Download Simple Alarms

7
Simple Alarms
1. Alarms using an electro-magnetic buzzer
To use the circuit shown below (left), the diode should be permanently
illuminated by a beam of light. When the light beam is cut, by an intruder,
the conductivity of the diode decreases and the alarm sounds. The alarm
will continue to sound, even when the light returns, until the reset switch is
pressed. The value of resistor R is found by experiment but will probably
be between 100k and 560k (start with 180k.)
If you want an alarm which sounds when the diode is exposed to light (for
example, in order to protect a bag, drawer, suitcase etc) you will need to
use the modifications to the circuit shown in the diagram on the right.
+
R
buzzer
330Ω
1
R
0·47µ
330Ω
3
2
22k
2
3
22k
1
4M7
-
Dark alarm
Light alarm
The NOR gate is one quarter of a 4001 chip. Transistor: BC109C or
similar.
Dark Alarm
Light Alarm
1
2. Alarms using oscillator driving high impedance earphone
In the following circuits, the chip used is a 4093 quad NAND Schmitt
trigger. These circuits will sound the alarm when the light goes off but it
can easily be changed to give an alarm when the diode is illuminated
simply by changing the places of the diode and resistor, R.
+
5
4
Si
2
3
6
R
output
1
0·47µ
C
R'
-
The components R’ and C determine the frequency of the oscillator and
therefore the “pitch” of the sound produced by the alarm; try 560k and
1·5n.
If the other two Schmitt triggers in the 4093 are used as shown below, you
can have an alarm which gives a series of “beeps” rather than a
continuous tone.
© David Hoult 2001
+
Si
5
6
R
12
2
11
8
10
3
4
13
1
0·47µ
output
9
10M
R'
C
100n
-
2