Casing for long range wireless video
Background
This wireless system provides a live video feed which can be accessed directly, without drivers, by any Windows, Mac or Linux computer. It uses a firewire interface to capture RAW DVAVI video which can be edited without conversion. The range is in excess of 50 meters where normal range is limited to 5 meters using a firewire (ieee 1394) interface. It was specifically designed for software packages that make it possible to watch instant replays during a live capture.
The goal of this project is create a custom made casing for a medium/long range wireless video system. The reason for this was that my prototypes had too many wires attached to them. This meant that it was very hard to do user testing on the prototype in a rugged outdoor environment - for which it was developed.
The needs of the casing were:
1. To be as small as possible
2. No wires!
3. Battery powered
4. Dissipate heat
5. Shield interference
6. Rugged
I decided on two aluminium cases which I ordered from RS components - these where designed to house electrical components. The cases needed to be modified to house the electronics.
Transmitter case
I choses a case that was as small as possible (for the elecronic components). In truth this was the third type of case I had ordered. They all cost about 5 euro. I got a friend to use a milling machine to create extra spaces for specific components.
Receiver case
The receiver casing was also modified by a friend of mine. I ordered the rectangular aluminium case (275mm x 175mm x 65mm) my desired height was 30mm (from 65mm) but I could not source it anywhere so this one was milled down to 30mm.
Extra holes were also added for the wireless receiver and for the computer connectors (USB & Firewire Neutrik).
Empty cast aluminium casing, 30 mm height, spaces for mounting components.
Fab Diary Week 1
Receiver
Now I had the basic shape of the casings, the components needed to be fitted. Essentially this meant working out exactly where each component went (I had a rough idea already) and fitting them in a way that they were rugged and secure but also removable. I chose to use metal 25mm metal spacers with nylon M3 screws. These are non-damaging to the print boards and do not conduct. The print boards were mounted upside down. The reason for this was to make sure each print board has enough space to get rid of excess heat.
I used Bison Kit metal on metal epoxy to create a convenient lip to lock the receiver into place.
The main problems I experienced where aligning the drilling holes to mount the PCBs. Even with careful measurement (and with unforeseen changes in placement), it was difficult to get the alignment exactly right. Larger holes allowed me to make small probe holes. Other problems where working out a good way to fix the cases in place whilst drilling.
Making a receiver unit provided the opportunity to incorporate extra technologies into the casing. Two HID (human input devices) were chosen for this casing.
Transmitter
Three holes where drilled for the RCA connectors to connect to a video camera. I also re soldered the connections to the antenna.
Fab Diary Week 2
Receiver
I added holes for two extra antennas on the receiving unit. The idea was to ensure there where no dangling cables outside of the casing. In retrospect, I really needed to test and do more research on antennas before I started drilling. A process of trial and error proceeded. The probe holes for the receiver case battery was aligned and tidied up. More holes are needed for the battery but I didn't dare making more holes. The aluminium was deeper than I expected and the battery LEDs were very small leaving no margin for error. I will solve this problem later.
Transmitter
The mounting bracket was attached to the transmitter lid. This was a little bit tricky because double side tape was not strong enough. Using the 4 mounting holes was the alternative but this meant the screw and bolt had to be flush on both sides - and correctly aligned. The antennae was attached with double sided tape.
Fab Diary Week 3
Receiver
The lid fitting screws where ordered countersunk M4 stainless steel hex head (rs components). The lid needed a bit more countersinking to get a flush finish on the lid.
Precise holes where drilled for the LED displays.
The power lead from the battery to the antennae was soldered. 2 x 2.1 mm DC jack. I had experimented with a more complex power setup with soldered connectors to the antennae circuit board with a diode.
The receiver case was reassembled.
Fab Diary Week 4
The system was extensively tested. There were a number of issues with the 12 volt battery packs. Universal battery packs are not built to be enclosed in casings which has caused a number of problems in the sender & receiver unit.
Receiver: HQ-UBT-8800 unit has the problem that it can only be turned on by attaching a DC jack. Unfortunately the unit turns on automatically when a cable is plugged in. This meant that this only means of turning the unit without it draining its battery.
The settings of this battery also have a design flaw (for my purposes at least). It can be set to 5.5 volts, 8.4 volts or 12 volts. Unfortunately when the battery test button is pressed (held) then battery reverts to 5.5 volt mode. Furthermore the battery test button does not work during use.
Transmitter: Anypower 12v 8400mah. I had chosen to disassemble the battery to relocate its display unit. This unit has a circuit protection system which rendered my arrangement inoperable. This meant that a new battery and design was needed.
Conclusion: Transmitter redesign was necessary.
Field test: To make sure that this setup achieved the necessary range required (50 meters +) a temporary version was cobbled together. The video transmitted reliably up to 120 meters. The two extra input devices (built into the receiver) did not perform well though 2-3 meters. Some modifications would be needed to boost their range to above 10 meters.
Transmitter redesign
The battery packs cause a number of problems when designing the transmitter and receiver unit. The main issue is the placement of the sockets, their function and the battery level display. Added, the cost of battery packs often exceeds 50 Euro. Any mistakes or poor measurement is costly and timely.
The redesign was based on a larger cast aluminium case. Less elegant, but large enough to house all components without modification. Added, no milling was necessary. The redesign only required careful measurement, HSS drill bits, Dremel and a hack saw. There was some rewiring to ensure the Transmitter could be charged and use the mains power using the same 12 volt 2.1 mm DC socket.
Reflection:
Building a complex hybrid device like this has been a pretty tricky experience. The casing was very much a process of trial and error. These designs I would classify as high quality prototypes which perform a unique, but specialist task. In terms of components the batteries gave the most troubles. In the end this means that both the Transmitter and Receiver need an explanation of how to charge and turn the device on and off. This is a little dissapointing because my intention was to make the prototype really easy to use.
My expectations from the project have changed. Now its main applications are in the elite sport sector, broadcasting and in specific areas of research and testing.
Personally this has been a great learning experience in compacting electonic components into an enclosure. That requires a particular type of thinking. By building existing on components I was able to develop a device that goes far beyond what I could have achieved by designing a product from scratch.
