Homemade switch: Two layers of plastic 10 mm combined X 200 mm supported by two metal strips 5mm X 2 mm at edges. Two short strips of Cu foil , one on base one on edge of bottom of plastic. It took 14 lb. to make contact. With a solid sheet of plastic, pressure will be greater.

 

Homemade switch: Two layers of plastic 10 mm combined X 200 mm supported by two metal strips 5mm X 2 mm at edges. Two short strips of Cu foil , one on base one on edge of bottom of plastic. It took 14 lb. to make contact. With a solid sheet of plastic, pressure will be greater.

14lb = 1 Stone in UK money, that's perfect. Do you have a picture?

 

@shortbus If you have a better idea then I am genuinely listening. I have a 10mm Acrylic sample here and it seems OK.

I just know from experience at work that plexi will shatter, and when it does the edges are sharp, sharp like broken glass. It was the go to thing for many years for machine guards and storm door windows. But all of industry and many states it it is now illegal to use it in either application.

 

Sorry, no picture. Try measuring deflection of 10 mm X 200 mm plastic with separation of 1.5 mm & 2 mm.
Put a slip of paper between sheets; increase pressure until paper will not pull out. Cu tape ,two layers is about .14 mm. This might also work for black keys?

 

I just know from experience at work that plexi will shatter, and when it does the edges are sharp, sharp like broken glass. It was the go to thing for many years for machine guards and storm door windows. But all of industry and many states it it is now illegal to use it in either application.

We used Plexiglass to make enclosures for containment until the enclosure blew up. A burst of about 1600 PSI of Hydrogen gas was introduced into the enclosure through a line rated for vacuum use. Normally the line would only see vacuum when doing a cylinder change, but a safety device failed, a excess flow valve, and when the tech did the "check the seat" thing by briefly opening and closing the cylinder that erroneously wasn't empty, The plexiglass did some serious shattering. I can still see in my mind the large gashes in the drywall.

After our investigation, enclosures are now made with Poycarbonate panels. The excess flow valves became orifices. Certain gasses got dedicated purge cylinders and the line rated for vacuum got upgraded. High purity gas regulator have the ability to pump out (vacuum pump) the regulator. And the Hydrogen alarm that was sitting in a box got installed. The cylinder valves got changed to air operated.

 

Made a small sample key with 5 mm plastic by running Cu foil from center edge to 1.7 mm spacer. Put a 1.7 thick mm section of rubber tu make a bump in foil. matching foil on base so plastic can be removed without disturbing contacts. Must pun.

 

@Bernard interesting. I plan to create a full scale key this week in MDF mostly to test my Routing skills! I will try your method, along with Tactile and micro switches.

 

The plexiglass did some serious shattering. I can still see in my mind the large gashes in the drywall.

In it's day Plexi was the go to. but was always overrated. The polycarbonate/Lexan is much better and is required here in the US for machine guards. I was involved at work in remaking/replacing guards on all of the machines that used them. It actually made some of the guards easier to fabricate, since Lexan can be bent in a sheetmetal type bending brake. Some guards could be made in on piece instead of gluing things together.

 

I can't find Lexan for sale in the UK and Polycarbonate Sheets seem to be available in clear only.

 

I can't find Lexan for sale in the UK and Polycarbonate Sheets seem to be available in clear only.

Lexan is just a trade name for polycarbonate. Polycarbonate plastics have a slight purple tint. You could use a thin diffusing plastic behind it, or spray it with a frosted finish (on the back).

I am not sure, frankly, if the lucite will be a problem in your application or not. I have used both extensively and the only thing I am sure of is that the lucite will be scratched up in use, and there are scratch resistant polycarbonate plastics available.

I think you need to test.

 

@shortbus You can bend plexi too with a heated brake. Somebody in the shop made one using heat tape.

Our experimental containment hoods were made with Unistrut and Polycarbonate. We either made sliding doors or panels with handles that fastened with 5/8-18 wingnuts. In one design, I integrated rack rails for the electronics.

 

You can bend plexi too with a heated brake

No heat needed with Lexan, just set the top leaf of the brake back double the thickness of the material and bend.

 

Just to round off this Thread, I thought I'd post the results of my testing.

Velostat didn't work. The voltage is gradual rather than on/off and it seemed to allow 1 Volt through without any pressure.

Tactile Switches seemed liek a great idea but they are soooo small it was impossible to cut a notch the correct size for it to sit in.

Microswitches. I cut a 4mm notch and put some 6mm neoprene tape in and attached the switch to the inside of the key it worked perfectly first time - I bet it doesn't next time!

The 9mm MDF that I used takes my weight, even with a little bounce so I am confident that 10mm acrylic will work fine. From here on in I will post in the Projects Forum.