RadioJay A1 Prototype

Altimeter and accelerometer tag prototype with Motus compatible radio transmission

The RadioJay A1 prototype served to test:

• STM32 microcontroller with FSK radio
• Solar cell performance
• Rechargeable LiMn coin cell battery performance
• Energy harvesting circuit
• RF circuitry
• Accelerometer

It was designed to have a circuit layout that approximates a tag but with the bottom components (battery and solar cell) folded flat to make everything more accessible for testing purposes. A row of pin headers provides easy access to signals and a (heavy) chip antenna avoids a long antenna wire.

Issues found

Insatisfactory battery performance

The battery chemistry, while having excellent power density, does not have the capability to power radio transmissions. This necessitates the use of a large capacitor to attempt to power the radio from the capacitor. However, this still places too high a current on the battery with the effect of reducing the usable battery capacity and leading to likely premature battery failure. The net result is that a different battery chemistry was chosen for the next tag version.

Start-up issues

While testing what happens if the battery is discharged below ~1.7V it was discovered that the solar charging has difficulties recharging the battery. The cause was tracked down to a misbehavior of a switching voltage regulator in the microcontroller that oscillates between about 1.65V and 1.68V and causes excessive power consumption. At the same time, the energy harvesting circuit is still in a bootstrap mode and cannot overcome the power consumption. A solution was found by increasing some capacitors, which may reduce low light energy harvesting performance but avoid a dead tag.