Some R/C model video applications require two video sources. They might be two cameras, or a camera and specialized video source combination. Ideal applications would be where a second camera is used to gather a quick downward view or perhaps peek at some cockpit instrumentation. The applications are only limited by your imagination.
Here are the main features of the R/C Video Switch:
This isn't a project for the average electronic tech. Advanced soldering skills are needed due to the fine pitched SMT soldering that is required. Do NOT attempt this project unless you have Surface Mount PCB rework experience and SMT soldering tools.
Lots of Chips, But Where's the DIP?
There are several specialized IC's used in the project. All are in tiny SMT (suface mount technology) packages. Sorry, but DIP versions are not available on most of them.
The dual 5V camera power is provided by the Maxim MAX8862L low dropout regulator. It is a perfect fit for powering up to two Panasonic CX161 CCD cameras. You can also connect most other camera models that accept 5VDC at 150mA or less.
Under the PIC's supervision, the MAX8862's 5V outputs (there are two) are turned on and off as needed. If you cannot use these 5V supplies then you will lose the intelligent power control feature. If you use battery voltages higher than 6V then you may need to add a heatsink to the MAX8862.
There is an unswitched auxiliary 5V supply too. Based on the National Semiconductor LM2937ES-5.0, it provides power to the board's electronics, as well as your external devices. You can safely draw up to 400mA with a 6V input (5-cell NiCD). Higher input voltages may require a heatsink on the LM2937 and good ventilation in order to prevent thermal shutdown. The need for a heatsink really depends on the input voltage and output current.
The video switching circuitry uses the Maxim 4313ESA video mux. It offers high bandwidth and the video paths have been optimized (via external components) for our 75 ohm video application. The buffered output has a slight DC bias, so if your video transmitter is not AC coupled then you will need to add a 330uF/10V tantalum cap in series with J4-1.
The electret microphone preamp uses the Maxim MAX4175 fixed gain Op Amp. It will boost the millivolt output of a typical electret condenser microphone to about 1V standard line levels. The amplifier is offered in several gains so you can tailor it to your needs. For glow or e-powered models the +6dB "AK" part is recommended. Gliders should use the +10dB "BA" part.
The device's brains rely on a MicroChip Technologies PIC12C508 microcontroller that uses custom firmware that you can download for personal use at no-charge. The PIC decodes the R/C signal for video source selection and controls the dual 5V supply.
The PIC is not a "Flash"
part, so you will need a traditional
PIC chip programmer to "burn"
the hex file's object code into the microcontroller. The SOIC-8 package will
require a socket adapter, so check to see if your programmer needs one. Be
sure to select the programmer's configuration fuse settings as follows:
By the way, after you program the PIC it will fail the verify cycle. Do not be alarmed -- everything is OK. Just ignore the "failure." Whatever you do, do NOT program the chip twice!
If you have trouble burning the PIC, then please check your programmer. Whatever the fault, it is not a hex file issue. The most common problem is that the user has forgotten to burn the PIC's four config fuses, as shown above. More programming information can be found starting here.
Connectors: The In and Outs of it All
The board's I/O is presented at the perimeter using several connectors. Here is a summary:
Each of the connectors summarized above have pinouts that are organized in a logical format. To aid in identifying the pins, the PCB's square solder pad indicates pin 1. Here are the J1 through J8 pin definitions:
It's All in the Details
I'm not going to offer detailed instructions on how to assemble the board. Instead, I will leave the board stuffing adventure up to you. All of the parts are in SMT packages, so you will need a good magnifying lens and tweezers, as well as the usual SMT soldering tools. If you do not have SMT experience then this is probably not a good project for you.
The area inside the
red box (see photo on right) is not stuffed and can be cut off if a smaller
board size is required. An overall size of 1.5" x 1.7" is possible if you
trim out the red area. These unused components are for the optional 12V DC-DC
switcher, which I'm still evaluating.
There are nine SMT components on the bottom of the board too. About half of them are for the Mic amp. They can be omitted if the preamp is not needed. However R8, R9, C11, and C12 are required and must be stuffed. You can click the image on the left for a closeup view or download the bottom silkscreen drawing from here: Layer2.
If your two cameras (or specialty video devices) are powered by the MAX4313, via J1-1 and J2-1, then the jumper pads at JP1 must be left open. If you are using another power source then the JP1 pads should be shorted together (a blob of solder works well).
Apply power to the board (5.5V to 8.5V DC). If LED1 is on, then your battery voltage is too low. The LED will light up when the battery is below 4.8V (which is too low to operate some cameras). The LED alert is only available when the Main Camera is on.
The video source is selected by the R/C channel that you have connected to J5. If the R/C signal is lost (out of range, R/C failure, etc.) then the video path will default to the Main Camera. This is a nice failsafe feature.
Because the VR1 regulator is shared, faults on the J7 Aux power output can affect Video Mux operation. Where possible, protect the J7 output from over-current issues due to your auxiliary devices (if any). If this is a critical installation then you should consider not using the J7 power or perhaps add fuse protection.
The technical details are available as file downloads. There is no charge for the firmware hex file when used in a personal hobby project. Commercial users and resellers must obtain written approval before use (license permission).
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. You are granted permission to post links to the web site's main page (http://www.rc-cam.com/). Please respect this simple request.
The Small Print:
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 find software bugs then please report them to me. I can only make corrections if I can replicate the bugs, so please give me enough details to allow me to witness the trouble.
I would enjoy hearing from anyone that uses the R/C Video Switch.