I have a wireless temp sensor module that currently runs off a 18650 battery. I am eventually going to get some PCBs made for it and I'd like to reduce the size of it to make it as small as possible so am trying to decide what batteries to use.
I think the 3xLR44 or a LIR2032 would be a good choice. Needs to be above 3V to run the DS18B20 and deliver enough current to work the radio. LIR2032 has a max pulse current of 75mA so should be fine, not sure about the LR44s.
lithium coin cells (as used in calculators) are good for about 1 or 2mA only, they will deliver more
with voltage dropping considerably (for instance they can power an LED as the voltage just falls
to the forward voltage of the LED before the current gets anything like large).
The NRF24L01 is a very poor choice for a remote sensor due to its high current draw and unnecessary complications.
The cheap 315 or 433 MHz TX modules draw only a few mA, have larger range and some will work at only 2V. Others won't work below about 3.5V. Unfortunately, at the moment I don't know how to tell the difference between the two varieties.
MarkT:
lithium coin cells (as used in calculators) are good for about 1 or 2mA only, they will deliver more
with voltage dropping considerably (for instance they can power an LED as the voltage just falls
to the forward voltage of the LED before the current gets anything like large).
The Li-Ion LIR2032 can handle much higher current. About 17mA constant and up to 75mA pulsed. They have a tiny capacity though.
I've ordered a couple to try out and see how well they do.
jremington:
The NRF24L01 is a very poor choice for a remote sensor due to its high current draw and unnecessary complications.
The cheap 315 or 433 MHz TX modules draw only a few mA, have larger range and some will work at only 2V. Others won't work below about 3.5V. Unfortunately, at the moment I don't know how to tell the difference between the two varieties.
Interesting, thanks. Might have a look at them even though I have some spare nRF24L01s ready to use with my project. Could be handy to save a bit of current draw if I use some tiny batteries.
Range is proportional to wave length unless you go to high-gain directional antennas - so 433MHz is
much longer range than 2.4GHz.
This is a simple fact of antenna effective area, which depends on wavelength squared - you gather more
signal power with a longer dipole.
Of course there are other factors, like available bandwidth, co-channel interference (2.4GHz has all
the microwave ovens blasting out on it for example).
If your transmit time is short infrequent pulses you could slowly charge a capacitor and then use the capacitor to deliver the current surge when required.
Amazingly my 15mAh NiMH battery has been running for just over 5 weeks and still going strong. The voltage has dropped to 3.5V so still a fair bit of capacity left in it.
I haven't had as much luck with the LIR2032 because my other sensors are on breadboards and keep getting loose connections and either stop working or drain the battery, but they should last ages if I make a reliable soldered board to use it with.