For my university project I'm making a certain sensor which has to (one-way) communicate with a smartphone. The sensor needs to be as cheap/simple as possible (aiming at +- €0,50 in total) and only has a 1.5V, 1.5mA max power source.
Now I want to send out a wireless signal from this sensor to a smartphone. Preferably without an extra receiver device. It has to have range of more than 1 meter. It only has to send out a few signals, no real data has to be transmitted, only a signal which the phone has to pick up.
I've looked into solutions like Bluetooth Low Energy (BLE) but this is already too expensive and consumes too much power.
Do you have any experience with building a simple, cheap wireless signal transmission to a smartphone?