Power PIR Alarm (Starlux)

Recently I designed a 12V (car battery) alarm system around a popular little PIR alarm, which is available worldwide under various names. Its factory name is the Starlux ST96 Sensor Alarm. I am impressed with this little alarm. But it is not designed as an “industrial” alarm. It has modest volume, and just a few seconds' alarm on triggering. However, with the addition of just a few components (shown), one may switch large loads (a lot of power), and adjust the duration, too. The IC on the left is what one sees on opening up the Starlux ST96 (one takes a single screw out of the battery compartment). Solder a control wire to pin 2, which is taken to the circuit as shown. Also connect pins 1 and 8 (a 5V supply) to the circuit as shown. The type of MOSFET TR1 is not important, except that it is n-channel, and the maximum gate threshold voltage should be 5V or lower (about 4V would be ideal). Also, check the MOSFET's maximum power dissipation in particular. This would typically be tens of Watts at 12V (quite a lot). Notice that the load (whatever you want to switch) runs off a 12V car battery, yet the Starlux ST96 runs off 9V. An LM7809 regulator circuit (pictured on the right, running off the car battery) would fit the bill for the 9V, or a micropower 9V regulator if desired. Note where there are connections in the circuit and where not (the black dots are connections). The circuit as shown gives just over a minute's power on triggering, but the values of C1 and R1 may be adjusted, say 470µF for C1 for about five minutes' power on triggering (in many countries, five minutes is the legal limit). This circuit may easily be fitted inside the case of the PIR alarm, but mount it outside, and the alarm will not stop if someone rips out the PIR. One does not need to wire up IC1b to IC1f, however this can save power where power consumption is critical. One can parallel multiple circuits using a common 9V supply and a common 12V supply, and by commoning the MOSFETs' s (source) and d (drain) leads, but the g leads (the gates) are separate for each circuit. OBSERVATION: For the load, one might power a 12V "sound bomb" or 12V LED floodlight, or transmit a signal, even trigger an (off the shelf) smoke bomb or explosion, but beware of fire hazard. Ideally, test the Starlux ST96 before you modify it. That way you will first get a feel for how it behaves, indoors or out. You may click on the image to enlarge. My design is © Copyright.