Home > Circuits > Timed Entry/Exit Alarm
Modified:21:47, 22 October 2013
CIRCUIT #1 DESCRIPTION

The problem with some simple alarm circuits is that they don't have an exit or entry delay. So for example when you switch them on, they are armed immediately and give you no time to leave. Or there is no entry delay, meaning that unless you can get to the keyswitch without triggering the alarm, it will always sound an alarm for the time it takes to disarm it. Also unless the alarm has a timer for the alarm duration, it's going to be sounding indefinitely or at least until the battery goes flat.

This circuit overcomes these problems by providing 3 timings.

EXIT DELAY
When first switched on the alarm is disabled initially, meaning it can not be triggered giving time to exit.
This time is set by R1 and C1 which keep the resets to the 555's low until the capacitor reaches about 1/3 of the supply.
The values showngive approx. 5 seconds, but it is recommended to change R1 to at least 150K if using a PIR. this will ensure it has time to settle after power on, in this case 15 seconds.

ENTRY DELAY
When the alarm is triggered by the PIR, the monostable IC1 begins timing giving time to diable the alarm with the key. This causes both plates of C3 to go low until it charges. This pulse is very brief but enough to trigger the second 555 monostable.After the monostable has finished timing the output goes low at pin 3

it triggers the second monostable activating the alarm. This delay time is controlled by R2 and C2 and follows the formula
t = 1.1 C R, standard for a 555.
The values shown give around 5 seconds.

ALARM DURATION
This is the length of time the alarm sounds for until it resets. This is controlled by R3 and C3 and again follows the formula
t = 1.1 C R so with the values shown about 50 seconds.

It should be noted that electrolytic capacitors often are considerably higher than their value - up to 40%. I have known 100uF capcitor measure at 146uF before when it was brand new. Maybe old stock from the manufacturer??


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CIRCUIT #1 PCB

The PCB layout is shown below, remember it is to scale but probably not 1:1.

Downlaod the PCB details for more accurate print out

COMPONENT SIDECOPPER TRACK SIDE


CIRCUIT #2 CIRCUIT DESCRIPTION

This version is very similar to the first but has some improvements.
Firstly the timing periods are all variable. The times are those using the values shown in the diagram.

  • Exit time = C1 and VR1, 0 to 10 seconds
  • Entry time = C2 and VR2, 0.5 to 11 seconds
  • Alarm time = C3 and VR3, up to 4 minutes

These can of course be altered. However take care never to connect pin 7 of either 555 timer directly to +v, eg by turning a potentiometer fully to zero ohms. If using a a variable resistor, it is a good idea to put a resistor in series of at least

100ohms.

Secondly an LED has been added to show when the alarm has been triggered by an intruder.

The option to have an active low or active high trigger has been added to the PCB.

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CIRCUIT #2 PCB

COMPONENT LIST

R1,2,6,7
4
10K
R3
1
47K
R4,8
2
4.7K
R5
1
330
VR1,2
2
100K preset
VR3
1
470K preset
C1,2,5
3
100uF elec.
C3
1
470uF elec.
C4
1
470nF poly.
C6,7
2
10uF elec.
Q1,2
2
BC337
D1,2,6
3
1N4148
D3
1
1N4003
D4
1
1N4001
D5
1
5mm Red LED
PCB dimensions: 97mm x 41mm

Download PCB details (PCB Wizard)

NOTES:
For a positive trigger - connect T+, for negative trigger - connect T-.
+v and 0v are accessible to provide power to devices such as a PIR or to easily connect switches as sensors.
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