Today I´ve nearly finished my 4 channel wireless firing unit. It uses a 433mhz
transmitter and receiver. The transmitter has to be paired to the receiver (Security function 1).
Moreover both units "talk" to each other with a rolling code system. The receiver
is of the "Rolling Code Superheterodyne Receiver" -type (Security function 2)
My system uses just one relay, which is not directly involved into the firing
circuit. It just diconnects and connects the MOSFETS from GND.
This leads to my I think quite unique security system (talking about a selfmade unit):
There is a double-timer integrated into the circuitry, which acts like follows.
1) MOSFETS are disconnected from GND all the time, that means an attached e-macth can under no circumstance be accidentally fired. Voltage across the MOSFETS is monitored by Led´s.
2) You press a special switch, which activates the security circuit
3) A timer starts and a pre-defined ammount of time will pass (30secs)
5) The circuit attaches the MOSFETS Source-pins to GND after that time
6) Only then the outputs are ARMED and can deliver the volts and amps
to fire the e-matches. You have to press Button A-D on your remote to do so, anyway 
7) Another timer-starts and stays active for 2...3minutes (matter of adjustment) After that time the MOSFETS are disconnected from GND: Circuit is armed again
The scenario is the following: You stand next to your "whatever" you want to test safely.
You can attach everything to your firing box without that gutt feeling of "Uuuuh, is there really
no voltage across that pins" you may have with certain homebrew-units.
When you have finished wiring everything you press that "arm" switch, which gives you 20 seconds (or more if you want more) to go away.
After 20seconds the circuit arms itself, giving you exactly 2 to 3 minutes to "do your duty"
After that time, the circuit automatically disconnects every voltage from the firing outputs.
It is now safe to go to your firing box, even if ther was a misfire or something. (E-match didn´t work etc etc)
Thats the unit:
Some things are still missing or incomplete. The caps and resistors are to be adjusted to get accurate timings,
Led´s have to be connected, some wiring work has to be done.
But it´s working flawlessly.
What do you think about that design?
