
Circuit Diagram of the Alarm

Item  Description  Qty 
R1, R2  6k8 0.25 watt resistors  2 
R3  220 ohm 0.25 watt resistor  1 
C1  0.15µF 15v capacitor  1 
C2  0.1µF 15v capacitor  1 
C3  100µF 15v capacitor  1 
D1  1N914 diode  1 
TR1  BDX35 transisitor  1 
IC1  555 timer IC  1 
LS  8 ohm 0.25w speaker  1 
  On/Off switch  1 
  9v battery and clips  1 

The circuit diagram of the alarm, together with a tabulated list of most of the components is
shown above. Notice that the table contains a more complete list  for example, the 9v battery and
clips  and gives more complete information about some of the components. For example, the circuit diagram
simply shows "R1 6k8" whereas its corresponding entry in the table shows "R1 6k8 0.25 watt
resistor." We'll be referring to the circuit diagram and the table of components on subsequent
pages so it may be worth printing this page.
Resistors
Resistors are simple electronic components which impede, or resist the flow of electrical current.
The degree to which they impede the flow is determined by their resistance value, measured in Ohms 
the greater the value, the greater the resistance and the less electrical current that flows. They're
available in a vast range of values from less than 1 Ohm to many millions of Ohms. R1 and R2, in our
circuit, are both 6,800 Ohms or 6.8kOhms  more usually written as 6k8. R3, towards the right hand side
of the circuit, has a much lower resistance of just 220 Ohms.
Different wattage resistors (pencil for size comparison) 
In common with many components connected across an electrical supply, they dissipate energy  they
get hot. In addition to being available in a range of resistance
values, resistors are also available in a range of powerhandling capabilities. Resistors designed for use in high
power circuits will be physically large so they can dissipate large amounts of surplus energy. Those in low
power circuits, such as ours, will be physically very small.
Very often, circuit diagrams don't give the power rating of resistors so it's worth working it out very
roughly. Imagine, for our circuit, a 6,800 Ohm resistor connected directly across the 9 volt supply.
The current flowing through the resistor would be 9 ÷ 6800 = 0.00132 amps. The power dissipated by
the resistor would be 9 x 0.00132 = 0.012 watts. So, in our circuit 0.25 watt resistors will be more than
adequate.

Contrast that with the 220 Ohm resistor. The same calculation results in a figure of 0.37 watts so, isn't
our specified value for the 222 Ohm resistor, of 0.25 watts, too small? Well, in this particular circuit, the voltage across
the resistor is switching on and off (at the frequency of the alarm sound) and it turns out that the
average voltage across it is less than 2 volts and, if you do the calculation again at 2 volts, you'll see that a 0.25 watt resistor is more than adequate.
OK, enough theory, let's get the project under way!
Continue..
