Security alarm system

Claim 1

A discriminator for actuating an alarm indicator in response to pulses having peak and base amplitudes deviating from predetermined levels, said discriminator comprising: an input terminal, a first transistor having a base connected to said terminal to be responsive to said pulses, means to bias said transistor to switch at an upper limit; a second transistor having a base connected to said terminal to be responsive to said pulses, means to bias said transistor to switch at a lower limit; a diode connected between the emitter electrodes of said transistors; said limits defining an acceptance band having a width correlated with the forward bias potential difference across said diode, and wherein the failure of said first transistor to successively switch in response to peaks of successive input pulses, and the switching of said second transistor in response to input pulse bases being each operative to actuate an alarm indicator, thereby providing an indication of the presence of a pulse without said acceptance band, and said alarm indicator being connected to the collectors of said transistors for providing an alarm indication when either said first of said transistors does not switch or said second of said transistors does switch, thereby indicating that the base, and only the base of said pulse does not fall between said upper and lower limits.