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- OT: Quiz Buzzer
October 12, 2012, 6:34 pm
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There are a lot of digital circuit designs out there that implement a
quiz buzzer. The basic layout is N buttons and N lights. Whichever
button is pressed first will light the corresponding light, and the
remaining buttons are locked out. There is generally an additional
reset button. Easy enough.
Now does anyone have any ideas how an RF wireless scheme could be
implemented with one of these circuits? Meaning, the physical button
would be replaced with a receiver sensitive to N transmitters within a
given range--hopefully around 40' or so.
The solution should be cheap as well--not pennies per transmitter cheap,
but I don't want to have to buy N wireless keyboards and tear them apart.
Thanks for your consideration of this problem.
Re: OT: Quiz Buzzer
If I was experimenting with such a system and given your constraints I'd
probably start at Sparkfun Electronics. They have RF data transmitters and
receivers which would probably do the job;
https://www.sparkfun.com/products/10534 for example. Couple a transmitter a
code generator, a button, a battery, and a few passive components and you'd
have the sending end to experiment with. Couple a receiver with an Arduino,
some clever code and a few high-intensity LEDs and you could cook up a
receiver with the proper priority encoding.
On the other hand one could start with some JeeNode bits and pieces from
Modern Device http://shop.moderndevice.com/products/jeenode-kit and you
could do essentially the same thing with possibly a bit less brain sweat
but for a bit more money. It all depends on how much time and effort you
are ready to put into the project.
Re: OT: Quiz Buzzer
Your application does have some tight requirements.
1) Transmitters cannot interfere with one another in such a way,
that one of the participants has an advantage over the others.
This eliminates some "shared radio channel" approaches. And
having watched "Reach For The Top" type competitions over the
years, yes, participants can be extremely concerned about that.
Any hesitation on switch input, is viewed as "cheating" on the
part of the person providing the infrastructure. This is why simple
minded wired schemes are preferred.
2) There cannot be too much variation in timing, or there could
be the concern as in (1). Imagine one person's wireless transmitter,
takes a few more symbol transmissions to be recognized, than the
person next to them. The latency doesn't have to be zero, but
the latency variation must be small. 101ms, 99ms, 100ms, from
button push, to "light" on the other side.
So actually, your problem is tougher to solve properly, than
you think. To make the participants think the quiz is fair, the
"magic button" has to be flawless, press after press, no latency
variation, and no bias at the resolver on the receiving side.
Imagine you press the button the first time, the channel "farts"
and the button press is not detected on the other side. That
infuriates participants. The solution has to be flawless, or
the person building it, gets labeled "an idiot" :-(
You can get cheap FM transmitters.
(Amazon.com product link shortened)
I don't know what the cheapest FM receiver is.
The transmitters in that case, can be tuned to an idle channel.
One participant at 98MHz, another at 102MHz, another at 106MHz etc.
If you're working in a room where you find an FM receiver
doesn't work (radio station signals blocked by walls), so
much the better. Otherwise, if there is external interference,
there may be a bias where some participants get their signal
through better than their opponent.
To communicate on the FM channel, you need a signal. This could
consist of a tone burst. It could be a single sine wave (since each
participant has their own FM channel). It could be a DTMF tone burst.
You need some electronics on the transmitting end, to send the tone.
And something on the FM receiver end, to detect the tone. You can
use a PLL for example, to detect a sine wave. The problem with
that approach, is some variation in lockup time (could add
about 100 milliseconds latency, just picking a number from
the air). I've experimented with NE567 tone burst detectors,
and performance wasn't that good at all.
Another approach, is to use a tuned filter at the receiving
end, and envelope detection. When effectively you see a
"carrier" coming in on the receiver, that is declared
as a button press. That might give a more predictable response.
I could see you wasting a lot of money, trying to perfect this.
There is a good chance the first one you build, will be rejected
by your users. If it isn't fair, if it doesn't work correctly
on each and every button press, you'll quickly get a "label" for
your efforts. So don't show them your design, until it's
thoroughly tested. And I mean thoroughly. Back in our
Reach For The Top days, there were accusations of less than
fair button systems. And those were wired ones. Throwing
radio into the mix, does not help matters. It only makes