Solar tracking transmitters (was: Stolen Bots)
- As I've noted, I know very little about radio, so I reserve the right to
fudge and make mistakes, but having had a look at some transmitter
specs, this is the situation as I see it.
What we want - a transmitter with a long range, consume little power,
consists of few components, quick and easy to make, will not drift
frequency. Bonus: Easily able to also transmit information about the bot
Crystal-locked transmitter: Extremely reliable lack of frequency drift,
though the circuits I've seen seem to involve quite a few components,
except for one built around a transmitter-on-a-chip IC, which wasn't all
that suitable for other reasons. If there is a transmitter-on-a-chip IC
that _is_ suitable however, that would be very cool.
Voltage-dependant transmitters: The frequency is dependant on the
voltage - very stable voltage gives stable frequency, but it will drift
if the voltage changes (eg a discharging cap). These designs usually
require very few components however.
Range: Generally it seems the higher the voltage, the longer the range
attainable. I haven't seen anything claiming a range over 800m that
doesn't require 9V. I have seen a nice simple (voltage-dependant) design
that claims 800m at 6v, and one claiming 1-400m at 3v. (Note: these were
made and measured by the same people, in a built-up area, and are
probably the furthest transmitting designs they have at those voltages.
For beam apps, a less-than-optimal antenna is likely to be desirable,
which would reduce the range, sometimes greatly).
Conclusion: I think BEAM tech could probably make a supply voltage
stable enough to reliably run a voltage-dependant transmitter (we are
the Voltage Trigger Enthusiasts after all :). This would significantly
simplify the transmitter circuit and could possibly require no extra
components than would be in the solar engine anyway. Super-simple
circuit idea: The transmitter is powered by a 1381 connected to a cap.
When the cap reaches trigger point, it transmits for the hysteris (sp?)
of the 1381 (ie a very brief pulse). While 1381's might vary from each
other, I'm guessing they trigger pretty reliably at the same voltage
each time. Are they much affected by temperature? Perhaps that cap could
be constantly being recharged by a 1F cap (in turn connected by a solar
panel) and thus be able to run day and night. How to slow down the
recharging of the drive by the 1F cap is something I haven't thought
much about yet.
Regarding range - is there any easy way to ramp up the voltage from the
1F cap (ie likely to be about 3V due to the solar cell)? If the "drive"
cap could charge at 9-12v, that would allow for a long range transmitter
(though the 1381s wouldn't be much use any more).
Regarding transmitting information, this is likely to be quite easy.
Even if the transmitter is only sending a beep every second or so, you
can easily an independently vary the beep rate, and beep tone, giving
two values about the robot status. Also, adding a microphone is
simplicity itself, but for a small solar powered bot, a continuous
transmission would probably not be sustainable 24/7.