Questions on security systems

1 Three switches are wired in parallel with each other and in series with a siren. The siren does not sound when:

2 Why is is preferable to use normally-closed switches on windows and doors?

3 What makes a reed switch close?

4 In the perimeter alarm circuit, what is the function of D1?

5 When any one of the switches SWz1 is opened, the effect on Q1 is to:

6 The length of time for which the siren sounds can be increased by:

7 In the light-activated circuit above, the voltage at node 2 falls when:

8 Gates a and b of the 74HC00 form:

9 The parts that make up the slow oscillator are:

10 The 'S'-shaped symbols on the gates indicate that they:


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