EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: TannerFrisby on September 28, 2020, 11:50:30 pm
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Quick description of the project:
I am trying to create a low current device (battery-powered) that listens for an RF signal and then performs a task. Both current consumption and cost are major factors.
Present implementation:
For the prototype, I am using an esp-32 (nominal 10ua deep sleep, enough io pins, Arduino IDE) and a HopeRF RFM83C (measured 5ma on, assumed nominal 3ua shutdown, acceptable RF sensitivity). Currently, the RF receiver is always on and the first 433mHz signal it hears will interrupt the esp-32 from deep sleep. While it should be possible for the esp-32 to interpret the remainder of the received signal and parse the instruction at the end, it is above my current programming level and I'll save it for future development. Instead, the esp-32 waits for the second sending, which requires the user to continue holding down the RF remote's button (less than ideal, but not really an inconvenience). As the first transmission from the RF receiver to the ESP-32 is lost (due to wake-up), I would like to lower the RF receiver's duty cycle to the point where it will wake up within the transmission window of one message (~35ms I think) "listen" for a few cycles of ~285us 1's and 0's and then be latched "on" by the ESP-32 for the next message if and only if it "heard" something causing an interrupt to wake up the ESP-32.
My request:
I don't know what an IC like this would be called? Wake-up timer IC? PWM timer IC? I would like the device to oscillate with a square wave of some duty cycle between high (active shutdown of RF receiver) and low (allowing the receiver to listen for a few cycles). Duty cycle and period adjustability would be ideal to help with tuning. Sub-milliamp current would be great. If it takes more than 3ma or so, then it's not even worth it.
Search results:
I found "wake-up" ICs, but they appear to be just a pulse (i.e. adjustable period, but near zero percent duty cycle). I looked at a 555 timer but the example I saw had 10ma draw and a handful of suport passives. Ideally the IC I'm looking for wouldn't require any support peripherals and would have very low current draw.
Thank you to all who have taken the time to read this and help me out,
Tanner Frisby
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I think what you are describing is a multivibrator which can be "gated".
Have a look at this page:
https://www.allaboutcircuits.com/technical-articles/how-to-design-schmitt-trigger-oscillators/ (https://www.allaboutcircuits.com/technical-articles/how-to-design-schmitt-trigger-oscillators/)
and, in particular, where you see this graphic:
[attachimg=1]
This article on Schmitt triggers might also be helpful:
https://www.allaboutcircuits.com/technical-articles/exactly-how-schmitt-trigger-oscillators-work/ (https://www.allaboutcircuits.com/technical-articles/exactly-how-schmitt-trigger-oscillators-work/)
The CD4093 is an extremely low power chip - itself only consuming in the single microamps. The power consumption of the RC timer will largely depend on the capacitor you use -- smaller will use less current.