1 set of remote control system includes receiver and transmitter
Transmitter:
1 Emergency Stop Swt.
1 ON/OFF key 
1 START key
6 Single keys
You could buy one transmitters and another standby at the same time.(that will be more favorable)

Model number: HS-6S
Power Supply:2×AA batteries
Operating Range:≤100m
LED:3
Frequency:422.4~438MHz
Response Time:0.0001s
Housing Material:PA66
Protection Degree:IP65
Operating and Storage Temperature:-30℃~+80℃
Power source of receiver: AC36V, AC110V, AC120V, AC220V, AC380V, AC/DC12V, AC/DC24V.
We have various dimensions and voltages for receiver that you can choose from.:)

Characteristics:
Tiny size & light weight,
Anti-jamming,
Anti- interfering,
Firm and durable,
Long natural life,
Improving workshop safety,
Long operation distance up to 100m,
300 installations in single area.

Application area:
Crane,
Conveyer,
CNC,
Coal Digging Machinery,
Generator Startup,
Concrete Pump,
Grab Bucket,
Other Machinery.