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 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.