Patent GB-1054975-A (GB)
IPC: G08B29/14
Abstract: 1,054,975. Transistor circuits. ROLLS ROYCE Ltd. April 13, 1964, No. 15145/64. Heading H3T. [Also in Division G4] A control circuit for checking the state of a fire detector cable of the type which becomes capacitivie at high temperatures, and acts as a battery when contaminated with a weak electrolyte comprises an oscillator which supplies pulses to the detector, an analysing circuit which actuates a fire warning alarm when a discharge from the detector, greater than the steady state discharges produced due to contamination, is produced during the space between oscillator pulses, and an arrestor circuit which cancels the effect of the first discharge only, so that the fire warning is cancelled if the discharges are due to contamination. The oscillator comprises a Schmitt circuit TR 1, TR2 and a further transistor TR3. If TR2 is conductive, TR3 becomes forward biased and Cl charges until TR1 conducts switching-off TR2 and TR3. C1 then discharges until TR1 switches-off, switching on TR2 and TR3 again to restart the cycle. The mark/space ratio of the oscillation is determined by the impedance between terminals 8, 9 which affects. the drive on transistor TR3, and hence the charging rate of capacitor C1. Fire alarm.-Pulses from the oscillator comprising transistors TR1, TR2, TR3 and fed from A via resistor R1, diode D1, switch S1, resistor R2, diode D4 to fire detector cable connected across terminals 8, 9. Under normal conditions the monostable circuit comprising transistors TR4, TR5 remains in the state with transistor TR4 cut-off and relay X unenergized. When a fire occurs the detector cable becomes effectively a capacitor discharging between pulses from the oscillator to switch transistor TR4 on energizing relay X which closes contact S2 energizing lamp 12. Relay Z also operates closing contact S4 to release relay X and extinguishing lamp 12. Capacitor C4 then charges, releasing relay Z. Application of the next pulse from the detector cable reenergizes relay X and hence lamp 12, but since capacitor C4 is now charged relay Z does not operate, and lamp 12 remains lit. High level contamination.-The cable connected to terminals 8, 9 becomes a short-circuit the potential applied to the base of transistor TR3 stops the oscillation with transistor TR2 conducting, capacitor C3 then charges allowing relay Y to operate, closing contacts S1 to light lamp 13. Weak contamination.-The cable becomes effectively a battery and discharges in a similar manner to the fire condition except that pulses after the first are of much lower level. The first output pulse causes relays X, Z to operate as for the fire condition, subsequent pulses are however too small to operate relay X immediately and the lamp 12 cannot be relit until capacitor C2 is charged by a number of small pulses. The lowering of the cable impedance due to contamination reduces the drive on transistor TR3 changing the mark/space ratio of the oscillation so that C3 cannot discharge between pulses. The charge thus builds up until relay Y operates to energize lamp 13 as before. Closure of contact S1 to energize lamp 13 opens the oscillator circuit so that no more pulses are fed to capacitor C2 and lamp 12 cannot be lit.