious personal liability issue with repairing such equipmentAurora

author: mcinque
author_rank: Unknown
timestamp: May 17, 2015, 03:47:48 pm
content: Quote from: Alex Eisenhut on May 17, 2015, 01:20:56 pmRight. We can't know why but your suggestions clarify my doubts.Quote from: Aurora on May 17, 2015, 01:21:36 pmWell, this is clarifying.QuoteExactly!QuoteChecked, replaced the two main caps because they bulged.Still the same behavior, it powers on but with no alarm or optical reaction from the detectors when I spray them with lighter refill.I should investigate if it's something in the detectors or in the circuitry after.I'm not an expert (nor in troubleshooting or engineering) and SMD PCBs with a lot of vias make the reverse engineering a little difficult to me, but it seems that the main power rail is STILL a little bit messy.QuoteYeah, I knew thatQuoteSince this device has 3 green/yellow/red status leds, I would expect to have the red one turned on in case of "self destroy".QuoteIndeed. I'm trying this repair only for fun.Thank you!

author: janoc
author_rank: Super Contributor
timestamp: May 17, 2015, 10:39:18 pm
content: Quote from: mcinque on May 17, 2015, 03:47:48 pmCareful, a natural gas detector usually looks for methane. Butane from a typical lighter refill may or may not trigger it, probably depends on the type of the sensor used. Better test it over your stove (assuming you have a gas stove too).

author: mcinque
author_rank: Unknown
timestamp: May 18, 2015, 11:26:58 am
content: Thank you for the advice; I've played with some methane detector sensors and they react very well with propane and other flammable gases. Of course not so much and quickly they should do, however this kind of test (lighter refill) is suggested in the service manual.