Wireless Trim Tab Switch

Work in progress

I want to try adding trim tab controls to my Donzi’s steering wheel. I have factory-installed Bennett tabs with the tab switch on the dash to the right of the steering wheel. Outdrive trim control is on the throttle, which I find perfect. However, I have to take my right hand off the throttle to adjust the tabs. I think tab switches on the left side of the wheel, which could be operated by thumb, would work well – hands remain on wheel and throttle. I'd like to give that a try.

I don't like the idea of messing with clock springs or brushes to make wired electrical connections through the wheel. I think wireless is the way to go.

Livorsi has a wireless setup with a radio transmitter in the hub of the wheel and switches mounted to the wheel. I considered this, but rejected it for several reasons: it's expensive, Livorsi insists upon doing it for you, and it's irreversible without replacing the wheel. Lectrotab has a wireless switch setup, but it's meant to be attached to the dash and draws power through a wired connection, so that’s out. I like the ergonomics of the Insta-Trim tactile switch. I imagine a smaller version of that switch mounted to the spoke of the wheel, integrated with a wireless transmitter. I think it would be easy to slide a thumb over and raise or lower one or both tabs without diverting the eyes with a switch like that.

I’m implementing this concept using two devices: a switch assembly with a battery-powered wireless transmitter for the wheel and a wireless receiver/controller behind the dash that attaches to the Bennett wired switch.

For the wireless link I’m using Linx components. Linx makes easy-to-use radio-frequency (RF) modules, ICs, and component building blocks, as well as a few complete transmitters and receivers.

Linx offers a few complete key fob transmitters for about $20 that are functionally perfect for this job. You probably have a fob like this in your pocket for your car. The fob uses small rubber switches that are not suitable for this application due to their size, shape, and tactile characteristics. I’m using the guts of the fob: a circuit board with battery holder and battery. I’m adding a circuit board on top of the Linx board that contains four tactile switches. On top of these I’m placing custom rocker switch caps. These components are all held in a custom enclosure. 

For the switch enclosure, I hit on the idea of using PVC plumbing materials from the local hardware store for this. The plastic facilitates RF transmission, and PVC is relatively easy to work with basic hand tools. The enclosure is a pipe end cap and the switch caps are harvested from the side of larger one, making use of the sidewall curvature. The switch/transmitter is about 2” in diameter and 1” tall. It’s a bit bigger than it needs to be, as the PVC is over 1/8” thick, way more than it needs to be. I think the diameter is about right. Smaller switch caps would probably be harder to use. But the enclosure could be a lot thinner (not so tall), especially if a single, custom circuit board were used rather than the standard Linx board and added switch board.

I think a billet aluminum enclosure would look sharp, but this is beyond my current skills and tool set (anyone interested?). As the Linx transmitter guts employs an integrated antenna, the case needs to allow a path out for the RF energy. I think a billet aluminum case with plastic switch caps and a plastic base would work, especially if the base is offset or overhangs the metal spokes of the steering wheel, which it probably will in my case. The Linx RF components have a range of up to 1000 ft with proper antennas. Since I will be operating at only a few feet at most, the RF transmission can be heavily attenuated and still have plenty of margin to operate. So an aluminum case is a possibility. A small external antenna – just a length of wire – could also be employed.

You can probably see that a wide variety of sizes and shapes of the switch caps and case could be used. For example, the new Bennett euro or the Livorsi switch shapes could be emulated if that were preferred.

The receiver/controller, mounted behind the dash, receives the RF signal from the integrated switch on the steering wheel. It decodes the RF signal and simulates Bennett switch closures to the stock hard-wired system. The receiver/controller employs a Linx receiver module, decoder IC, some discrete components, and relays. The receiver/decoder converts the port/starboard up/down button presses into the proper pump active/pump reverse and port/starboard valve contact closures. I’m using common automotive relays in the controller, as these are capable of handling the Bennett system current loads and are relatively inexpensive.

Clearly, safety is a concern, which I have taken into consideration as I chose my design. Transmitter and receiver frequency of course must be the same.  Any transmitter not on the same frequency is ignored.  The frequency that I'm using is 418MHz. Here's what Linx says about this frequency choice:

"418MHz is a good frequency to use in the U.S. as it is not very crowded. This gives the least likely chance for interference, therefore the best performance"

In addition to the frequency, a digital address code is sent by the transmitter and decoded by the receiver.  I'm using the MS series encoder/decoder. The receiver's decoder will only respond when the preprogrammed digital address or code matches. The possible address combinations are over 16 million. It's extremely unlikely that another transmitter's address would match and could "take over" and move the tabs up or down. More likely but still rare would be another transmitter on the same frequency stepping on my transmitter. This is unlikely because of the physical proximity of my transmitter to the receiver versus the distance to an offending transmitter. Received power falls off at the square of distance, so the receiver sees 25 times the power from my transmitter 2 feet away as from an identical transmitter just 10 feet away. However, if another transmitter were broadcasting at the same time on the same frequency with sufficient power to interfere, without a digital address match the receiver would simply not respond. If that were to happen, I still have the wired switches on the dash, which would of course remain operational.

I completed assembly and checkout of my receiver circuit board. I’m just using a stock breadboard for now, so it’s not the prettiest, but functional. The RF module is on the underside of the board out of view. The board will drive four automotive relays, one each for pump pressure; pump reverse, port valve, and starboard valve. I temporarily connected some LEDs in place of the relays to verify operation. A simple ¼ wave wire antenna on the receiver seems to give more than enough range. The logic on the receiver board converts a single transmitter switch press into the two relay activations needed – e.g., press the top of the port rocker switch and the receiver board activates the pump pressure and port valve relays. Of course both port and starboard tab switches may be activated simultaneously, and the receiver responds with pump pressure, port valve, and starboard valve relays all activated. The relays will be wired so as to prevent an attempt to activate pump pressure and pump reverse simultaneously.