A Port Elizabeth company has fought off stiff competition from multinational firms such as Goodyear, Monroe and Bosch, to win a place in the top eight finalists of a national innovations competition.
At the Automechanika South Africa Show in Johannesburg, High-Tech Technical Components was highly commended for its towbar wiring harness tester, a cost saving product that provides wireless communication to test trailer socket wiring at the towbar.
High-Tech owner Ian Ward said: “We did well to beat off some of these internationals to gain highly commended status. We are particularly proud of being placed in the top eight out of 24 entries, especially as we were up against household names such as Monroe, Tenneco, Shatterprufe and Goodyear.”
Organised by motor trade magazine Automobil, the event was in keeping with similar competitions held at other Automechanika shows throughout the world, the biggest being in Germany.
The concept for the towbar wiring harness tester was borne from a patent held by High-Tech for wireless communication to trailers and caravans. Electronic design on the project – which constitutes a hook-up-and-go system with no trailing wire – was undertaken by Racoon-Tech, another Eastern Cape company with technological know-how.
Racoon-Tech’s design brief was to develop a product that was both simple and cost effective. The company shared some of its design insights with Dataweek:
Frequency bands considered were licence free 403 MHz, 433 MHz and 2,4 GHz. 403,55 MHz was chosen for a number of reasons: firstly, it is now relatively uncluttered as most home and automation remote control devices have moved either to 433 MHz or, more recently, 2,4 GHz. For this application, the required bit rate is low and the address and payload packets are small, so it was decided that the bit rates provided by 2,4 GHz was not necessary, over and above the fact that 2,4 GHz needs more expensive instrumentation and is far less sensitive to construction methods.
The main disadvantage of using 403 MHz is that it needs a relatively long antenna (about 18 cm for ¼ wavelength). As the range required is short and data is repeated at regular intervals, the unit is highly tolerant of poor reception as well as signal reflections. A simple way around the antenna problem was to use a helical antenna, which readily fitted into the available space. Helical antennas are very sensitive to their immediate environment and calculation of their design is highly complicated, so empirical testing was a big factor in the antenna’s design.
The transmitter employs the straightforward but well established Colpitts LC oscillator solution using a track on the FR-4 PCB as an inductor. The oscillator is driven by an HF transistor and kept on frequency by a 403,55 MHz SAW resonator. The method of data transfer is OOK (on/off keyed modulation), which was chosen over ASK (amplitude shift keying) as the system was to be kept simple and highly tolerant of component variations.
Monitoring of the tow-bar socket is done by an Atmel AT tiny2313, supplied by Arrow Altech Distribution, with an internal RC oscillator used to minimise parts count. Power is taken by a diode array from all possible lamp connections and regulated, so that no matter which light is on, the unit receives power. The lamp pins are also sent to the tiny2313 where they are read and turned into a data packet, onto which a unique address header and error checking byte are added.
Once the packet is read, it is sent to the transmitter via an open collector transistor, which controls the oscillator’s base bias to stop and start oscillation. The packets are transmitted by a proprietary PWM method designed specifically to be very tolerant of noise and poor signal quality. In addition it has the advantage of allowing the receiver to continuously auto-synchronise; no preamble is needed.
Once again, the receiver was designed with a minimum parts count in mind. The device used is Micrel’s RF011, supplied by MB Silicon Systems, about which the designer is extremely complimentary, emphasising in particular its ease of use, readily available application data and the fact that it is equally at home on double or single sided printed circuit boards. The RF011 has a PLL, which is driven by a crystal in the 6 MHz range. Mixing, AGC and data slicing are all done in the same device.
Once demodulated, the data stream is fed to another AT tiny2313, which decodes the PWM signal, checks for errors and then presents the information on the wiring checker’s six LEDs (see product picture).
The receiver is powered by a pushbutton on, auto off circuit. Due to cost constraints, the simplest possible method was employed, which in this case turned out to be an SMD P-channel MOSFET coupled to a tantalum capacitor. When the on key is pressed the capacitor is discharged, so holding the on key gives the option of keeping the unit on, or restarting the timer. Once released, the capacitor begins to charge. After about 30 to 40 seconds, it reaches the supply voltage and the MOSFET turns off.
As the MOSFET has such a high input impedance, only a small (10 μf) capacitor is required to obtain a reasonable time frame. The tiny2313 has a brown out detection fuse which enables it to cease functioning for the short period when the voltage on the capacitor approaches the supply voltage resulting in undesirable random switching. The MOSFET is directly connected to the battery supply, so the regulator wastes no current when the unit is off.
The supply for the receiver is a normal PP3 9 V rechargeable battery. The life expectancy of the battery is obviously highly variable depending on usage, but the unit only uses about 3 mA with the LEDs off and 10 to 15 mA when they are on. Ultra bright LEDs were used for this purpose, because when biased with high value resistors, they provide adequate light without using much current. Current consumption when the unit is off is in the nano- to micro-amp range, so battery life is maximised.
Proving that every good engineer is really a masochist at heart, the software that runs the tiny2313s was written in assembler in this case. The GNU compiler (WINAVR) used is available for free on the Internet and is a good platform for devising everything from the simplest to very complex systems. As it falls under the Linux umbrella, there are many people constantly adding to and improving the compiler.
For more information contact Ian Ward, High-Tech Technical Components, ants17@iafrica.com, Craig Pratt, Racoon-Tech, capratt@telkomsa.net
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