You just better hope the car coming toward you isn't an LEO. You could end up paying a hefty fine, especially if he learns you have the photocell disabled.
Headlight modulators are DOT legal in all 50 states.
USA - Department of Transportation:
FMVSS 108 (Federal Motor Vehicle Safety Standards) (49 CFR Part 571.108 S7.9.4) allows motorcycle headlight modulation systems in all 50 states provided they comply with the standards set forth in Section S7.9.4.
Title 49 USC 30103(b1) (US Codes) prohibits any state from forbidding a system that conforms to FMVSS 108. No State or Local ordinance shall usurp the authority and enforcement of the Federal Motor Vehicle Safety Standards
S7.9.4 Motorcycle headlamp modulation system.
S7.9.4.1 A headlamp on a motorcycle may be wired to modulate either the
upper beam or the lower beam from its maximum intensity to a lesser
intensity, provided that:
(a) The rate of modulation shall be 240 [plusmn]40 cycles per minute.
(b) The headlamp shall be operated at maximum power for 50 to 70
percent of each cycle.
(c) The lowest intensity at any test point shall be not less than 17
percent of the maximum intensity measured at the same point.
(d) The modulator switch shall be wired in the power lead of the beam
filament being modulated and not in the ground side of the circuit.
(e) Means shall be provided so that both the lower beam and upper beam
remain operable in the event of a modulator failure.
(f) The system shall include a sensor mounted with the axis of its
sensing element perpendicular to a horizontal plane. Headlamp modulation
shall cease whenever the level of light emitted by a tungsten filament
light operating at 3000[deg] Kelvin is either less than 270 lux (25 footcandles) of direct light for upward pointing sensors or less than 60 lux
(5.6 foot-candles) of reflected light for downward pointing sensors. The
light is measured by a silicon cell type light meter that is located at
the sensor and pointing in the same direction as the sensor. A Kodak Gray
Card (Kodak R-27) is placed at ground level to simulate the road surface
in testing downward pointing sensors.
(g) When tested in accordance with the test profile shown in Figure 9,
the voltage drop across the modulator when the lamp is on at all test
conditions for 12 volt systems and 6 volt systems shall not be greater
than .45 volt. The modulator shall meet all the provisions of the standard
after completion of the test profile shown in Figure 9.
(h) Means shall be provided so that both the lower and upper beam
function at design voltage when the headlamp control switch is in either
the lower or upper beam position when the modulator is off.
S7.9.4.2(a) Each motorcycle headlamp modulator not intended as
original equipment, or its container, shall be labeled with the maximum
wattage, and the minimum wattage appropriate for its use. Additionally,
each such modulator shall comply with S7.9.4.1 (a) through (g) when
connected to a headlamp of the maximum rated power and a headlamp of the
minimum rated power, and shall provide means so that the modulated beam
functions at design voltage when the modulator is off.
Rules Regarding Motorcycles and Motor Driven Cycles
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(b) Instructions, with a diagram, shall be provided for mounting the
light sensor including location on the motorcycle, distance above the road
surface, and orientation with respect to the light.