A Dozen Switches on a Panel for Two Quid

[karla]

This sim switch is a simple, easily constructed, single pole, single throw, momentary contact switch. It has been designed especially for PC simulation situations and should only be employed on low voltage and current circuits.

Twelve momentary switches were constructed to interface with a keyboard control card following Robert Prather's article FlightSim.Com - How To...Build A Flightsim Keyboard Interface. A panel of these switches were assembled as a set up for a generic GA aircraft with a bonus of the three Num/Caps/Scroll Lock LEDs as indicators. Use the following as a guide for the construction of switches from materials that you have available.


Description

The Lever moves on an Axle and the Spring loaded, rounded Piston rubs along the Base with a metal Contact strip in the centre. At rest the Lever is at an angle with the Piston in one of the corners of the Box. When the Lever is switched, from one side to the other, the Piston quickly rubs over the Contact in the centre of the Base. This creates a momentary electrical path between the upper Axle and lower Contact.

The image shows the switch without top, one side and one end. The Lever is in the vertical (momentary) position half way between open switch positions. The Lever is rendered transparent to reveal the internal Spring and Piston.


Materials

1. Lever - strong plastic or metal e.g. polystyrene, nylon or ABS rod, brass rod or metal tube with end cap.
2. Box/Base/Lugs - plastic or insulating material e.g. polystyrene, nylon or ABS sheet, acrylic sheet or thin plywood.
3. Piston - metal rod e.g. steel needle, iron nail, brass screw shank or brass rod; aluminium tends to stick.
4. Axle - metal rod e.g. brass rod, copper wire, iron nail or metal coat hanger.
5. Spring - high tensile wire e.g. cut length of spring or coiled steel guitar string.
6. Contact - metal sheet e.g. brass sheet, tinned steel food can or copper sheet; aluminium lager can will tend to stick.


Design

These are approximate requirements -

1. Decide on Lever upper length; personal preference.
2. Make lower length of Lever half upper length; good leverage.
3. Lever diameter determines size of Box inside edges; adds stability.
4. Make internal width of Box to accommodate total Lever throw of 55 degrees; good switch action.
5. Internal Height of switch Box greater than lower length of Lever + Piston diameter.
6. Piston length equals 0.7x lower length of Lever; gives smoother Piston movement.
7. Spring length equals half of lower length of Lever.
8. Contact strip width equal to Lever diameter; less tends to give less reliable switch operation.
9. Contact strip in shallow slot on Base; gives smooth action.
10. Contact strip on undersurface of Base; gives electrical path and to add strength.
11. Spring to be relatively soft on compression. Best to adjust number of turns and spacing to obtain best performance.
12. On dry assembly, aim for Spring to be under almost full compression when on Contact.


Dimensions

dimensions (in mm) are given as a guide only -

Lever diameter - 6
Lever external length - 24
Box width - 16
Box internal height - 15
Piston diameter - 4
Piston length - 10
Spring wire diameter - 0.012
Spring length - 6
Spring number of turns - 4


Construction

1. Cut Lever slightly over-length.
2. Drill Axle hole correct length from top.
3. Drill internal hole in Lever for Spring and Piston.
4. Assemble Box sides.
5. Add Lugs.
6. Cut Axle notches.
7. Cut shallow slot in top of Base.
8. Cut Contact strip over-length.
9. Glue Contact to underside of Base and allow to cure.
10. Tightly pull Contact over top of Base, glue into the shallow slot and pull round and glue under Base.
11. Leave to cure then trim off surplus strip.
12. Lightly sand and polish Base and Contact to smooth surface.
13. Glue Base to Box sides.
14. Acquire or make Spring:
14.1 Make good length of Spring by winding wire around suitable size mandrill (the Axle for example).
14.2 Optional: anneal Spring by heating to red heat in gas flame for 3 seconds, drop into motor oil, wait 5 minutes, remove and leave for 10 minutes.
Flash off oil by burning in match flame. Cut to length. Annealing like this makes spring several times stronger.
14.3 Cut spring to length.
14.4 Turn ends of Spring inwards slightly to give positive contacts on Piston and Axle.
15. Round and polish bottom end then cut Piston to length.
16. Solder wiring to Contact and Axle.
17. Trim lower length of Lever to clear Base
by roughly quarter of Piston diameter.
18. Dry assemble Switch and test operation.
19. Repeat trimming, adjusting strength of Spring and dry assembly, if required.
20. A very small amount of grease on the Base, Piston and Axle will make for smoother operation.
21. Choose whether to add suitable metal or plastic top now or simply fix open-top sim switch directly to rear side of panel using adhesive or screws into Lugs.


Mass Production

1. Cut lengths of material of width equal to width of Box side.
2. Cut lengths of material of width equal to internal width of Box end.
3. Assemble rectangular sectioned lengths identical to plan view of Box.
4. Allow assemblies to cure properly.
5. Cut assemblies equal to internal height of Box.
6. Also cut Bases of Boxes from (1).
7. Can also use (6) for tops, or make oversize tops suitable for mountings - or fix switches directly onto a switch panel.


Operation

The sim switch is an Off-On-Off momentary switch. Most events will be logically controlled, for example: on/off and open/close operations. Some events may require two or more switches, for example: increase, decrease. Practically, most events are logically operated, for example: undercarriage operation - down, up, down.


Performance

1. The sim switch should be only used on low voltage (< 12 v) and low current (< 500 mA) circuits.
2. With a 6 mm wide Contact the sim switch is activated for about 100 - 150 ms.
3. A sample sim switch was tested by operating over 1000 times and still functioned.


Cost

Nothing - if you're lucky; otherwise up to GBP 2 (two quid) for an old keyboard. A full set of sim switches for a jetliner cockpit should cost no more.