# Cheap Low Power Signal Transmission To Smartphone

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?

• Certain sensor is a bit vague, can you do with a nfc tag. Which just reads out information. Or do you need a constant unidirectional stream of data? ( If you want to get really hackish and ugly, try the other embedded sensors most phones have. Use the embedded light sensor of the phone to detect light pulses and bit bang some data. Or build a very rudimentary Infrared receiver if your phone doesnt already still supports infrared. – Remco Vink Aug 22 '18 at 9:53
• You're limited to what the smartphone can support. As far as I know the only one-way radio-based wireless communication smartphones support is GPS and FM-radio. All other standards require 2-way communication, at least during setup (as BLE). But please correct me if I'm wrong and I forgot about a possible standard that does fit your needs. – Bimpelrekkie Aug 22 '18 at 9:54
• As you've already ruled out the go-to solution of bluetooth. How cheap does it need to be? Other options would be NFC (if very low distance) or having to get another device to convert from something to whatever you want on the mobile (probably Bluetooth or USB). Alternatives I can think of would be infra red, maybe some really complicated set up using the microphones or optical sensors on your phone. – Puffafish Aug 22 '18 at 9:54
• For the sensor cheap, simple and low power almost immediately points you in the direction of 433 MHz transmitters. Many cheap wireless temperature sensors use this standard. 433 MHz uses OOK modulation which can be done by any microcontroller. To connect 433 MHz to the smartphone you will need to make a converting device as I know no smartphone that can receive 433 MHz signals. – Bimpelrekkie Aug 22 '18 at 9:59
• Optical PCM with the camera as a sensor, basically morse code with an LED? Or go acoustic, a few tens of bits per second should be easily doable using a piezo disk at 10KHz or so, just need to make the data rate low enough to avoid room reflections screwing things up. The real trick is to use that 1.5mA to charge a honking great cap and to not transmit too often, if you only transmit for 10 ms out of every second you can have 100mA or so for the length of your transmission which is much more useful. – Dan Mills Aug 22 '18 at 10:03

One-way beaconing is part of the BLE specification, supported by both iOS and Android, and can be done for less than $2 (single quantity) in parts (likely well less than$1 in quantity), specifically an nRF24L01+ clone chip, and an MCU that pre-munges the data buffer to account for the differences between BLE formatting and the nRF24 approach on which it is in part based (Nordic has a key role in defining BLE). With a little web searching you can find Arduino demonstration code to do this, and re-implement it on a cheaper MCU.

Power consumption is primarily a function of transmission frequency - ie, the less frequently you transmit, the more opportunity you have to charge up a capacitor slowly, and then use that to cover the brief power spike of transmitting. You'll likely need a small degree of boost conversion as well, but well within reason.

Alternatively, if your range is short and the phones are Android type, you may be able to use NFC. In theory this can be done with no device side power, though it is easier and more flexible if you have a small power source.

See what connectivity your smartphone has:

1. Analog audio input (mic) - it is wired
2. BLE - the most obvious solution
3. WLAN - more power hungry than BLE, your sensor could be an access point and run a web server
4. USB host on Android - it is wired
5. 3G/LTE - you'd have to connect your sensor with a proprietary radio to a gateway box, connect that gateway box to the internet somehow and send data to an app on the smartphone
6. ANT - maybe slightly more power efficient than BLE, but your smartphone has to support it
7. Built-in microphone - you could record ultrasound from your sensor

Keep in mind that if you have 1.5mA current limit it may still be possible to use BLE with a sufficient interval between measurements - add capacitors or supercapacitors and don't transmit too often.

• Note that the phone has a microphone which can be used for non-wired communication! Including in frequency bands above those audible to humans. – pjc50 Aug 22 '18 at 10:13
• @pjc50 indeed, a microphone is possible. That said, having implemented a dual ultrasonic/BLE system, the piezo-based audio portion consumed four times the power of the BLE portion. – Chris Stratton Aug 22 '18 at 13:31