Previous post on this project is here <link>.
This little project has certainly taken on a life of its own. I've been at this for several weeks now watching what goes on with batteries and various components and software. My latest set of revisions is almost a start-over.
I removed the Ardweeny I was using and replaced it with an Arduino Pro Mini. The little Pro Mini is designed for folk that want the power of an Arduino without the bells and whistles. There's no place to plug in wires, no screw holes to mount it, take a look:
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The disadvantage is that it has an LED and a voltage regulator on it. Voltage regulators waste power by having a ground reference and a small amount of current through it that can run my batteries down. However, this regulator is a SE5509BALG and it has a ground current of only 21 uA, so that may not be a problem. I've run across folk that pull the regulator out to decrease the current draw, but I need some kind of regulator to supply the XBee with 3 volts. More on that later.
The LED is no problem, I can just pull it off the board if I can see it well enough.
I also switched from the TMP36 temperature sensor to an 18B20 one wire device. The reason I didn't start with the 18B20 was that I didn't have one to try. During the testing of battery operation, I ordered some and decided that now was the time to try them out. My decision to switch was partly curiosity and partly laziness. When I tested my code on the Pro Mini, I found a problem with the analog to digital converters. It seems when you switch them around to read the processor voltage, they don't settle out correctly on the different version of the 328 chip on the Pro Mini. I was going to research the problem and work around it, but realized that the one-wire 18B20 didn't need analog conversion, it's all digital. Problem solved.
Then, I went to three batteries. All the experiments indicated that I just didn't have enough voltage range to get long battery life. The AA batteries supply power very well all the way down to about 0.9V, and then they drop off pretty rapidly. With two batteries, that means I can't get close to the maximum capabilities of the batteries because my circuitry drops out at just under 2.8V. I can only drag the batteries down to 1.4V each and that means I'm wasting almost half a volt of useful power.
Using three batteries and a 0.9V limit I can drag most of the power out of the AA batteries and do useful things with it.
Now do you see why I need the voltage regulator? Three AA batteries will give me 4.5 volts and a very hot XBee. With the low dropout, low current voltage regulator on the Pro Mini, I should be able to get a long period of life before I have to change the batteries and still power the XBee just fine.
This is what it looks like right now:
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I tried to hide the LED so the picture would be clearer. The 18B20 is on top, bent over a bit so it can catch whatever breeze may happen along. I used the same rubber band, but it may have to be replaced soon, starting to dry out and crack. Things are really crowded on purpose, I'm trying to keep all the circuitry on a half sized protoboard. At some point I'm going to solder all this stuff down and actually use it.
So, it's in service right now measuring temperature every minute and sending it to my house controller. I loaded it with dead batteries from the previous attempts. Not only is that saving me money, it should also die more quickly from draining the batteries. Similarly, I left the LED in to consume more power to get to that point. Battery operated tests take a long time to learn what happens at the end if you don't do something like that to hurry things along.
The compromise is that the processor voltage is after the voltage regulator. That means my measured voltage is going to be level until the batteries are nearing the drop out point. I could get around that with some clever rewiring, or a separate regulator for the XBee, but this test may give me all the information I need. I'm just going to look for a level voltage reading until the end where the voltage regulator drop out happens. With the LED running, it shouldn't take too long. If it does take too long, I'll think a bit about a voltage divider from the unregulated voltage to give me a sample measurement. If anyone's interested the code will be in GitHub in about an hour or so.
More later.
This little project has certainly taken on a life of its own. I've been at this for several weeks now watching what goes on with batteries and various components and software. My latest set of revisions is almost a start-over.
I removed the Ardweeny I was using and replaced it with an Arduino Pro Mini. The little Pro Mini is designed for folk that want the power of an Arduino without the bells and whistles. There's no place to plug in wires, no screw holes to mount it, take a look:
The disadvantage is that it has an LED and a voltage regulator on it. Voltage regulators waste power by having a ground reference and a small amount of current through it that can run my batteries down. However, this regulator is a SE5509BALG and it has a ground current of only 21 uA, so that may not be a problem. I've run across folk that pull the regulator out to decrease the current draw, but I need some kind of regulator to supply the XBee with 3 volts. More on that later.
The LED is no problem, I can just pull it off the board if I can see it well enough.
I also switched from the TMP36 temperature sensor to an 18B20 one wire device. The reason I didn't start with the 18B20 was that I didn't have one to try. During the testing of battery operation, I ordered some and decided that now was the time to try them out. My decision to switch was partly curiosity and partly laziness. When I tested my code on the Pro Mini, I found a problem with the analog to digital converters. It seems when you switch them around to read the processor voltage, they don't settle out correctly on the different version of the 328 chip on the Pro Mini. I was going to research the problem and work around it, but realized that the one-wire 18B20 didn't need analog conversion, it's all digital. Problem solved.
Then, I went to three batteries. All the experiments indicated that I just didn't have enough voltage range to get long battery life. The AA batteries supply power very well all the way down to about 0.9V, and then they drop off pretty rapidly. With two batteries, that means I can't get close to the maximum capabilities of the batteries because my circuitry drops out at just under 2.8V. I can only drag the batteries down to 1.4V each and that means I'm wasting almost half a volt of useful power.
Using three batteries and a 0.9V limit I can drag most of the power out of the AA batteries and do useful things with it.
Now do you see why I need the voltage regulator? Three AA batteries will give me 4.5 volts and a very hot XBee. With the low dropout, low current voltage regulator on the Pro Mini, I should be able to get a long period of life before I have to change the batteries and still power the XBee just fine.
This is what it looks like right now:
I tried to hide the LED so the picture would be clearer. The 18B20 is on top, bent over a bit so it can catch whatever breeze may happen along. I used the same rubber band, but it may have to be replaced soon, starting to dry out and crack. Things are really crowded on purpose, I'm trying to keep all the circuitry on a half sized protoboard. At some point I'm going to solder all this stuff down and actually use it.
So, it's in service right now measuring temperature every minute and sending it to my house controller. I loaded it with dead batteries from the previous attempts. Not only is that saving me money, it should also die more quickly from draining the batteries. Similarly, I left the LED in to consume more power to get to that point. Battery operated tests take a long time to learn what happens at the end if you don't do something like that to hurry things along.
The compromise is that the processor voltage is after the voltage regulator. That means my measured voltage is going to be level until the batteries are nearing the drop out point. I could get around that with some clever rewiring, or a separate regulator for the XBee, but this test may give me all the information I need. I'm just going to look for a level voltage reading until the end where the voltage regulator drop out happens. With the LED running, it shouldn't take too long. If it does take too long, I'll think a bit about a voltage divider from the unregulated voltage to give me a sample measurement. If anyone's interested the code will be in GitHub in about an hour or so.
More later.