Normally I don't comment on these blogs, but I wanted to write anyway on the off chance you read it.
I think your invention has great potential, but it needs an added safety feature:
Put a thermometer in the device to gauge the temperature in the car. Then have a remote monitor that the dog's person can take with him that lets him see this temperature. If the temperature reaches or exceeds your predetermined "safe" temperature, then the monitor should beep. Ideally, the monitor would be small enough to fit on a keychain with the car keys.