Bit-Switch

Micro-Sized R/C On-Off Switch

In case you were not aware, MicroChip has a family of tiny microcontrollers that are about the size of a grain of rice. Called the PIC10F family, they are available in a six pin SOT-23 SMD package. It seemed to us that these little fellows would be perfect for use as a dedicated R/C signal decoder for On-Off control applications, especially where size and weight is a concern.

We are happy to report they are an excellent itty bitty microcontroller for such tasks. All that is needed to finish the job is a 0.1uF decoupling cap, resistor, and some good eyesight for the soldering efforts.

What you and your R/C system control with it is up to you. For example, you can use it to remotely turn on aircraft lights, drop a payload, switch video signals, enable a UAV control system, and much more. The logic level outputs can be adapted to switch high currents, so there is really no limit to what you can do.

Features of Bit-Switch:

• DSP analysis of incoming R/C servo signal prevents false On/Off detection.
• Both high stick and low stick control outputs.
• Lost servo signal status output (for older AM/FM PPM type radios).
• Designed for R/C models and robotics. Requires 3.25VDC to 5.25VDC.
  

Smallville

Bit-Switch can use the Microchip PIC10F202 or the PIC10F206 IC. They are available from most large parts suppliers, including digikey.com and mouser.com.

If you are used to working with DIP packaged components, then the size of the PIC10F will be somewhat of a shock. For those that don't need the small size, the IC is also available in a 8-Pin DIP package. But where's the fun in that? Regardless of the package you choose, the decoder is designed to allow easy integration into your R/C project.

Ignoring the power connections for a moment, there are only four I/O lines on the microcontroller. So it is hard to get lost when using it. In a minimal installation, you would only need to use two of them, the R/C Signal input and one of the On/Off outputs.

The following table summarizes each pin.

There are two On/Off outputs. If your R/C transmitter does not have channel reversing switches, then choose the output that turns on with your preferred stick or switch position.

The Lost signal Alert output is normally logic low, but it will go high whenever the R/C servo signal is missing or corrupt. This optional output can directly drive an LED (as shown in the schematic below), or it can be buffered with a transistor and used to turn on a lost model siren.

By the way, the three logic level outputs can only drive about 20mA each. The available output voltage is about the same as the PIC's VDD input voltage. Many applications will need to buffer the outputs in order to switch higher current loads. Below are just some of the many ways to interface to Bit-Switch.

Switch Example S1: This circuit shows how to connect a solid state relay for up to 350mA loads. This isolated interface is a great way to to control the shutter on digital still cameras (remote photography).
Switch Example S2: This circuit shows how to connect a small DPDT relay. This could be used for a variety of applications, including video camera signal switching (Camera A and Camera B).
Switch Example S3: This circuit shows how to switch very high currents using a MOSFET. Inductive loads should include a protection diode across Q2.

Beauty is Only Skin Deep

As nice as the little PIC10F's look, they are just mere specs of silicon when you get them. The magic part is the software, which must be loaded into the chips. You will need a PIC chip programmer to "burn" the provided hex file's object code into the microcontroller. Be sure to select the configuration fuses during chip burning as follows:

	WDT:	Enabled
	MCLR:	Disabled
	Code Protection:	Disable

The PIC's Hex file is designed to automatically instruct the programming hardware to chose these values. However, it is always a good idea to check them for accuracy. Be sure to setup your programmer so that it does NOT overwrite the factory stored OSCAL value! Please do not ask me how to do that -- I will not know how to operate your equipment.

If you have trouble burning the PIC, then please check your programmer. Whatever the fault, it is not a RC-CAM hex file issue. The most common problem is that the user has forgotten to burn the PIC's configuration fuses, as described above.

Please keep in mind that there are restrictions to using the hex file. Permission requirements are found in the readme file that is provided with the hex data.

Board Construction:

Unlike most of the other RC-Cam projects, this one does not have pictures or details to an assembled board. That is because Bit-Switch is intended to be a component in YOUR custom R/C project. If you use it, please send along some clear photos and a description of your DiY hobby project. We will post photos of any that help demonstrate Bit-Switch's versatility.

Check it Out

To test your work, just follow these three simple steps:

1. Turn on your transmitter and verify that moving the R/C transmitter stick (or toggle switch) reliably controls the On-Off output.
2. Turn off the transmitter. Verify that the Lost Signal Output immediately turns on. If an On-Off output was on, it will turn off while the R/C signal is missing. Note: This feature is not compatible with PCM/DSP or 2.4GHz R/C systems..
3. Turn on the transmitter and verify the Lost Signal LED turns off. The On-Off output will turn back on if the "On" stick/switch position is active.

Design Documents:

The technical details are available as file downloads. There is no charge for the information when used in a personal (hobby) project. Commercial users must obtain written approval before use.

Please be aware that the information is copyright protected, so you are not authorized to republish it, distribute it, or sell it, in any form. If you wish to share it, please do so only by providing a link to the RC-CAM site. Please respect this simple request.

	Schematic Files: PDF file of the Bit-Switch circuitry. Revision: Rev A, dated 08-28-2006
	PIC Object Code: Firmware Hex files. Revision: V1.0, dated 08-28-2006.

The Small Print:

I do not work for, nor represent, ANY supplier of the parts used in Bit-Switch. Any reference to a vendor is for your convenience and I do not endorse or profit from any purchase that you make. You are free to use any parts source that you wish.

All information is provided as-is. I do not offer any warranty on its suitability. That means that if you build and use this device, you will do so at your own risk. If you have technical questions or comments about this project then please post it on the rc-cam project forum.