Two-wire fire sensing and receiving system

Claim 1

A two-wire fire sensing and receiving system comprising; a plurality of sensing devices comprising a capacitor, a fire sensor connected between the terminals of said capacitor and which is operated by discharging current of said capacitor to produce fire signals, and a terminal short-circuiting line including a switching means which is rendered conductive by the application of such fire signals; a receiving device comprising a d-c power supply, a first switching element connected in series, in a short-circuiting trunk line connected between the positive terminal and the negative terminal of the power supply, with an alarm control device; a two-wire circuit extending from the positive and negative terminals of said short-circuiting trunk line, said switching means comprising a second switching element the positive and negative terminals of the terminal short-circuiting line of the second switching element of each of the sensing devices being connected thereto, such that said sensing devices and said receiving device are coupled together; a transistor connected between the positive terminal of said capacitor and a positive terminal of said terminal short-circuiting line, a Zener diode connected between the control electrode terminal of the transistor and a negative terminal of said capacitor, and a detector resistance included in either the circuit between the output terminal of said transistor and the positive terminal of said capacitor or the circuit between the negative terminal of said Zener diode and the negative terminal of said terminal short-circuiting line, wherein a voltage differential is created by an electric current flowing through said detector resistance; and by-pass means of which one terminal is connected to the control electrode terminal of said transistor and of which the other terminal is connected to a low-potential terminal of said current detector resistance, such that the potential to said control electrode is varied in dependence on a predetermined potential difference across the two terminals of said current detector resistance; wherein the electrical current fed from the d-c power supply is controlled to be not greater than a current which is necessary for driving said fire sensors and which is determined by said current detector resistance incorporated in said by-pass means.