Hello! I do not know if I am the first person to create sensors in mcpe with command blocks, but this is my take on them.

To install the command creation copy exactly these 2 commands:
/execute @a ~ ~ ~ /execute @e[type=shulker,name=Sensor,r=5] ~ ~ ~ /setblock ~ ~ ~ lever 13

/execute @a ~ ~ ~ /execute @e[type=shulker,name=Sensor,rm=5,r=10] ~ ~ ~ /setblock ~ ~ ~ air

The first one goes in a repeating command block set to always active and the second in a chain set to always active aswell.

To create a Sensor, summon a Shulker and name it "Sensor". Also you cannot place a block on top of each shulker because shulker teleport when they have a solid block on top of them.

Sensor emit a redstone signal on all 4 sides when a player gets in 5 blocks of proximity