Filtered Wakeup Sensor Circuit

Forum 2005-2010 (read only) Hardware Interfacing
1

Hello,

I'm using an Arduino in a mobile project that goes to sleep all the time to save power - it only needs to be up and running while the device is moving. To do this, I have a tilt-switch-esque sensor which essentially open and short circuits itself on vibration. It is normally closed, but opens and closes on vibration / rotation. In order to properly use the sensor, it needs a filter on it to both save power and not be overly sensitive.

You can see the datasheet for the sensor here: SQ-SEN-200%20Omnidirectional%20Tilt%20and%20Vibration%20Sensor%20Datasheet.pdf - Google Drive

I've got a filter circuit that uses this device to hold the signal low until movement, at which point the TO_MCU line (see schematic) goes high. Unfortunately, to wake up, the ATMega's need a LOW level interrupt, not a high one like this circuit provides.

The schematic for the filter circuit is here:

So my ultimate question is: how would I go about modifying this circuit so that it works in the opposite direction (ie - regularly high on TO_MCU, but goes low on movement)? I'm pretty much a complete electronics noob, so I don't even know where to start.

Thanks for any help at all you can provide!

2

If you want to keep your filter as is you could invert the output by adding a small logic level FET to the filter output.

Filter output to transistor gate, source to 10k resistor and micro controller input pin, other end of resistor to 3V3 (if that is your logic supply voltage) and drain to Gnd.

3

Thanks, that's an excellent idea! Much simpler than trying to modify the rest of the circuit.

Cheers!