An Introduction to Solid State Relays

When we think of relays, we tend to think of those big mechanical things that make a satisfying ‘click’ when activated. As nice as they are for relay-based computers, there are times when you don’t want to deal with noise or the unreliability of moving parts. This is where solid-state relays (SSRs) are worth considering. They switch faster, silently, without bouncing or arcing, last longer, and don’t contain a big inductor.

Source Fotek SSR Specifications Sheet

An SSR consists of two or three standard components packed into a module (you can even build one yourself). The first component is an optocoupler which isolates your control circuit from the mains power that you are controlling. Second, a triac, silicon controlled rectifier, or MOSFET that switches the mains power using the output from the optocoupler. Finally, there is usually (but not always) a ‘zero-crossing detection circuit’. This causes the relay to wait until the current it is controlling reaches zero before shutting off. Most SSRs will similarly wait until the mains voltage crosses zero volts before switching on.

If a mechanical relay turns on or off near the peak voltage when supplying AC, there is a sudden drop or rise in current. If you have an inductive load such as an electric motor, this can cause a large transient voltage spike when you turn off the relay, as the magnetic field surrounding the inductive load collapses. Switching a relay during a peak in the mains voltage also causes an electric arc between the relay terminals, wearing them down and contributing to the mechanical failure of the relay.

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A Guide For Building Rubber Dome Keyboards

Let’s talk about computer keyboards for a second. The worst keyboards in the world are the cheap ‘rubber dome’ keyboards shipped with every Dell, HP, and whatever OEM your company has a purchasing agreement with. These ‘rubber dome’ keyboards use a resistive touchpad to activate a circuit, and the springiness of the key comes from a flexible rubber membrane. Mechanical keyboards are far superior to these rubber dome switches, using real leaf springs and bits of metal for the click clack happiness that is the sole respite of a soul-crushing existence. MX blues get bonus points for annoying your coworkers.

Mechanical key switches like the Cherry MX, Gateron, or whatever Razer is using aren’t the be-all, end-all mechanical keyswitch. History repeats, horseshoe theory exists, and for the best mechanical keyswitch you need to go back to rubber domes. Torpre switches are surprisingly similar to the crappy keyboards shipped out by OEMs, but these switches have actual springs, turning your key presses into letters through a capacitive touchpad. Is this a superior switch? Well, a keyboard with Torpre switches costs more than a keyboard with Cherry MX switches, so yeah, it’s a better switch.

It seems everyone is building their own mechanical keyboards these days, and the recipe is always the same: get a few dozen Cherry MX (or clone) switches, build a PCB, grab a Teensy 2, and use the tmk keyboard firmware. There’s not much to it. DIY Torpre boards are rare because of the considerations of building a capacitive switching PCB, but now there’s a DIY guide to making the perfect rubber dome keyboard.

[tomsmalley] put together this guide after reviewing a few amazing projects scattered around the web. Over on Deskthority, [attheicearcade] is building a custom, sculpted, split Torpre board and a split Happy Hacking Keyboard. These are projects worthy of a typing god, but so far there has been no real beginner’s guide for interfacing with these weird capacitive switches.

As far as circuitry goes on these capacitive boards, the PCB is the thing. Each key has a pair of semi-circular pads on the PCB to serve as plates on a capacitor. These pads are connected to a microcontroller through an analog mux, with a little opamp magic thrown into the mix.

With a relatively decent guide to the hardware, [tomsmalley] has also been working on his own firmware for capacitive switches. Shockingly, this firmware is compatible with the Teensy 3.0, which will provide enough horsepower to read a bunch of analog values and spit out USB.

Mechanical keyboards are great, and we really like to see all these hardware creators pushing the state of the art. You can only see so many custom sculpted keycaps or DIY MX boards, though, and we’re really eager to see where the efforts to create a custom Torpre board take us. If you’re building one of these fantastic keyboards, send it in on the tip line.