GB1315681A - Burglar alarms - Google Patents

Abstract

1315681 Substation equipment ROBOPHONE Ltd 19 April 1971 [10 April 1970] 17218/70 Headings H3T and H4K A burglar alarm comprises means to detect an intruder means to dial out over a telephone line in response to the detection and a microphone arranged to be actively connected to the telephone line via an amplifier. As described operation of the device can be triggered by the reception of sound of predetermined amplitude by the microphone 20 or by the displacement of a window or door mounted magnet causing closure of a reed contact "REED". Sounds picked up by the microphone 20 are amplified at A and via conducting transistor Q1, capacitor C2 and rectifier circuit D1, D2 charges capacitor C3. If the sounds are of sufficient level as set by variable resistor VR1 transistor Q2 conducts triggering SCR1 to its conductive state. The anode potential of SCR1 drops to a level at which it cuts off transistor Q1 and Zener diode DZ2. Q1 isolates the input while DZ2 ceases to charge a capacitor C4. Transistor Q8 turns off and transistor Q9 turns on thus enabling a relay D the contact D1 of which loops the line. Cutting off of DZ2 turns off conducting transistor Q4 causing capacitor C5 to charge via resistor R4. After about 3 seconds to allow for the return of dial tone the level of charge causes Zener diode DZ3 to conduct energizing a relay x via a transistor Q5. Contact x1 of this relay energizes a tape drive motor 22 and contact x2 shorts the line transformer X2. Dial pulses in the form of tone bursts are picked up from the magnetic tape by a read head TH. The tone bursts are amplified by transistor Q6 and Q7, rectified at D3, D4 and charge a capacitor C4 which discharges between each burst via a resistor VR2 controlling the dial pulse mark to space ratio. Each burst causes Q8 to conduct blocking Q9 causing relay D to release its contact D1 to transmit a pulse to the telephone line. At the end of the dial signals a foil strip on the tape bridges contacts S.F.C. causing relay X to release. Return of contact X3 to the position shown connects a previously charged capacitor C6 to SCR2, which is triggered turning off transistor Q10 and turning transistor Q11 on connecting the microphone to the telephone line. Triggering of SCR2 also turns Q12 off allowing a capacitor C7 to charge. After 1 minute 40 seconds to allow the called subscriber to respond to the call and listen in to the sounds picked up by the microphone the charge on C7 reaches a level at which Q13 and Q15 conduct cutting of Q3 and Q16 resetting the apparatus. Triggering of SCR2 also causes diode D8 to conduct preventing Q5 from reoperating relay x. A capacitor C8 is connected so as to charge when the apparatus is switched off and to prevent operation of the apparatus for three minutes after switch on to allow the user to lock up the protected premises.