Hi all, I’m a complete newbie with the imp. I have followed an instructable and built the imp tempbug using the april dev board and so far so good, i.e. it is logging the temp data to Xively and sending alert emails via Zapier.
I would like to add a battery monitor, but it needs to be able to handle a range of battery voltages exceeding 3.3V, so I need a voltage divider. To conserve battery I want to gate the voltage divider. Now for less that 3.3V, I know I can just use a GPIO pin to gate the bottom of the divider to low when I want to measure, which is how the tempbug is currently measuring the thermistor voltage divider. But if I want to read a higher voltage, say a 9V battery, when the GPIO pin goes high, the divider will still be drawing current and/or, the ADC input will be over the 3.3V limit. So I guess I need an active voltage divider? I saw a post on the forums about measuring battery voltage on the Hannah board using the motor driver enable gating function to actively gate an FET switch (part of Hannah board) to gate the battery measurement. I can try to implement a similar switch, but I’m hoping there is something more clever and clean to implement on the April board. I have even thought about repurposing the battery reversal FET on the April board, but that is probably a bad idea. So if anyone has a sweet way to add this function to the April board I would love to hear about it.
Another question I have is about the possibility of a simple rig to use dual power supply for an April board, i.e. the USB 5V when the mains power is on, and a battery when the mains power fails. I am thinking that a forward diode from the USB 5V header pin to the Vin header pin, combined with using a 3xAA (4.5V) battery supply should be enough to do this very simply, if the existing reverse voltage protection on the battery supply line will block the higher USB 5V from the lower battery voltage. Does anyone know if the reverse voltage protection FET on the battery supply line would be adequate to keep the USB input from trying to charge the batteries?