Two - dual switches

Here are two approaches to making a dual switch that requires minimal motion and minimal activation force:

1. The first is a mechanical switch setup using small "subminiature snap switches" like OMRON #SS 5 (Digikey SW868-ND). I mounted the two switches in a small plastic box (like digikey SCR4B-ND) so that the actuator stick seen in the photo below is moved left or right to actuate one switch of the other. Using even smaller switches like those commonly found in mice will work and may yield an even more precise switch. The switch is shown mounted on a 6mm rod using a binder clip. The binder clip is just screwed into one of the plastic halves of the box with a tiny sheet metal screw (or a small machine screw and nut) through a hole drilled in the binder clip.The binder clip holds securely enough, yet permits easy positioning and removal (for repairs about once every 2 years.) The actuator stick consists of two parts - a brass tube and a piece of music wire that slides into the tubing in a snug fit. This tubing and music wire are available in hobby stores that cater to radio control car or airplane enthusiasts. The brass tubing is about 3/32" or about 3mm. The tube is squeezed slightly out of round so that the music wire slides in or out but remains where you want it. The blob at the end of the actuator stick is a small vinyl screw protector glued on. The switch is easy to activate with the lips or chin either left/right, or up/down if that is your preferred orientation.

Inside the box you can see how the switches are mounted with small machine screws and nuts. The switches are oriented so that the actuator buttons face each other. The actuator stick has a wire soldered into the end of the brass tube that wraps around the post that holds the box halves together. The post may have butresses that need to be shaved away. I also put short lengths of clear vinyl tubing (like the stuff used in aquariums) to support the actuator wire from either half of the box to assure it remains centered between the halves. A small plastic tube is fitted over the actuator stick to take up the distance between the snap buttons on the switches. I position all the parts in the box before marking and drilling the holes for mounting the switches.The edges of the box have been trimmed to accommodate the actuator and the wire where they pass outside the box. A user with some head movement can actuate two dual switches side by side on a 6mm rod supported on a rig much like a harmonica holder. I fashioned such a setup using music wire for my most long-term client. He has been using this setup for more than 10 years daily with only few repairs. I keep a switch setup ready to go at all times for quick repairs.

2. The second switch is based on a Quantum Research Qprox IC QT113 which is a capacitive proximity sensor (the closest is Digikey 437-1132-2-ND which is a two switch IC) . Unfortunately, I don't know where my schematics are. But, it wasn't that hard to figure out how to use these little wonders with the product sheets. In this unit I use two proximity sensor ICs. I think the only required components are a capacitor (I used a .01 uF) and a sensing electrode. The two proximity sensor chips and two solid state relay chips (NEC PS7141-1A, Digikey PS7141-AA-ND) are wired to a proto-board that resides in one half of the battery box shown below. The two screws shown on the battery box are the sensing elements. Clear tape covers the screw heads so you can't touch the metal parts, but just by touching the tape on top of the screw heads the output relays are activated. The bigger you make the sensing elements the greater the sensing distance allowed. I used the smallish screws because I wanted a sensor you pretty much had to touch.

The blurry photo below shows how the circuit is fit into the battery box so that there is still room for two AA batteries needed to power the circuit. The two AA batteries will power this circuit for months due to the very low power consumption of the Qprox sensors. The Qprox sensor outputs are active high and directly drive pin 1 of the opto-coupled solid state relays via 220 Ohm resitors. Pin 2 is connected to the - side of the batteries. The battery box I used does have a built-in switch I use to turn off the box when not in use (box = Digikey SBH-341 AS-ND). The total cost for the dual switch proximity sensor parts including the 3.5 mm plugs on 6 foot wires and batteries is less than $20.

As shown below I am touching the screw on the left. The meter is monitoring the output associated with the screw on the right and indicates an out of range value - more than 20 Meg Ohms.

When I touch the screw on the right the meter reads less than 90 Ohms. If the batteries were fresher that value would probably be less than 50 Ohms. In either case the output resistance is low enough to activate a switch input for a computer ( like an adapted gamepad or adapted mouse). In spite of the close proximity of the two sensing screws the isolation is pretty good. You can position your sensors wherever you want. But the longer the wires are between the Qprox IC and the sensing elements the more the wires themselves act like sensing electrodes and may cause both sensor to be simultaneously activated due to cross coupling. If I find my schematic I will add it later. The response of the Qprox based switches is a little sluggish. I had an avid gamer give this technology a try and it wasn't responsive enough. You can probably expect to get maybe 3 activations a second reliably.