double pulse positive to single pulse neg

I have a single positive siren output that pulses single positive on arm and double positive on disarm. I want to use this output to drive my negative trigger door locks. The lock is simple enough. The unlock is my issue. I need something that will activate with a double pulse positive only to generate a single negative pulse. A point in the right direction is all i need. Thanks in advance.

Not exactly cheap ( $18 ) but the PAC TR-7 will do what your want.  Use Feature #14, the Green wire for the (+) input pulses from the Siren, the White wire for the (-) output pulse to the Unlock wire and program for two input pulses within 3 seconds.

Here is a link to the PAC TR-7 guide  :  https://www.the12volt.com/installbay/file.asp?ID=1031

Soldering is fun!