PIR Alarm System

I went looking for a 12V PIR alarm system yesterday (one which detects motion). As was to be expected, I found a 12V PIR unit (a common item), and a 12V piezo Sound Bomb (also a common item) -- but to bring the two together was expensive. So I designed a crude interface myself (see the diagram) for about $5 / R50. "Crude" means approximate timing, and no hysteresis -- but it works. The internal switch of a PIR is normally closed. Points A and B in this diagram are wired to this normally closed switch (S1). The PIR unit, together with this circuit, are wired to a 12V power supply -- this could be a 12V battery. The power MOSFET may be virtually any one -- say an IRF510 or BUZ10. VR1 limits current consumption of this circuit to about a milliamp on standby. A value of 1M for R1 switches the Sound Bomb on for two or three minutes. Adjust VR1 to see what it does, but not turning it fully clockwise or anticlockwise -- note that at some settings the Sound Bomb will always be on. OBSERVATION: In some situations, one needs two PIR units to monitor each other. In this case, wire up two of these circuits separately, with two PIR units, but wire the same Sound Bomb to both circuits. One could wire a concealed switch in series with the Sound Bomb, to switch the alarm on and off. This small system may not only be used for home security, but for vehicle security too. Note that a hooter may draw too much power to replace the Sound Bomb.