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Hydroaeropropulsion Rocket Research Labs
homepage!
16 november 2002
To measure the height of our rockets, we will try and build a little electronic
circuit that emits sound pulses at regular intervals. If we attach this
to the rocket, we can determine the height of the rocket just by listening
at which instant in time the pulses arrive at the ground (preferrably using another
piece of electronics we still have to build). The principle is simple: we know
exactly at what time the rocket emits a sound pulse (because we made
the circuit in the rocket to do this at very regular time intervals),
we know when the sound pulse arrives at the ground (using the second piece
of electronics) and we know the speed of sound. And if you know a time
difference and a speed, it's simple to calculate the distance (in this case:
the height of the rocket).
The plan is to let the rocket emit about 1000 sound pulses per second.
Because it's very likely that the detection circuitry on the ground
will miss some pulses now and then (because of background sounds)
we don't emit every 16th pulse. So we emit 15 pulses of sound and then
do nothing for the duration of another pulse. This way, we get a
"synchronisation silence" every 16/1000th of a second (this is about 60 times
a second). So we should be able to cope with background noise that obscures
the beeps for about 1/60th second. Since this is not a very long time,
we repeat the same trick: we emit 15 blocks of pulses (i.e. each block
consists of 15 pulses and one "synchronisation silence) and then
remain silent for one block. This way, we get a "large synchronisation silence"
every 16*16/1000th second, or about 4 times a second.
Whether any of this will work out in practice remains to be seen of course ...
This is a piezo-electric beeper we just started dismantling.
You can see clearly the electronic circuit.
From left to right: power suplly wires, electronic circuit, and the beeper
itself. The electronic circuit is what we don't like: it converts
12V DC into pulses (so the thing beeps when you put 12V at it). What we want
is to drive the beeper itself directly, so we can control the frequency very precisely.
As a side effect, the beeper is a lot flatter without all that plastic packaging
and circuitry!
Shame they don't sell beepers without all this circuitry.
We need only a little piece of breadboard.
Building in progress.
This is the status at 17 november. Getting all those wires right is a
story of much fiddling and a bit of careful planning.
