Hello All,
This is my first post on this forum. I am in a
science competition next month and have to create an
electric, propeller-driven car that travels a meter
in an amount of time that won't be divulged until
after our vehicles are impounded by the judges.
Therefore, we must be able to adjust the car on the
fly at launch time. We have tried a million things
and haven't had a lot of luck. We used a rheostat to
adjust the voltage to the motor. We adjusted the air
intake area of the propeller and even the angle of
the propeller relative to the car but the results
aren't near as constant as we need, sometimes
varying by 30% between runs. We had an epiphany
last night but don't know how to implement it.

At full speed, our car can travel the meter in
about 2 seconds. We thought we might be able to use
an adjustable circuit (maybe an RC with an
adjustable pot) to create a delay between cutting
our car on (the timers start then) and when enough
voltage is delivered to the motor to make it go
(around 6.5 - 7volts). We don't have a lot of
electronics experience so any help would be greatly
appreciated!

Here are the details:
Time requirement: 5 to 15 seconds
Voltage: 9V DC
current: App 4A
Motor/prop: standard speed 400 brushed motor and 5"
prop for small hobby planes.
NO ICs allowed (I guess that means we can use
resistors, capacitors, inductors, diodes etcetera).

We tried studying up on simple RC circuits and even
experimented a bit but we couldn't get the motor to
turn for some reason. We built a simple series
circuit with a 10 ohm resistor and 1000uF cap. We
attached the motor in parallel with capacitor and
calculated that it should've taken app 12sec to
start the motor but the prop just barely turned.
Again, we appreciate whatever you can do for us.

 

use full speed/high power fan setting and direct half the air backward and half forward for complete stop. direct all air backward for full speed and vary the mix for any speed in between. simple air vane mixing valve and thumbscrew for locking. Lots of trials should let you develop a scale for the mix that can be applied to show where to set the valve for any time wanted.

 

Do you start the run with a switch?
These timers can use a scaled rheostat.

 

how about just cutting down the size of the prop till it meets the timing specs? make sure you have spare props first.

 

how about just cutting down the size of the prop till it meets the timing specs? make sure you have spare props first.

I can't see that being very scientific; I'm picturing a kid with a pair of dykes chopping off whatever "looks good" and then crossing his fingers. The only way your suggestion would dependable is if he were to go through tens of props to generate the logarithm table required to know exactly what diameter fan creates what run time, and then bring along a benchtop lathe to machine the prop to the exact diameter needed at launch time. I'm sure you'll say "well it's simple, bla bla, wind speed, bla cube of diameter, bla bla just use an equation bla." Well, if it were me, I wouldn't leave the fate of the golden trophy to a formula for theoretical force of a perfect blade, not accounting for the fact that the blades don't extend all the way to center, or rolling resistance or drag coefficient of the car, et. al..


I think he's settled on the best solution, so long as there's no penalty for sitting idle at that starting gate for the first period of the race.

 

Kermit2 & Alfacliff: I didn't mention the other problem with the rheostat and other "thrust adjustment" methods in my original post. At the long end of the required time (above, say, 9 sec), a weird thing happens. The amount of thrust required to move the vehicle at all (i.e. overcome the static friction of the car) will then move the car way to fast to reach the required time. Conversely, the setting of the Rheostat that will provide the precise amount of thrust needed would then require a slight nudge of the car (a big violation) to get it moving. Therefore, we abandoned the thrust-adjustment methods.

Inwo: I'm not very familiar at all with Mosfets. I'll have to chew on that one for a while.

Strantor: Your closing sentence brings up an excellent point! I'm not sure if the judges would consider the car as having been "started" if it sits there initially as if it is completely turned off. I wonder if, instead of a square wave where Vcc is suddenly present after a period, if there could be an initial, slow ramping up of the voltage. That way, the propellers would immediately start turning (indicating to the judges that we have started) but not go anywhere until the voltage gets to the required level.

Thanks for your posts!

 

Can we leave the battery at the starting line & run on capacitors; maybe 100,000 μF with adjustable charge voltage, OR'd with another cap. of about 20,000μF charged to full 9 V as a kick starter. Adjust V with a pot calibrated in time vs 1 meter. Tried with small stationary motor driving 5 in prop ?? Or calibrate charge on a Ni-Cd. Might use a NC lever SW with a latch, [ like a pin on a grenade ], to keep SW open untill manual operation takes over at start. This would allow a release at start rather than a push to start.

 

Inwo: I'm not very familiar at all with Mosfets. I'll have to chew on that one for a while.



Thanks for your posts!

It's just a time-on or time-off delay.

Transistor circuit works just as well.

Start under full power and stop powering fan after xx sec.

Or if you prefer:

Delay start xx sec.and power all the way to finish.

 

If you want a specific timer circuit using parts that you have.
Someone will put a diagram together for you.

 

Use large diameter hard wheels if you want to avoid static friction at the beginning of your run. CDs or DVDs make excellent wheels for these "races".

 

Don't over think this:

1 - make car go as fast as practable

b - add turn on time delay circuit with a variable time

III - set time to race day rate minus fast transit time

 

Don't over think this:

1 - make car go as fast as practable

b - add turn on time delay circuit with a variable time

III - set time to race day rate minus fast transit time

That's exactly what he said he was going to do in post 1. I dont know why anybody is suggesting any other solution.

 

That's exactly what he said he was going to do in post 1. I dont know why anybody is suggesting any other solution.

I agree.
I was just suggesting methods that might follow the rules and ability of OP.

 

Sorry if I offended... glossed over most posts when it seemed all were PWM methods.

Now where did that post go with a 555 timer done with discrete transistors go...

 

Now it will be a race for the simplest discreet component timer.

 

And my entry is:

A relay with a 250 ohm coil can have a bank 3300 μf switched across it.

Each 3300 will give about 3 sec of run time. (delay off)
Or ≈ 1000 per second.
Jumpers or a 10 position selector?

No solid state components at all.

 

Here is a fairly accurate tested delay timer. Battery is 7 AA Ni-MH , 9.2 V unloaded, 8.88 V loaded. One glitch, if motor is not loaded enough , SCR can cut out; with 5 in prop , SCR held in most of the time, added 120 Ωs, R6 with no further problems. PUT, programmable unijunction transistor, is also available from Electronic Goldmine as is an SCR sim. to the R-S.

 

Why does it need a timer?

The vehicle speed will be based on motor RPM. Motor RPM is based on load and applied voltage.

For a stable applied voltage the load will be stable as it is a fan.

So all he needs is a good adjustable voltage regulator. Maybe 3 transistors.

Then; adjust the volage = adjust vehicle speed = win.

 

I still believe that Piddler's idea of delayed start with 2 sec run will be the most reliable; for some indication that a start has started, how about a flashing LED? One more PUT & darlington transistor, MPSA 14, or FET, BS 170 used for test. Pulsing the motur is a little more complicated, but is possible. Schematic on request.

 

Maybe a shutdown circuit? Delivers enough power to get started, reach speed, then coast the remaining distance. Also what is the goal of this competition? Speed? Innovation?