I am using this RTC clock module (http://datasheets.maximintegrated.com/en/ds/DS3231.pdf). The DS3231 has alarm function that can be programmed twice a day. When the alarm goes off it makes the interrupt pin an open collector (according to the data sheet). The particular board I am using can be found on eBay , and here is the schematics for the board I am using https://edwardmallon.files.wordpress.com/2014/05/rtc-goodparts.jpg?w=640 as you can see, the interrupt pin "SQW" is not pulled high by default). I have already verified on a 3V circuit (with arduino) that the interrupt pin works as expected. For my use case, I am using the DS3231 from the battery Vcc , and not steady 3.3V vcc from a regulator.
What I am trying to achieve is to utilize the alarm function to drive a 9v circuit. I have a few questions:
1) My first attempt is to use the IRF9510 P channel MOSFET. The result is MOSFET is always on . Pull up 9V resistor is used at the gate. Gate-Ground voltage is 0.19v when interrupt is active low. Gate-Ground voltage is 3.5v when alarm is not triggered (interrupt is not open collector) , which explains why the MOSFET is on, but I don't understand why, as the pin should be high impedance when not active low ? (note my battery is at 8.65V, spec sheet says maximum input range to be 5V and 5V + 3.5V is 8.5V quite close to 8.65V)
2) My second attempt is to use a small 3v cr2032 battery and a P-channel MOSFET (I have also tried the same setup with a PNP transistor and that works too). The active low interrupt can drive a 2V+ signal to another N-channel MOSFET gate. (ofcourse, the common ground are established.) That N-channel MOSFET has its source connected to 9V. Setup 2) works except for the current consumption for the 3V cr2032 battery. I measure 0.11mA when the interrupt is not activated, and 0.4mA when interrupt is low. Given 200mAh typical capacity, this only amounts to approximately 1/2 a month of runtime (assuming average 0.55 mA draw) I should mention, both the MOSFET-MOSFET setup and PNP-MOSFET setup draws about the same amount of currents, (MOSFET-MOSFET is better, at 0.1mA when not activated and 0.25mA when active low)
3) for setup 2) there are only a handful of pull-up resistors and gate resistor pairs that result in a working system. I have tried many combinations and my conclusion is anything from 1k ~ 4.7k works very reliably. The current draw is a function of the resistors used but at higher resistance value, the N-channel mosfet (the second mosfet) will not turn on appropriately, why is that?
any suggestions ? thanks