ARC WELDER SIMULATION
Detailed workshops and maintenance yards can be the highlight of many model railways or dioramas. Unfortunately they usually suffer from one common problem. They are static. At the scale involved, it is not an easy problem to overcome either. Moving parts at this size present quite a challenge.
However making things move is not the only way to animate a scene. One classic sight in any industrial workshop or construction site is a man hunched over something he is welding, a fairly slow process that doesn’t require much movement. None the less, anyone glancing in that direction is sure something is going on because of the flashes of light caused by the process.
Simulating such flickering is not difficult in electronics, and with the advances in LED technology, can be quite convincing
The arc welder simulator is made up from several functional blocks: five oscillators, two gates and the output driver.
IC1A, IC1B and IC1C are all wired as square wave oscillators, each operating at a different frequency under 100 Hz. The three outputs of these oscillators are gated together by a NOR gate consisting of three 1N4148 diodes, a 100k resistor and IC1D. Only when the output of all three oscillators are LOW, is a HIGH present at the output from IC1D. This output is a pulse of semi-random duration and occurring at semi-random intervals. This is used to generate the occasional bright flashes associated with arc welding.
IC1E is also wired as an oscillator operating at a similar frequency to the previous three, though unlike them, its pulse length is adjustable from nearly 0 percent to 50 percent. This is used to generate the consistent flicker. The adjustment is provided to give some control over brightness, which is particularly important when using a lamp as the output. If a fixed mark space ratio was used, some lamp filaments would not achieve enough heat to glow during the short pulse.
The outputs of the NOR gate (IC1D) and the output of the flicker oscillator (IC1E) are gated together by an OR gate consisting of two diodes. This combines the flicker and flash, feeding them to the base of the Darlington driver transistor via a 1k5 resistor.
An onboard LED is provided to allow monitoring of the output.
The circuit as described above would result in a never-ending welding effect, and that would be as bad as not having any animation at all, so the remaining Schmitt inverter IC1F was added to the circuit to switch the effect on and off periodically. IC1F is wired as a square wave oscillator with a cycle of several seconds. Its output is fed to the same OR gate as the flash oscillators, and also to the flicker oscillator via 1N4148 diodes. When the output of IC1F is HIGH, the output of IC1D and IC1E are both forced LOW, preventing any output, thus darkening any LED or lamp connected to the output. When the output of IC1F is LOW the output of both the flicker and flash sections are enabled, giving the welding effect.
While this cycle is predictable because of its square wave nature, the period is long enough that is isn’t that noticeable.
One other note about this oscillator: unlike the other oscillators in this circuit, the timing capacitor is between the input of the Schmitt inverter and the positive rail. This has been done so that when the unit is first powered, the discharged 4u7 capacitor will hold the output of IC1F LOW, allowing the effect to start immediately.
As the arc welder simulator is designed to be used with the uncontrolled DC output of model railway transformers, a 1N4001 diode has been used to provide polarity protection, and a simple zener/transistor regulator has been included to limit the voltage to the chip to around 12 volts. Without this, the voltage of the model railway transformers could push the power to the chip to over 15 volts, destroying it. The uncontrolled DC output is usually rectified, but unsmoothed, and may be as high as 15 to 18 volts, despite being labelled as 12v on the transformer.