Making an AND gate with relays…

In my post about the EME R3.S, I listed a number of challenges. One of them was a real disappointment and needed some follow-up ingenuity.

The RT’s parking light is simply an added front facing light that is normally on when your switch is set to just the parking lights. This was more common on non-US bikes where the main headlight could be turned off and you use the parking light position on the switch. What I see most often is these light during low-beam use on US bikes as well, but don’t quote me on that. The original light is a low wattage bulb that does not do much and the RT and RS position this light above the main lamp. Other models include a small bulb in the main lamp housing for this function.

The full fairing version offers a rather large area to use and there are compatible LEDs for the socket. The versions I have tried all create a bit too much of a spot-light appearance and I was unhappy with the results. This led me to get more creative and I looked for more options. There are countless options on Amazon and other Chinese import sites that may work, but I opted for a product from Diode Dynamics. The make semi-flexible LED strips in 3 and 6 inch lengths. They also offer them in red or white/amber switchback versions. Clearly, I am not using red, so I picked up some white/amber versions.

The amber switchback option got me thinking about the parking light function. Since I am using the R3.S, I have the option of built in hazard light functionality which glasses all four turn signals as expected. I became set on the idea of using the LED as a flashing amber when the hazards were active, gaining some additional functionality. Since the R3.S claims to support type 2 switchback functionality, I figured this would be a simple setup. Alas, I was wrong about that.

The LED strips from come with a dedicated driver and a nice long wiring harness for install. There are three wires between the driver and the strip, but four wires for the light control. Those are Ground, Low Power, High Power, and Amber. You have the option of using only low, only high or both on the strip, so I chose to splice those into the headlamp high and low wires as appropriate. The R3.S has a signal wire for “HALO/DRL” which is also used for parking lights. The claimed functionality is that this can trigger a switchback with a change in one of the dip switches on the R3.S PCB. After attempted to follow the instructions I was able to get from NWT, I could not find any combination of connections and switch settings to produce this function. So, rather than continuing the frustration of working with NWT on this, I opted to make a solution that leveraged what was actually working.

What I needed was a way to trigger the switchback on the LED only when the left AND right turn indicators were lit. With the R3.S, this condition is met when the bike is initially powered on and when the key switch is moved to the first position. So, the simple answer was to make a relay based AND gate to trigger the switchback. I grabbed some ultra micro relays from Amazon and made a custom housing for two of them.

Wiring up two of these relays is a clever, but simple process. You need one relay for each signal input which is why I used two. My signals are coming from the turn indicator signal wires. You also need to have a 12v input to power the gate, a ground lead and an output lead to act as the switchback signal line.

Wiring up the AND gate is straight forward. I used 5 post relays, but this can be done with 4 post relays as well. If you aren’t familiar with the standard relay terminal numbers, here’s a simple key:

30: Power Input
85: Ground
86: Signal Input
87: Power Output
87a: Power Output

My relays normally pass power from 30 to 87a and switch the power output to 87 when the relay gets power on 86. So, we need to connect two relays in series so power can route from terminal 30 on the first relay and out terminal 87 on the second only when both relays have power on 86. Here’s a diagram that does this:

When relay 1 receives power on the signal (86), power routes from 30 to 87. This feeds power to 30 on relay 2. When relay 2 receives power on signal (86), it feeds power out 87 to signal the LED switchback. The LED only gets power when both signal wires are powered. So we simply splice the left and right indicator wires into a signal wire on the relays. It doesn’t matter which indicator is connected to which relay, as long as they each get their own relay.

This all works nicely and will live inside the headlight bucket. One side effect is that every time an indicator circuit is active, you will hear the mechanical relay’s solenoid activate.

My final relay based AND gate in the 3D printed housing. It is now mounted inside my headlight bucket.