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More  Model Train CircuitsLED Circuits; Flashing Lights; Models

HO Train Lighthouse Flasher Revisited
December 24, 2012
designed by David A. Johnson, P.E.

HO train sets often have authentic looking scale model homes and buildings.  A while back someone asked me to design a flashing LED light, which he could mount inside a model lighthouse and have it operate so it would appear to rotate and flash.  I designed a working circuit but I was never pleased with the results.  Iím still not 100% pleased but the revised circuit below seems to operate a bit better than the older circuit.
The circuit below can be mounted inside a model lighthouse and powered by 4 AA alkaline cells or a 5v to 6v supply.  The circuit drives a single LED lamp with a pulse width modulation (PWM) ramping signal.  The ramp simulates the rotating light from a lighthouse beacon.  The circuit uses two CMOS versions of the 555 timer and a common LM393 dual voltage comparator.  The triangle signals generated by the two timers converge at the inputs to the comparator.  The slow 0.14 Hz lower oscillator makes a complete cycle in about 7 or 8 seconds.  The second oscillator runs at about 10KHz. The PWM output of the LM393 is connected to a FET switch.  The PWM signal at the FET gate causes the average light intensity of the LED to swing from about 5% to 95%, with R4 limiting the current to about 4 milliamps.  At the peak of the PWM signal, a pulse from the lower 555 timer is routed to a second FET switch.  The signal applies a 50ms pulse to the LED through a second resistor R5.  The result is a LED light, which gradually grows brighter, then flashes even brighter before dimming down again. This produces a light, which somewhat simulates a rotating lighthouse light although I think a llogarithmic waveform would produce a better intensity ramp than a triangle waveform.   Perhaps that feature could be added later.
HO Train Model Lighthouse  HO Train Model Lighthouse

Click on Circuit Below to view PDF of Schematic

HO LightHouse Flasher Revisited designed

 by David Johnson, P.E., Dec 24, 2012

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