Intro: Servo Reverser and LinkIt One
The LinkIt One board by MediaTek and Seeed uses the Arduino IDE but the board comes with a lot more connectivity features built in. Your connected device may want to use multiple servos, and if you're like me, code is harder than the hardware. I used the same Servo Sweep sketch that comes with the Arduino IDE and I created this instructable to show you how you can get a reversed signal from the one output.
When connecting servos to the board, it is important to note that the LinkIt One only has two PWM output pins (D3 and D9).
The signal reversal is accomplished with a quad nor logic gate.
Step 1: Servo Sweep
To use servos with the LinkIt One, attached the servo's signal wire (yellow wire) to your digital pin (D9) and ground wire (brown or black) to a ground pin.
Then, attach the servo's power wire (usually red) to a 5 - 6V supply capable of delivering 1 Amp. The LinkIt One board is not built to deliver high currents through its pins and the servo or board may malfunction if you attempt it. I suggest using a 4x AAA battery pack (6 volts) as your servos power supply.
Join the ground of your separate supply with the ground of your LinkIt One. The servo and other electronics we'll build must all reference a common 0V ground.
Find the free Servo Sweep function from your Arduino IDE and upload it onto your LinkIt One.
If you can't find it in the examples menu, right click the application, show package contents, and navigate through the folders until you find the example sketches.
You can see from the oscilloscope gif that the output signal from D9 of your LinkIt One sweeps from ~.75ms to 2.25ms so the servo fully rotates.
Step 2: Reversed Signal
In order to reverse the servo, we need the signal to be 2 ms when the LinkIt One is delivering 1 ms and then our reversed signal must be 1 ms when the board is sending 2 ms.
The quad nor logic gate can do just that. With a 50k potentiometer, the reversed signal can be tuned precisely.
Build out your circuit to match the included diagram and protoboard photo.
You can see that the signal from the LinkIt One is sent directly into the base of an npn transistor. The collector is attached to your 6V battery and the emitter is connected to a resistor and your new signal source. I added this part to insure the logic gates got a strong enough signal.
The original signal can be tapped off at this point as well if you want to keep one servo rotating in the normal direction.
The signal passes through 3 nor gates of your quad nor IC (CD4001). The 50k potentiometer used should be set around 42k. You can adjust it to make sure your servos are perfectly reversed, or not! It's up to you.