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I would like to detect when the postman put mail in my box, so I was thinking about a simple attiny with tilt sensor (which would wake up the attiny) and a 433 MHz emitter:

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The problem is that one cell coin (3V) is really low for this kind of emitter, it is good with at least 5V. So if I use two cell coins I have 6V and above max attiny voltage. If I plug it with one cell and emitter with two cells, the data pin will be only of 3V which is not good.

If I plug all on 6v, I have to use a zener diode or a classic diode to get below 5.5v but this diode will drain my battery even when the attiny is sleeping. How could I get a good voltage for the emitter without draining my battery?

Nightmerker
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bodtx
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  • This is a very broad class of challenge. You may want to use a very low quiescent current 3v3 regulator for the ATTiny, especially one with an enable where you can wire it to fire via a series capacitor and the switch and then have software hold it on as long as needed. In terms of translating the modulation signal level, you would have to understand the nature of the circuit you are driving. Note there are far better 433 MHz radios out there which will work fine on 3v3, from the likes of SiLabs or TI. Or you can go to LoRa. If range is not too far, maybe nRF24 clones. – Chris Stratton Dec 14 '18 at 15:20
  • The bigger issue is designing a data frame and decoding that frame at the receiver. Have you considered this aspect? – Andy aka Dec 14 '18 at 15:23
  • Decoding with the horrid regen receivers is indeed tricky, but I believe there are off-the-shelf Arduino or whatever libraries for that. One can get an example of viable data frames by looking at existing remote controls. But OOK is a primitive modulation, and theses SAW TX / Regen RX solutions are very crude compared to an SPI-interfaced synthesized digital packet radio with FSK (as well as OOK) capability like the Si443x or CC1101 or something with more advanced modulations like the SX127x or a garden variety 2.4 GHz chip like the nRF24L01+... all of the latter operate fine on 3v3. – Chris Stratton Dec 14 '18 at 15:27
  • @ChrisStratton indeed I'm using virtualwire lib in my home I have many of theese receviver emitter for my lights and heaters. If I change for another emitter, I would have to dedicate a receiver with special lib or special code managment for that. I would rather keep my virtualwire thing. I will have a look into a good 3.3v regulator but I have a doubt. – bodtx Dec 14 '18 at 15:33
  • The 3v3 regulator will be fine for the MCU, the questions you will need to solve are if you need to cut power to the transmit module when not in use, and how to translate the modulation signal - for that you'll have to electrically understand the input to the transmitter. A better 433 MHz radio like the Si443x or CC1101 can transmit an OOK frame compatible with what you are using now, it just isn't limited to that. – Chris Stratton Dec 14 '18 at 15:38
  • @ChrisStratton for the emitter consumption, two solution: maybe the emitter consumption is really low when not in use, I did not find that on the [datasheet](https://cdn.sparkfun.com/datasheets/Wireless/General/TWS-BS-3_433.92MHz_ASK_RF_Transmitter_Module_Data_Sheet.pdf) or I power it with a digital output pin of the attiny because it draws 8mA at max – bodtx Dec 14 '18 at 16:11
  • Typically for those transmitters the current into the VCC pin is virtually zero if the data in is at ground - they just consist of a single transistor oscillator with a SAW resonator to set the frequency. You can feed 6V (two cells) to the transmitter VCC with a tap at a single cell to power the micro controller. – Kevin White Dec 15 '18 at 19:11
  • @KevinWhite I will have a try but I thought data pin should have the same voltage than the VCC. Another way is to go for solar panel without battery, up only in the day, I have to check how the attiny goes when input current is very LOW, maybe BOD fuse works on this case – bodtx Dec 16 '18 at 22:16
  • @bodtx - Most of the schematics I have seen of similar transmitters use a single transistor oscillator with the input data just going to the base through a resistor - there is no requirement that the voltage on the data pin is the same as that on VCC. For example see http://jap.hu/electronic/rf/tx434.gif. – Kevin White Dec 17 '18 at 02:57

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