Here is an inexpensive circuit of a wireless cashbox security alarm realized using generally available low-cost components. Why it is inexpensive when compared to readily available wireless security alarms? The answer is simple. Here, a wireless door bell is hacked (and tweaked) to rig up the system. Needless to say, the prototype is wired around a cheap (chinese-made) wireless door bell.
First of all, note that there is nothing to do with the “Receiver”box of the wireless door bell. Just insert two AA (1.5V x2) cells in the battery compartment and keep it handy. Now carefully open the “transmitter” box (ie doorbell switch), and remove the PCB from the enclosure. Now lift the push switch from the PCB as it is not necessary for the project,and extend three wires from the suggested PCB points as shown in Fig 2. Next, follow the schematic diagram to complete the security alarm. The transmitter section uses a 12V (A23 type) battery as its power source. Fortunately, we can power the whole circuit from this single battery.
After finishing up the construction,house it in a small tamper-proof box leaving a little window for the LDR. Now fit the unit inside the cash box with the LDR pointing towards the door of the cash box. Working of the circuit is simple. If the cash box is closed, the interior will be dark and transitor T1 (BC547) is disabled by the high resistance of the LDR (threshold determined by P1). If someone tries to open the door of the cash box, light-most probably from the burglar’s pen torch falls on LDR fitted into the cash box. As a result, LDR conducts and T1 is forward biased. This switches the next transistor T2 (SL100B) and +12V supply is extended to the transmitter through T2. Finally, an aural indication (a chime with a fixed duration) is avaiable at the remotely located ( 10 to 100 metres) wireless receiver (doorbell) box.