It is true that a basic astable 555 timer cannot produce a 50 percent duty cycle. However, unless you can visually see the difference between 50 percent and 50.34 percent at 1 Hertz then the 555 circuits shown will do just fine as a grade crossing signal flasher.
Below are three calculated values of the duty cycle for the values given in the circuits below.
R1 = 10K / C1 = 1uF
Flashing rate: 2.0Hz / R2 = 355K / Duty Cycle = 50.69 Percent.
Flashing rate: 1.0Hz / R2 = 710K / Duty Cycle = 50.34 Percent.
Flashing rate: 0.7Hz / R2 = 1.0M / Duty Cycle = 50.25 Percent.
The following are Low Power - Grade Crossing Flasher circuits based on the LM555 and LM556 timer chips. They are designed to drive Light Emitting Diodes but could be used to drive low current incandesent lamps.
The first circuit is a basic version that can be used in many situations.
In the basic flasher the power to the oscillator and LED's are turned on and off by the control circuit.
The second circuit could be used where there are multiple crossings that are controlled separately. The circuit allows the lights at individual crossings to be powered from one oscillator and thereby have them flash in unison. This might be beneficial in places where the crossing lights can be seen at the same time as it would be less distracting if the lights flashed at the same rate and on the same side for each crossing. Or it might look interesting.
In the synchronous flasher control the oscillator is running at all times and the power to each set of flashers is turned on or off by the control circuit for that crossing. This allows any number of flasher sets at different crossings to controlled individually and yet flash in unison.
Two versions of the flasher output control are shown.
In OUTPUT 1 the lights are turned on when the control circuit goes to the common of the circuit.
In OUTPUT 2 the lights are turned on when the control circuit goes to the positive supply of the circuit or some other supply source.
In either case the PNP and NPN transistors above and below the LED's are turned on and the crossing lights will flash.
IF more than one set of flashers is used at a particular crossing the LED's could be wired in series so as to keep the number of output control circuits to a minimum. This is shown on the basic flasher schematic.
The circuits can operate from a 12 volt power supply and will provide a current of about 10 milliamps to the LED's. A lower or higher voltage supply could be used if the 1K resistors were adjusted to suit the voltage..
The "CROSSING CONTROL CIRCUIT" shown on the schematics is representative of the detection circuit that controls the flashers as the train proceeds through the crossing. The "control" function could be performed by a transistor, relay or a toggle switch.
The third circuit uses an LM556 dual timer in a push-pull output arrangement. The advantage of this circuit is that only two wires would be needed for a flasher, a advantage when working in small scales as the structure could be one of these wires.
The flashers can be turned on and off by using S1 to control the power to the circuit or by using S2 to RESET the timers. Resetting the timers will make both their outputs go to a LOW state.
This following circuit shows a simple grade crossing flasher with enough current capacity to drive 12 Volt lamps with 2 Amps current draw. The circuit could be used for open house displays, etc.
The flasher is driven by an LM556 dual time configured in a Push-Pull arrangement.
The flash rate can be adjusted by varying the value of the 500K ohm variable resistor. The flash rate is calculated as being from 0.54 to 3.1 Hertz.
The only real concern with this circuit is the need for a 12 volt power supply large enough for the current required by the lamps. The #1156 automotive lamps specified are rated at 2.1 amps but as only one light is on at a time the current draw for the circuit would be just slightly above 2.1 amps. Other lamps such as the #1141 draw less current.
The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.
If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.
Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.
27 March, 2017