If you want to make your artwork look like it came off of an old dot-matrix printer, there are two ways to create the effect. One way is to create a dot pattern using a Halftone Filter on a new canvas and then apply it to your artwork with a Paint Brush or Paint Bucket pattern. A second way is to apply a combination of filters directly to your artwork. Sometimes, a single Patchwork filter with the right settings will give you the effect you want. But most of the time you will want to apply additional filters to make the the pattern look more authentic.
Making a Quick Dot-Matrix Pattern
A quick way to create a dot-matrix brush or filter pattern is to apply a Halftone Pattern to a solid-black layer. To find the Halftone Pattern, open the "Filter Gallery" under the "Filter" menu and then click the "Sketch" folder. Set the Size at "2" and the Contrast at "4" and click "OK." This creates white dots on a black background, so press "Ctrl-I" to invert the colors, which gives you black dots on a white background. Select the pattern by pressing "Ctrl-A." You can then create a brush pattern for the Brush Tool by selecting "Define Brush Preset" from the "Edit" menu. To create a pattern you can use with the Paint Bucket Tool, select "Define Pattern" from the Edit menu.
Making a Denser Pattern
In itself, the dot pattern created using the Halftone Filter is quite sparse. To double the amount of dots, just double the pattern. After creating the dot-matrix pattern with the Halftone Filter, drag the Background layer on to the "New Layer" icon in the "Layers" panel to create a duplicate layer. Click the white field of the top layer with the Magic Wand Tool and press "Delete" to make the field transparent. Press "Ctrl-D" to deselect the field, click the Move tool and then press the left or right arrow key to move the current layer's dots so they are centered inside the lower level's dots. Press "Ctrl-E" to merge the layers and then use this denser field of dots as a dot-matrix pattern.
Applying a Dot-Matrix Filter
A patchwork filter often works for a dot-matrix effect on black-and-white images. You can apply this filter directly to your image, so merge the layers if necessary after making a copy of your artwork. Open the "Filter Gallery" from the "Filter" menu and select the "Patchwork" filter in the "Texture" folder. If your layer includes text, you're prompted to rasterize it first. Set the Square Size and Relief setting both to "0" and click "OK." If this gives you more of a checkerboard effect than a dot-matrix effect, a Half-Tone Filter and Crosshatch Filter will fix it up.
Improving the Dot-Matrix Effect
On some images, the Patchwork filter creates more of a checkerboard than a dot-matrix pattern. After using the Patchwork filter, open the Filter Gallery again and select "Halftone-Pattern" from the Sketch folder. Set the Size at "1" and the Contrast at "0." Click "OK" and then open the Filter Gallery one more time, selecting the "Crosshatch" filter located inside the "Brush Strokes" folder. Begin with a Stroke Length of "3," a Sharpness of "10" and a Strength of "1." This should work well for most images, but you can tweak the Crosshatch settings as desired while the Preview option is on.