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Clock Battery Monitor Circuit

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bitogre:
I am working on a personal project using a Teensy 4.1 to monitor/log info in my RV.  When the RV is in use, the Teensy will be on, powered by the RV's 12V lead acid battery.  But I remove the lead acid battery when in storage, so, to keep the Real-Time Clock set, I will have a smaller battery (lithium coin cell or 2 AA batteries to power the RTC).  I want to use a spare ADC input on the Teensy to monitor the RTC battery and let me know when it is low so I can change it before it dies.  Problem is, it looks like the Teensy ADC input will put too much load on the battery when it has no power (other than for the RTC) which will kill my RTC battery in a month or so if I connect it directly.  Will the following circuit help prevent the extra load on the RTC battery when the Teensy has no power (effectively pulling the ADC Input and Enable Logic pins to Ground)?



If so, any recommendations on the FET?  This is a one off project so through hole components are preferred.  I have no experience with FETs so not sure if I got this right.  The key thing I am looking for is as low as possible Drain to Source current (micro-amps or less) when Enable Logic is pulled low and turns on restively (can have a resistance as high a 1K ohm) or known fixed voltage drop when Enable Logic is pulled high.  I am not worried about the current draw when enabled as I can minimize how long it is enabled to when I am checking the voltage (a couple milliseconds every hour/day or so).

I am also open to other suggestions for other circuits that would work using through hole components.

madires:
A JFET would be the wrong transistor for that purpose (because of the depletion mode). Better use a low-leakage small signal p-channel MOSFET or a PNP BJT.

bitogre:

--- Quote from: madires on July 18, 2020, 08:23:01 pm ---A JFET would be the wrong transistor for that purpose (because of the depletion mode). Better use a low-leakage small signal p-channel MOSFET or a PNP BJT.

--- End quote ---

The problem I see with using a PNP BJT is that the Teensy 4.1 uses 3.3V logic so the Enable logic high voltage may only be a couple tenths of a volts higher than the battery voltage I am trying to measure.  I am having problems figuring out how to amplify that signal enough to provide enough Base to Emitter voltage to get an accurate voltage reading.

As for using a MOSFET, as I said, I do not know much about FETs (of all types) so that seems reasonable.  Would it be more or less a drop in replacement to the JFET in my circuit diagram? Or would the circuit need changes to support the MOSFET?

madires:
The PNP or p-ch MOSFET would have a pull-up resistor to prevent them from conducting and the MCU would pull the base/gate down for switching on. But that switching will draw some power from the battery. Another idea would be to use a CMOS switch. Basically you're looking for a low power switch with a low leakage current.

bitogre:

--- Quote from: madires on July 18, 2020, 09:03:13 pm ---The PNP or p-ch MOSFET would have a pull-up resistor to prevent them from conducting and the MCU would pull the base/gate down for switching on. But that switching will draw some power from the battery. Another idea would be to use a CMOS switch. Basically you're looking for a low power switch with a low leakage current.

--- End quote ---

I do not think an active low solution will work.  The problem I see with that is that the MCU, when powered off, effectively is a 1.5K pull down to ground.  What is to keep it from turning on in that case?  I was looking for an active high solution so that a low on Enable is off (so the turned off MCU and intentional off are the same).

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