
2
MMV1 (Monostable Multivibrator) of IC3. The reset time of MMV1 is controlled by the combination of
C15, R22 and P1.
The multivibrator FF2 is also a D F/F and it is the «24 hour protection» circuit of the alarm system,
as it can provide instant triggering by means of a conveniently placed push-button (S2). (This is a
function similar to the «panic button» found in shops and bank cashiers, and will cause an immediate
alarm which will be completely independent of the state of operation of the rest of the circuit).
FF1 and FF2 trigger the transistor TR1 through the diodes D16 and D15 which are there to isolate
them from the transistor, and the transistor is then used to operate the relay which controls the siren.
The monostable MMV2 in IC3 controls the exit time of the system and is triggered at the moment the
circuit is turned on (GO function) and for as long as it is latched keeps all the other multivibrators of the
circuit locked in the reset state thus permitting the user to leave the protected areas without triggering
the alarm. This time is controlled by C14, R21 and it is adjusted by P2. For the duration of the
operation of MMV2 the yellow LED D13 is turned ON to show that the exit time is counting.
The monostable MMV4 in IC4 is the circuit that controls the repetition time of the alarm, it sets the
pause duration between two consecutive alarms. MMV4 takes part in the operation of the alarm if the
switch S4 is set to close the contacts between the pins 12 and 13 enabling the use of the alarm time
function of MMV1 and if the switch S3 is closed, shorting pins 3 and 4 together. When MMV1 is reset
and the siren is silenced, MMV4 is triggered through D19 and produces the pause time. When MMV4
is reset MMV1 is again set through S3, C9 and TR4. These states of MMV1 are in turn used to control,
through S4, FF1 which in turn controls the siren. For the duration of MMV4's operation the LED D14
glows indicating the pause time.
The reset period of MMV4 is controlled by C10 and R17, and it is adjusted by P4. Please note that
when S3 is open, the alarm will sound once for the duration of the alarm time and it will then stop till the
next excitation of the circuit by one of the sensors. If the switch S4 shorts the points 13 and 11, the
siren will sound continuously until the circuit is manually reset from the lock-switch S1.
As it is shown in diagrams 1 & 2, there are 4 sensor inputs which will trigger directly (no delay) the
multivibrator MMV1. These sensors are connected to the input of MMV1 through the isolators N1, 2, 3
& 4 and the diodes D1, 2, 3, 4 respectively, and they trigger MMV1 by means of TR4. Triggering one of
those inputs will cause an immediate alarm. There are two more input circuits which through N5 and
N6 control MMV3 and produce delayed triggering of the circuit. These sensors are used in the area
which must be protected but it must also be used as an entry and exit route for the user. The delay
provided allows the user to enter and leave the protected premises without triggering the alarm. The
delay time is controlled C7 and R13 and can be adjusted by P3. When MMV3 is reset MMV1 is set and
consecutively the siren sounds if the switch S1 is not in the meanwhile turned to the standby position.
The key-switch S1 can set one of the following three modes:
a. Stand-by.
b. Activation (GO).
c. RESET
When the switch S1 is open the circuit is activated and the green LED D18 glows indicating the
operation of the circuit.
When S1 is closed, the circuit reverts to STAND-BY operation where it is only possible to trigger the
alarm by means of the «Panic Button» which as we have already mentioned allows instant triggering
on a 24 hour continuous basis. If during the operation of the circuit there is an alarm, turning the switch
S1 to the STAND-BY position and then to the GO position the alarm circuit will be RESET and the siren
will stop. If the alarm system is not needed, the switch S1 must be returned again to the STAND-BY
position.
If you observe the circuit diagram carefully, you can see that there are two types of sensors which
are drawn as «Normally Open» (N.O.) or «Normally Closed» (N.C.) switches. The name indicates the
state of the sensor if it is represented as a switch while the circuit is in operation but it is not triggered.
(Toggling the state of the particular contact activates the alarm).
If one of the inputs becomes triggered during the operation of the alarm will cause its respective
LED indicator to glow thus giving an indication of the state of all the input circuits at any moment.
Diagrams 1 and 2 show clearly the relationship between the input circuits and the indicator LED's D7,
8, 9, 10, 11 & 12.
In the diagram 2 it is also shown the use of the switches S1, 2, 3 & 4. Diagram 3 gives examples of
use of the alarm system with various peripheral devices, like an electronic combination lock, remote
control, and various N.C. and N.O. sensors.
Please note that if an input of the N.C. type is not used, its input pins must be always shorted.
The circuit also incorporates a power supply stage which only requires a 220 V/10 V mains
transformer to provide the necessary 10 VAC/50-60 Hz which are necessary for the operation of the
circuit. The power rating of the transformer should be considered in relation with the capacity of the
backup battery and the power rating of the siren. For example a battery having a capacity of 1.5 Ahr