For free ? :) Vasile On 1/24/07, Rolf wrote: > Hi all. > > I am looking for some comments and criticisms on the design of a 418MHz > wireless transmitter for a hobby project. > > I am nearing the end of my first "Major" project. Consists primarily of > an "advanced" shutter control system for my camera. Will control long > exposure photos, time-lapse, and other modes. One 'mode' is to have a RF > wireless remote for the device. The camera has an IR remote already, but > this requires the operator to be in front of the camera, and is limited > range (about 5 meters - with the OEM remote - my home-built IR remote > has about 30m range). > > The wireless component is new to me in many ways. I decided to go > 'medium simple', and use the chips from LINX tech: the TXM-418-LR and > the RXM-418-LR as 418MHz transmitter and receiver respectively, using > the LINX "splatch" antenna. I have carefully implemented everything as > much as possible according to the datasheets and application notes from > LINX. The microstrip trace on my proposed PCB connecting th TX and > Antenna will be less than 0.2", there is as large a ground-plane as > possible, etc. > > LINX 'advertise' that if you have a 1-wire 'serial' communication link > between to processors, you can simply replace the wired link with the TX > and RX modules, and it will work up to 10kbit. Well, I have the > prototype system running with a single wire (in a breadboard), and it > works, so I want to now implement it with RF instead of the wire. My > plan is to build the 'final' version of the remote control (TX), and a > "break-out board" receiver module to help me debug the RX side on a > breadboard. > > The attached circuit diagram is of the transmitter, as well as the top > and bottom board layers. The entire board is 1.9" x 1.2", and fits in > the C-4 box from Serpac. All produced with Eagle 4.16 > > The Major components are: > PIC10F206 > TXM-418-LR TX chip > the matching LINX "Splatch" SMT antenna > A Momentary switch N.O., pulled low with a weak pull-down. > a CR2032 Li. Cell. > a Schottky diode (will be explained later) > a 5-pin In-Circuit PIC programming header (The pin order is non-standard > - it is easy to route on the PCB though). > > I have implemented a concept of an "Analog Ground". All the ground > connections for both the TX and the antenna are fed through to the drain > of an NMOSFET. The ground-plane of the entire board will be this > "Analogue Ground", and NOT the negative terminal of the battery, or Vss > of the PIC. > > The basic operation is as follows: > The PIC10F206 sleeps in normal operation. This consumes 0.1uA typically. > MCLR is pulled high with internal weak pull-up. GP0, GP1, GP2 is set as > high-impedance input and will be pulled low. > Normally the NMOSFET Gate is pulled low by the 1M to ground. This > "disconnects" the "Analog Ground" from GND, and isolates the transmitter > and antenna from the rest of the circuit. > > Pressing the button wakes the PIC, which sets GP1 and GP2 to output. It > sets GP1 high, saturating the FET, and connecting the TX and antenna to > ground, thus powering up the TX (and illuminating the LED). > After a delay to allow the TX to stabilize, the PIC bit-bangs a serial > data stream over GP1 to the TX (2 bytes + sync at 1200bps is less than > 20ms). > After the transmission, it reverts back to it's high impedance sleep > mode (effectively powering off the TX, and disconnecting it's ground). > > There are three major considerations I have when I designed the circuit > this way: > The TX is Vmax of 3.6V. > I will almost certainly need to re-flash the PIC a number of times as I > work out issues (in fact, the first program of the chip will be in > circuit - SOT 23-6). > Power consumption must be low. > > In order to keep low power consumption I use the MOSFET to low-side > switch the entire TX system. Only leakage current will happen. > Additionally, all inputs are put in high-impedance, and pulled to ground. > MCLR is pulled high (internal weak pull-up). > > The ICSP header will be available for (re-)programming easily. The > Schottky diode protects the 3V cell from the 5V programming voltage. > > My real concern is the 3.6V Max for the TX chip. When programming I have > 2 concerns: ICD2 will pull Vdd to 5V. Because the ground part of the TX > circuit is isolated by the MOSFET, I believe the TX will be isolated > from the 5V. There are three possible problems with this though: > First, will GP2 be set output-high at any point by programming the chip? > Second, After programming, the PIC will be reset (remeber, Reset is what > happens after a sleep in the PIC10F series - it does not continue from > the instruction after SLEEP), before starting the transmissin process, > the PIC checks to ensure that the trigger pin GP0 is high, but perhaps > the ICD2 will leave the data line high (or will maybe pulse it or > something). > Third, 5V programming Vdd will charge the capacitor... activating the > circuit after programming will cause the capacitor's charge (at 5V) to > potentially destroy the TX. > > Is there a better way to ensure that programming the PIC does not damage > the TX? > > Thanks for any comments/critiques of the board. > > I know this is a big request to put on the list, and I appreciate that > this is a long post... but, the next step is to build a 3" x 4.5" board > which will be much more densely populated, and I need to get the process > right before I mess something up there. > > Thanks all > > Rolf > > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > > > > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist