Electronic Design

Anti-Theft Alarm Lookalike Protects Parked Motorcycles

Sometimes, a simulated anti-theft device can deter a would-be thief just as well as an actual alarm system. To that end, the circuit in the figure automatically flashes two super-bright LEDs, mounted in the rear-light cluster of a motorcycle, to simulate the warning commonly used to indicate an active alarm. A mercury switch activates the circuit when you lean the motorcycle to rest on its kickstand.

Forming the heart of the circuit is an automotive linear regulator with an internal reset and watchdog circuit (MAX5023MASA). The watchdog flashes the LEDs by asserting the Reset pin, which drives the gate of a p-channel MOSFET. Any p-channel device with a logic-level gate threshold and a 50-mA continuous-current rating is suitable. The example circuit used an MTD20P03HDL from ON Semiconductor.

The red LEDs (Lumiled HPWT-DD00) are designed for automotive applications. They require about 40 mA for flashing, but the circuit drives the LEDs with an on time of 200 ms and a period of about 1.6 seconds. This duty cycle of about 1/8 minimizes the battery drain.

Clearly, the circuit should not be powered when the motorcycle is upright and should be turned on—by the mercury switch—when the bike is resting on its kickstand. Finding the appropriate mounting plane and adjusting the angle of the switch with respect to the motorcycle is occasionally a challenge. The electrical performance of the mercury switch isn't critical, but a switch designed for automotive use is more robust and easier to adjust.

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