Hi, I have a project that needs to work in a remote location with battery power. I need to install the system, have it switch on 11-V power (very low amperage) once after a 2-month wait (precision to a day or two), and preferably switch the power off after another 4 months. That's all I need to do, but it must be very reliable and as small as possible.

With earlier help on this forum I have set up a PICAXE 08M2 controller (on their "starter kit"), a Grove DS1307 RTC, and a Grove MOSFET. This combination works, but I can't figure out the power consumption--so I know how long it'll last. I am also not clear on whether the MOSFET will continue to operate as the battery voltage drops. The battery pack that powers the PICAXE is 3 alkaline AAs.

Someone has suggested that I use a latching relay to save power. I see that the Grove 2-Coil Latching Relay advertises low power consumption. But it also says that its minimum operating voltage is 4.7 V, which I think is too low for the AAs. So may this won't work unless I figure out a separate battery for the relay. But at 5 V, the battery option doesn't look easy.

I guess my main problem is figuring out the power consumption of the various components. And it's hard to test the system, because the critical part is that it last for the 6-month period! I have been looking mainly at the assembled kits because my knowledge about putting together the boards is inadequate.

If the MOSFET power consumption is low enough maybe I can keep that. But I don't know what that is. If anyone can help I'll appreciate it!

Thank you!

 

I can't figure out the power consumption

Use a meter to measure the current?

 

Use a meter to measure the current?

I will try that. I was thinking that the current would be so small that I wouldn't have the equipment for that.

 

OK, I tried the voltmeter and found a problem that may indicate I can't use this system. Without the mosfet and without the clock, the Picaxe chip and board (I have the AXE003U Starter Pack board) shows 12.44 mA of current draw. When flashing an LED the draw increases to 17.34 mA. While the Picaxe 08M2 chip itself is supposed to (according to Picaxe) draw 1 mA at most, my measurement of 12 is consistent. Perhaps the board that the 08M2 is on draws the power difference (although it has on it only minimal components on it); there are not specs for the Starter Pack board. Or perhaps I am measuring the current incorrectly. What I am doing is pulling off one of the battery contacts and replacing that circuit with the voltmeter probes. I have the probe plug in the "mA" socket on the meter, and have the dial turned to mA, DC. Does it sound like I am doing things correctly? If so, this system will run on 3 AAs for only 2 weeks, and I need 2 months.

 

OK, I solved this. It was the Picaxe 08M2. I tried another chip and it draws just 0.62 mA at idle; 6.45 with the LED on. So apparently I was measuring the current correctly; I just hadn't thought that one chip would draw 20 times more current than another.