Hacks n Mods – CIA (Clock Intruder Alarm)


Can you remember the movie “Entrapment” wherein the lead actress (Katherine Zeta Jones) tried to defeat the security of the building to get a very valuable item? In the movie she had to practice very well only to cross an area guarded by invisible lasers that once the line is cut it would trigger the building’s alarm and that would alert the all the building’s guards. Hollywood spy movies have given those optical alarms (infrared laser) an impression of being a state of the art technology. Though in reality it is very simple, it is still one of the projects many hobbyist would love to build.

On my CIA project I’d like to show how simple (6 components, an alarm clock, and a laser pointer) it is to build one of these devices.

Schematic Diagram…

CIA (Clock Intruder Alarm) gtx


Circuit Description…

The circuit is basically a monostable multivibrator (active high) compost of resistors R1 and R2, capacitor C1, and the diode D1.

The phototransistor Q1 act as triggering device that at normal condition pulls the anode of D1 to ground when there is light (coming from a laser pointer source). This would divert all the current flowing across R1 to the ground. The value of R1 was carefully selected so it would dissipate power as low as possible since it is supplied by a singe 1.5V cell.

If the light entering the phototransistor Q1 is blocked, the phototransistor would open and this would allow the current to flow through D1 and therefore will charge the capacitor C1. The voltage across the capacitor will increase until it can produce enough potential to drive the transistor to turn on.

The transistor Q2 inverts the signal from the monostable mulivibrator and because the Alert# pin is active low this would set the clock’s alarm when it is pulled to ground.

If the blockage is removed, the light would trigger the phototransistor again and would pull R1 current to ground. At this point the capacitor will stop charging and will now enter discharging phase. Energy stored on the capacitor will slowly discharge through R2 and the transistor Q2. Voltage at the capacitor will slowly decay and there would be a point where it can no longer produce enough current to drive Q2. Q2 will then turn off and this would also turn off the alarm.

I measured the time the alarm will stop after it was triggered and I measured a considerable period of 1.5 minutes. This period can be easily increased by increasing the resistance value of R2.

C2 capacitance may also be increased to increase the alarm period but this would also slow the charging time down. If this happens, the sensor’s sensitivity may be affected. In order to correct this, the value of R1 can be lowered.



I first constructed my circuit on a breadboard to quickly test if my design concept will work on actual. Using breadboard also allowed me to easily trim the component’s values to achieve desirable response.

Breadboard gtx

After successful testing I finally decided to assemble it on top of the alarm clock’s board.

Assemble Board gtx

Care must be taken so it wont interfere with other components originally there. I also have to strip a few parts only to make the circuit fit inside.


Bill of Materials…

Circuit Code




WPT440F Phototransistor

Php 15.00


9014 General Purpose NPN Transistor

Php 2.50


27K, 1/4W

Php 0.25


680K, 1/4W

Php 0.25



Php 1.00


1uF, 10V

Php 1.00

Total Price:**

Php 20.00

* Prices are true as of 20091001 and may vary depending on vendor and location.

** Total Price does not include the price of the tools that will be used and other constructing materials like solders and wires.

The total price does not include the price of the alarm clock and the laser pointer because it varies greatly depending on the vendor and location. I bought the alarm clock at an Ukay Ukay store for only Php 37.00 bringing the total project cost to Php 57.00. Daiso sells these clock for only Php 66.00. Cheap laser pointers may be bought on stores along Quiapo for as low as Php 30.00. I bought mine at Shopwise Cubao for Php 90.00 (with LED flash light).



I spent almost a lifetime designing circuits for high voltage sources (minimum of 5V) and never thought designing a circuit for 1.5V source would give me great challenge and more learning. At these voltages the effect of diode drops and the parasitic resistances becomes very significant.

Results of my simulations were far from the actual response, nevertheless my circuit worked perfectly.

Final Project gtx

Right now I have plans to assemble my CIA version 2. Yes you heard it right, I already designed version 2 and I’m very excited to blog about it. It is already working on my simulations.

Improvements are to use infared light instead of ordinary red LED and it is also capable of accepting a pulsed light for greater efficiency and would prolong the battery life of the light source.


~ by glutnix_neo on October 5, 2009.

23 Responses to “Hacks n Mods – CIA (Clock Intruder Alarm)”

  1. Sir, what about a modulated light. IMO, if it is a visible light, intruders can easily fool the sensor by directing enough light on it so that the alarm wont trigger.

  2. ^^ I think visible light is fine.

    I had a design before using a direct light and I saw in the “mythbusters” of Discovery Channel that the sensor can be fooled by directing another light from a laser pointer. With this, I recalled one system I had deployed from a client and created a minor change by sending a pulsed light (receiver and transmitted are in sync).

    nevertheless, glutnix_neo’s project is a low-cost version of intruder alarm which is nice. bravo. ganda points for you! oopss 😀

  3. ^^ mga impakta!!!,hehehe

    I wish I have the luxury of time to improve my project but right now everyday are Mondays, (-:

    If you wish to have a circuit that accept pulsed light you can add a capacitor across R1. Charging and discharging of that capacitor would keep voltage at D1’s anode near ground level.

    Preventing a continuous light from the circuit can be easily accomplished by adding a DC blocking capacitor in series with D1.

    It worked on simulations but I still need to try it on actual application. (-:

  4. hey sir cant you create a higher voltage of your experiment and plizz send me a diagram of it tnx 1.5 is a not a louder one
    that you can hear send me plizz the loudest one ok tnxxx

    • OK I’ll create another one for you. The one using the Alexan’s Buzzer.

      Same circuit but using different values

  5. sir how much is the bread board???

  6. sir how many volts to be applied??

  7. ok sir ill wait for it ok tnnxx

  8. sir do you want to send you a diagram at about a 9v laser alarm trigger if you want i can send you a diagram that its not a fake ill wait for it tnx…..

  9. sir sorry i cant send a diagram cause i dont know how to make a diagram via pc

  10. sir..the liGht is visible.Isnt it?.is there any way to make it becoMe invisible to the eye of intrudER?

    • Yes it is, only from the source(laser pointer) and the light hitting the sensor. It is also visible if it hits an obstacle but if it does, it will be too late it will trigger the alarm.

      You can use IR LED instead of red LED to make it invisible, it should work on the circuit but you may have trouble directing the light to the sensor because you cannot see it.

  11. tol san coconect ung circuit sa alarm clock? san ba ung alert# pin? reply nmn asap,need ko lng ngaun,salamat! 😀

    • There three large pads on the alarm clock, the +supply, -supply, and the one with metal contact touching it, this is the alert# pin.

  12. light intensity sensing maybe added through simple window comparator, more or less intensity of the light will trigger the siren.

  13. thanks sir! baguhan pa lang ako sa electronics tamang tama to susubukan ko to gawin.

  14. hi sir

    i made a similar project minus the alarm clock
    my problem is the laser pointer,pano ba eto para always on lagyan ng tape sa on button? i need a laser ppinter na psupply powered not battery powered kasimadaling mag drain ang batt pag alwys on ang LED

    ano ba flow na sayo?


    • yes, you have to place a hard object(PCB cutout) before wrapping the switch/button with electrical or masking tape.

      You can use a cellphone charger to supply the laser pointer, simply series a 470Ohms resistor when you connect on the battery terminal.

  15. problem ko sir ang pag dis assemble ng LED pointer , pag natattamaan ng init ng solder ang LED pointer asssembly nag iiba ang laseroutput nya lumalakai eh

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