I have a number of standalone ATmega chips, with a cheap 433MHz transmitter from Ebay, and I use VirtualWire to transmit the temperature every minute.
Is there a schematic for a very efficient low current 9V to 5V step-down converter ?
I would like to run the Arduino for 1 or 2 years on a 9V battery (9V, 400 mAh). The average current would be about 30uA.
Perhaps using CD4000 series cmos logic (they run at 9V without problem) with a coil. For example the CD4011 uses 5pA (pico Ampere) quiescent current.
The transmitter at 5V needs 20mA turned on.
I could power the transmitter directly with the 9V. So only the ATmega needs to get 5V a short time every minute.
The ATmega chip needs less than 10mA when running.
The step down converter converts to a lower voltage and decreases the current from 9V. Do you need that? When you are happy with the 30uA average go with linear regulator - they may have 1.5uA current today (ie MCP1700).
Run the transmitter from +4.5/-4.5.
Run the atmega from virtual Gnd (connected to Vcc) and -4.5 (connected to Gnd).
Many transmitters will go sleep when their input is not being toggled. Check if yours does. If not, find one that will.
Running the VirtuaWire library, it will send a few bytes to get the receiver in sync, and encode your message, the receiver will decode it to ensure a good message came in.
How do you wake up the C every minute to send something?
Saving power:
Maybe don't send if the temperature didn't change.
Or only send every ten minutes so the receiver knows the transmitter didn't croak.
Change the power source - use a 3xAA battery pack intead of 9V. 434 MHz Tx modules will transmit fine with that.
I have an RF remote control, its based on 8 MHz promini and 434 MHz Tx module, runs from 1000mAH LiPo directly. Starts at 4.2V and drifts down to 3.7.
MAX1811 charge control chip charges it from a 5V supply when I remember to plug it in every 4-5 weeks (or, when I go to use it and it won't send (LED blink) and I realize its been a while). Press a key, it wakes up & sends the key data, goes back to sleep. Someday will rebuild to be full standalone; no power LED, no regulator being reverse driven from the battery, and maybe a transistor to ensure the Tx module is unpowered. My multimeter measurements show ~9mA when in sleep mode, ~15mA while transmitting, so got some room for improvement.
9V batteries have another surprise, 8.4V is a common terminal voltage and it gets worse with rechargeables.. 7.5V is common and all are about 100 mAH capacity. After all the stuff is said the 3 AA batteries is the best deal. The rail splitter (data sheet attached) TLE2426 requires 170 uA quiescent current.
The last caution is to place the batteries somewhere that the corrosive battery electrolyte won't damage anything. Old (+ 1 to 2 years) batteries Will leak. The internal chemicals will rot out the casing and your electronics too if the stuff contacts the board.
On that page I described a "room temperature" monitor:
That was powered by 3 x AA NiMH batteries. I set up a test on 18th March this year, and today (27th June) it is still going strong. The combined battery voltage has dropped from 4.02V to 3.86V over the 3 months, with it flashing its LED quite often because this room is usually either too hot or too cold. That means it is waking up and doing something every second usually.
Since the NiMH batteries are mostly drained at around 1.2V (and thus the combined voltage would be 3.6V) it looks like it has a way to go yet. For example from my log it has 3.89V on 19th May.
If I wanted it to last longer I would use Alkaline batteries which have a much lower self-discharge rate. On this page I am exploring the use of a MOSFET to control external devices (like your transmitter) whilst keeping current consumption low while it is off.
I recommend using batteries in the required voltage range (eg. 3 x AA batteries) rather than a 9V battery and then waste power converting it back to 5V.
Lithium 9V batteries are sometimes 1200mAh. For example smoke detectors will run for many years on those. I only use good quality batteries (GP, Varta). Corrosion of the circuit can be easily avoided with pcb spray (I use SK10 flux spray).
The cheap 433MHz transmitter uses less than 10uA (I could not meausre it) when not active. So that is okay.
For waking up, I think the WatchDog is the best option. Or an internal timer. The Arduino would wake up a lot, but would transmit only once per minute (or per 5 minutes).
3 AA batteries would be the easiest solution. But I have to do more tests, since the transmitter needs at least 5V for outdoors range. I have now 100% reception at 5V and missing a few temperatures is no problem.
A 9V battery is smaller... My plan is to use a standalone Arduino without pcb, just solder a sensor and the transmitter to an ATmega chip. Uploading a sketch with hooked clips to the ic pins.
I would like to use batteries and not Li-ion cells, since I'm not very good at making something for outdoors. Either a complete closed box, or one that has an open bottom, whatever I make seems to collect moisture.
My plan is to test 3 AA batteries, if that is not good enough a single AA with step-up converter, or as third option a 9V with LDO.
Docedison:
9V batteries have another surprise, 8.4V is a common terminal voltage and it gets worse with rechargeables.
i use a few hundred alkaline and over a thousand zinc (super heavy duty, lol) 9v batteries a year in my business and if any measure less than 10v the batch is refused. ive never seen 8.4v but that is common for the rechargeables though.
imo biggest issue with this plan is that cheap ook tx cannot be simply turned on for few microseconds. dc settling and other issues require very lengthy sync headers before even starting to transmit data. and data rate is very low. in their original application (garage/car dongles) transmission may take several seconds for even tiny 1 byte on/off packets. doing the math it may prove difficult to get a year even from lithium types. practical options would be to use more modern rf modules like 2.4ghz 24l01 which are now even cheaper than the old 433mhz ones. and orders of magnitude higher throughput. if you can be pried away from 9v world but space still an issue then 3 aaa is quite doable with these and no regulator required.
John1993, I keep using the cheap 433MHz modules (1.50 dollars for TX + RX module inclusive shipping).
The VirtualWire library doesn't use a single start pulse, but a number of short pulses to synchronize. It also doesn't repeat the message. I have leds pulsing on the TX and the RX Arduino (only for testing) and as soon as the TX Arduino has ended transmitting, the receiver pulses for good reception. The more I use VirtualWire, the more I like it.
I have 6 bytes + 3 or 5 overhead = 9 or 11 bytes at 2000 bits per second.
hey, if it works and you get the range, reliabilty, and battery life you need then problem solved. for me at 60 cents ea imported from china the 24l01 modules do the trick. strangely the bare chips cost 2x that. two of my products are powered off button cells and the ook just didnt work out. i do use them for other applications though where extreme range is required. 433mhz has higher output and better penetration than microwave. 310/318mhz better still but some certification issues.
For sleep mode, I use SLEEP_MODE_PWR_DOWN. The Power-Down almost shuts down the complete chip.
To wake up, I use the WatchDog.
The WatchDog causes a reset, so counting with a variable until a minute is reached is not possible. Therefor I use the EEPROM to count. I spread the counting over the whole EEPROM, to prevent EEPROM wear.
When it is time to measure the temperature, I use an Arduino pin to power an LM35 or DS18B20 and transmit the temperature.
I don't know what the average current is, it is hard to measure.
Perhaps I will use 10 ceramic 100nF capacitors parallel. They don't leak and I need some capacitance for when the batteries are getting weak.