Well, the cost of a receiver in each box is about $8, but then you have
to add additional code, etc.

Digikey apparently carries some citizen oscillators, CSX-532T SMT TXCO
which have +/-1.5ppm at 25+/-2C and 2.5ppm over the entire temp range
(-30c to +75C).

2.5ppm means they could be off by +-9mS after one hour.

They cost $9 each, though.

But for small teams (10 on each team) and relatively fast transmitters
with small data packets you could easily get an update from each
transmitter more than once a second.

However, I figure that a two-way system is probably better overall
anyway.  It does make the code a bit harder (synchronizing slaves to the
master is non-trivial - doing so over radio is harder), but the benefits
can outweigh the cost
* Have a light on the player's control telling them who has control.
* No need to hand synchronize them.
* Overcome issues such as spurious reset of the control without
retrieving the box.
* Put flash in them, and remotely update for a new game type
* They can contact each other (interesting uses)
* for simple games, no master needed - one 'slave' can assume
mastership, and the lights can tell who pressed first.
* Add an LCD so players can see 'secret' information

For one off cost would be:
Radio (RX and TX) $10 (cheap laipac modules)
Flash PIC, crystal, LED, PCB $10
Project box with membrane keypad $8

So for under $30 per box you can build a full system with all the
flexibility one could want.  Add some simple encryption routines so it's
not trivial to fake a fastest press...

If I only had the time.  Wired quiz show boxes are sold for over $300
for 8 players in two teams.  Just have to make them really durable...

-Adam

Bob Ammerman wrote:

>>A more complex, but deterministically simple method is to have a PIC in
>>each transmitter.  Each PIC has a crystal for accurate time keeping.  At
>>the start, all transmitters are given the current time (to the
>>millisecond, perhaps) and a time slot in which they are able to
>>
>>
>transmit....
>
>I'm afraid this won't work too well as is. Assuming the time is reset in the
>transmitters once an hour, and that the timing is controlled by a standard
>30 ppm crystal, then the error after one hour could be as large as:
>
>    3600 seconds * 30ppm = 0.108 seconds
>
>Obviously this is *not* good enough.
>
>Let's see if we can use this trick to move in the right direction however.
>
>I'd look at using a rfPIC12F67x transmitter and rfRXD0x20 receiver in both
>the slave and base units.
>
>Now the base unit can regularly send out a poll that informs the
>transmitters of the current time.
>In response to that poll each transmitter sends a packet in a particular
>timeslot. That packet can include the most recent keydown time.
>
>A couple neats tricks here are based on the fact that the polling occurs
>continuously. This allows the slaves to keep track of the current time. If a
>slave misses  a poll or a few due to noise that is ok. It still transmits in
>its standard timeslot. Similarly, the master can run thru several poll
>sequences before accepting what looks like the first-down from all the
>slaves. This allows for packets missed by the master.
>
>Finally, there is a neat way to set up the Ids in each slave:
>
>In configuration mode, the master repeatedly sends out a special packet
>that, if effect, says: "Setting up unit N".
>
>When you press the button on a slave while the master is transmitting that
>special packet, the slave then simply stores "N" in its eprom to track its
>ID (which determines its timeslot).
>
>Note that everything runs on one frequency.
>
>Bob Ammerman
>RAm Systems
>
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