Trying to fit the functionality into the existing ebay trigger receiver housing seems to be a royal pain in the butt, especially trying to work in with the batteries, so after a few days of PCB redesigns and juggling compments around I have put that particular design on hold and moved forward with my original PIC16F873A design, also I did not like the fact that there was 2 circuits to maintain, a transmitter and receiver, this then leads me to the next problem. I want something easy for the end user, something similar to the Pocket Wizard Plus that is really a no brainer for the user to drive, i.e. a switch to select operation mode and channels, a 3 and 4 way slide switch respectivly, well do you think I can source any 4 way slide switches easily? Seems I can get some 3 way easily enough, and I guess I could use a rotary switch but these are too clunky for my liking, but still an option.
I’ve done a bit of further research and I think I will use 2 push buttons (one for each of the functionality) that will just toggle throught the channels and mode, and some LEDS to display the current setting using charlieplexing in which I can use 3 I/O lines to control 6 LEDS, this gives me 4 for the channel display with 2 additional that I may use for the mode, but with charlieplexing it’s really only useful for driving a single led at any one time, not a combination, i.e. good for just the channels, then another 2 I/O may be used for the 2 mode LEDS. So for my UI Feedback, I’ve used a total of 5 I/O, which then begs to question, with another I/O pin I could add a LCD running in 4-bit mode that would be totally more useful. I may add an optional LCD so it can be built using LCD or using LEDS.
I’ve reworked the strobe trigger circuit to now handle 400V isolated, which should be plenty of protection for any strobe, old or new, also isolated the input from camera/manual trigger circuit.
Now started reading up more on the spec sheets on the HopeRF module that arrived last week and if I get time this weekend will started playing with it and see how I can get it talking to the demo unit that I also purchased, that way I can test the Tx and Rx side of things, and start testing ranges, throughput, interference handling etc.
This has then got me thinking of how I can handle interference which will almost always be there at different frequencies due to the nature of the ISM bands. The easy option is a number of channel selections that the user selects if there is noise on the channel, but how many do you have for the user to select? and at which frequencies? In theory the current ISM band I’m working in, 902-928Mhz, could have anywhere between 25 and 50 channels depending on the channel bandwidth that is chosen. Aparrently the FCC (need to find out Australian equivilent) have set the number of channels must be at least 50 if the bandwidth is less than 250Khz, and at least 25 channels is the bandwidth is greater than 250Khz ? So I though some sort of frequency hopping that is transparent to the user. I would like to implement this in the unit as I know it will be more robust against interference. So instead of say ‘X’ fixed channels that the user can select through, I could have ’X’ different hopping schemes that are user selectable, so really these are virtual channels that cover different random frequencies in the current band, but the receiver will need to somehow find and syncronise to the transmitter.
The model of HopeRF trancievers that I’ve purchased states that it is 915Mhz, which is the centre frequency of the ISM band that covers the 902Mhz – 928Mhz band, from the specs they look as though they can have selectable frequencies so in theory I should be able to select any frequency within the band, but need to look into it further.
Anyway with all that in mind I’m endeavouring to start playing with the RF module and the PIC this weekend and at least get things talking (depends what the boss has planned I guess lol)
- RFM12 Tutorial – Part1
- RFM12 FSK Library – Alpha Release
- Frequency Hopping
- PIC, SPI and the RFM12