Intruder Alarm

INTRUDER ALARM In today"s society, it makes good sense to provide some form of intruder alarm system in the home, if for no other reason than the family"s peace of mind. Effective, reliable and simple to control, the intruder alarm system described in this month"s article uses readily available low-cost components only. E. Chicken, MBE, BSc, MSc, CEng, FIEE Apart from its low current demand from a battery during non-alarm conditions, the alarm is also noteworthy for its sys- tem-test bleep on switching on and on leaving the house, its pulse drive of the external sounder to economize on battery power, and automatic time-out of the in- ternal and external sounder to minimize social disturbance. The block diagram given in Fig. 1 shows the various stages of the circuit, their interconnections and related signal routes. The way in which the stages inter- act in detail is explained below. Circuit description Power supply As shown in the circuit diagram of Fig. 2, the alarm is powered by...