Ultra low power receiver, little data, long operation - Page 1
Forum section: electronics / rf-microwave

author: Johannsen
author_rank: Regular Contributor
timestamp: April 28, 2020, 04:30:53 pm
content: Hi,I'm looking for a solution to send a short information, 8bit number to a battery powered receiver.between pressing a button/trigger and receiving the data should not exceed 2ms. Range of 300m would be nice.Most of the receivers I found earlier used around 20mA in receiving mode.(like rfm69/rfm95 etc) My idea was to use them in duty-cycle mode but either the delay was too long or the power consumption too high.Would be nice to power the receiver with max of 4 AA batteries approx 6 month or longerNow I found the following from Semtech. The IC LLCC68 even uses <5mA in RX mode.Do you think one of the 3 chips could be helpful for my purpose?https://www.mouser.de/pdfDocs/DS_LLCC68_V10-2.pdfhttps://www.mouser.de/pdfdocs/sx1276_77_78_79.pdfhttps://www.mouser.de/datasheet/2/761/DS_SX1261-2_V1.2-1626941.pdf

author: mark03
author_rank: Frequent Contributor
timestamp: April 28, 2020, 06:44:21 pm
content: Let's do the math:AA battery capacity is ~ 2000-3000 mAh according to the great google; let's call it 2500 mAh.2500 mAh / (6 * 30 * 24 hours) = 0.58 mA continuous current over six months (at 6V).Maybe you only need 3V so could put two batteries in parallel, which would increase your current budget to ~ 1.2 mA.I'm not an expert on wireless but I think you will be hard-pressed to find off-the-shelf receivers which can operate at such low power. As you guessed, the usual "trick" is to run them at a low duty cycle, e.g. a sensor node which wakes up and reports some data once every minute or every hour. If you require a 2-ms response time, at any random time of day, I don't think you have any alternative except to stay awake. It may be counter-intuitive, but radio receivers are inherently power hungry, more so than transmitters at sufficiently low power.Maybe if you built something from scratch (discrete components) there is a way, but I'll let the real experts weigh in on that.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 29, 2020, 09:56:23 am
content: The biggest factor in determining power draw is latency - if you can relax the latency requirement then that will help a lot.Do you actually need low latency, or just to know when the signal was activated ? If the latter you can use repeated transmission with timestamps

author: Kalvin
author_rank: Super Contributor
timestamp: April 29, 2020, 04:24:08 pm
content: Would it be possible to arrange some sort of early warning transmission from the transmit side to the receive side when the button press / trigger is likely to happen withing a few minutes or seconds? In that way the receiver could be running at slower duty cycle when button press / trigger is not going to happen, and become more active when the button press / trigger is probable. Also, if the button press/trigger is not going to happen during night time for example, the rx could be turned off completely during the night time, effectively doubling the battery lifetime.

author: Johannsen
author_rank: Regular Contributor
timestamp: April 29, 2020, 06:07:35 pm
content: Thank you all for your profound answers!The latency requirement is because I want to sync a flash with it's camera and therefore need a fast response. 2ms is the average what I measured on available remote trigger.QuoteThat's maybe a good workaround as the camera itself has a delay around 10ms between pressing the shutter and firing the flash.Do you have a clue how I can calculate the min/max duty cycle for a maximum latency of 2ms?What do you think of the mentioned receiver like LLCC68 which only needs approx 4mA. Would that work in duty-cycle-mode?(built a rf receiver from scratch I wouldn't dare to do)

author: Johannsen
author_rank: Regular Contributor
timestamp: May 06, 2020, 12:06:59 am
content: Another receiver I would like to mention is the CMT2210LSW. This one is a OOK stand-alone RF receiver.Would there be any advantages? As I understand and tried it it could be easier to toggle the power and switch it on/off according a duty-cycle.The more sophisticated chips like from Semtech need some time after powerdown to start again.Some china remotes with a long runtime but long response time use this CMT2210 I figured outhttps://www.hoperf.com/ic/rf_receiver/CMT2210LSW.html