GB1208997A - Improvements in or relating to transmitters for use in alarm systems - Google Patents

Abstract

1,208,997. Alarm systems. HONEYWELL Inc. 28 March, 1969 [3 April, 1968], No. 16320/69. Heading G4H. In an alarm system of the kind in which a number of transmitters are connected into a two-wire loop constituting a common transmission path to a receiver, the loop having a supervisory current passed through it by the receiver, the transmitter comprises means for sensing the occurrence of an alarm condition and responsive to this condition to switch on a motor to drive code contacts connected into the loop to produce coded pulses in the loop there being a PNI (positive non-interfering) circuit in each transmitter for sensing coded pulses in the loop to inhibit operation of its motor, so that a transmitter will not produce voltage pulses if another transmitter terminal is doing so. A.C. power applied at the terminals 10, 11 is rectified at 14 to energize an oscillator including unijunction transistor Q7 which oscillates at 120 cycles per second. Flip-flop 33 divides this frequency by two and alternately switches transistors Q6 and Q10. A transistor Q12 switches on when Q6 is on and transistor Q11 goes on with Q10. Transistors Qll, Q12 convert the D.C. on line 20 to A.C. for energizing the A.C. motor 35 through series switch and relay contacts. The output from Q6 charges capacitor C3 in integrator 82 rendering transistor Q3 conductive and allowing a pulse to pass through relay winding 1K to operate the relay. This relay closes contact 1K6 to allow the sensing current to pass from terminal 53 through external loop 54, resistor 56, contacts 1K6, external loop 60, closed contacts 29a of reset switch SW2 and resistor 63. The voltage drop across resistor 63 is sufficient to turn transistor Q1 partially on but the current is not sufficient to pick alarm relay 2K, but turns on transistor Q4. Relay 1K holds over contacts 1K3 and transistors Q4 and Q3. If the power supply fails the relay 1K resets in the same way. Contacts of relay 1K set the transmitter in its normal operating condition. If an alarm condition occurs, causing contacts 64 to close in the external circuit the resistor 56 is shorted and the supervisory current increases, increasing the drop across resistor 63 and rendering transistor Q1 more conductive so that alarm relay 2K picks and holds with contacts 2K6. Contacts 2K3 open and relay 1K drops out. Contacts 2K1 close and the motor starts, driving the code wheel 38 to transmit the coded train of pulses. Cams, also driven by the motor, rotate once while the code wheel rotates six times and operate switches M3, M4, M5 and M6. Cam switch M3 allows four rounds of the coded pulse train to pass to the receiving panel 117, Fig. 2. Contacts 2K5 close to ring an alarm bell at the transmitter. Lamp 75 lights as relay 1K drops out. The positive non-interfering circuit PNI prevents more than one transmitter being allowed to transmit at a time. As the code wheel contacts M1 open the supervisory current from the receiving panel closes and the supervisory relay responds to reverse the polarity contacts 118. Line 114 then becomes positive and line 113 negative. The reverse flow of current causes the alarm relay to operate. In a transmitter which is not transmitting the reversal of polarity on line 114 is detected by transistors Q15 and Q14 and a signal applied to switch on transistor Q13 and actuate relay 3K which latches over contact 3K2. Contacts 3K5 light lamp 75 and contacts 3K6 open to disconnect the motor 35 so that another transmitter cannot transmit after the transmission; the PNI circuit is reset by a timer generating pulses every 26 seconds. The PNI circuit belonging to the transmitter actually transmitting is prevented from operating to lock the transmitter out by contact M1b of the transmitting switch M1. This applies a short circuit to the SCR Q18 which becomes non-conductive and prevents operation of the lock-out relay 3K. If the A.C. power fails the stand-by battery 10 is automatically connected. If the supervisory current fails transistor Q1 becomes non-conductive, followed by transistor Q4 and relay 1K drops out. The motor runs and transmits one round of its coded pulse train. Specification 1,188,090 is referred to.