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Extreme newbie needing project advice

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davets

New Member
Hello all. I'm not really experienced enough in electronics to even ask these questions but I've read many posts on here and I don't think I could find more qualified help. If you have time and can respond I would appreciate it.

** I'm building an electric pinewood derby car. I need 2 switches that operate with light pressure like micro switches but are in the ON position until depressed. One switch will be in the nose of the car. The weight of the car on the starting ramp pushing forward against the starting pin will hold the switch in and OFF. When the gate drops the switch will pop out and turn to ON position.

** The second switch will be on the bottom of the car and when the car reached the end of the track, a raised center rib in the middle of the track will depress the switch and turn the power OFF.

** I'm looking at finding a small, dual shaft motor, maybe 1.5-3v ,that would have a shaft running out both sides that I could attach the rear drivewheels to.

** I need a battery to operate the motor. I see 4.0v, 2200 mah electronic receiver batteries for rc cars. Would this work with a 3.0v motor? Can it be used for a lower voltage motor? The block of wood that this will all be mounted in is 1 3/4" wide. I've got a couple old 7.2v rc car motors and batteries but these present 2 problems:
1) size of both motor and battery
2) If I ran straight 7.2v to the motor I think it would be way too fast.
This would also require an axle with spur gear which means more space needed.

Thanks for any time you could give me. Even if you just know of a good source for components that might work I would appreciate the direction.
 

alec_t

Well-Known Member
Most Helpful Member
I need 2 switches that operate with light pressure like micro switches but are in the ON position until depressed.
Standard microswitches have both a NO (normally open) and NC (normally closed) contact. You would use the NC contact for your purpose.
Can't you use 2 AA cells to drive your 3V motor?
 

dr pepper

Well-Known Member
Most Helpful Member
Welcome to the forum.

You can overvolt a motor a little for a short time, the life might be lessened but for this it might be worth a try.

You can get V3 or V4 microswitches with either a button or maybe better a leaf actuator, a piece of metal that actuates the switch, you can bend it to suit.

You'll also need a relay to keep the motor on till the stop switch is operated, a similar idea to a on/off switch for a pillar drill.
 

davets

New Member
Thank you. I went to a surplus electronic store this morning and found the microswitches I think I'll use. One has just a button which will work on the bottom of the car and the other is like was mentioned above; it has a metal lever that pushes against the button. I'll use this one in the front so the starting pin has something substantial to push against.

The store owner used an ohm meter and showed me how the switches can be wired to be either ON or OFF when depressed.

I bought a housing for a small, 3v pancake battery. I'm not sure how long the battery will last running the motor down the track for around 3 seconds but I'll experiment. I don't know the voltage of the motor I found there but its a dual shaft. The store owner ran it at 3v and she really wound out. I'll try it with the 3v battery and if the run time is OK I'll add weight to the car to slow it down if need be. I may also try a 1.5v battery.

What is this relay you're talking about and how does it work? Where in line does it get wired?
 

dr pepper

Well-Known Member
Most Helpful Member
I dont know if this is too much for you or not, have a look at the first schem on this page:

https://drstienecker.com/tech-332/5-ladder-logic/

The start and stop switches are your microswitches, you could put your motor directly across the relay coil, 3v relays however are uncommon.

You could do it with a c106 thyristor and a maybe resistor if your feeing adventurous, scroll half way down this page and you'll see a schematic exactly what I said.
 

rmn_tech

Member
Why would a relay be needed ? a simple circuit surely. 2 n/c switches, motor and battery in series will do the job as the o.p. stated.
one switch is off (depressed) until the start pin on the ramp is removed this will then turn on. The second switch turns off when it is depressed at the end off track center rib.
 

dr pepper

Well-Known Member
Most Helpful Member
Right, I made an assumption maybe incorrect, I assumed the rib that switches off the motor wasnt continuous, if it is then forget the relay amd thyristor rmn is right.
Just wire the motor through the normally closed contact on both switches, so either of them pressed stops the motor, simple.
Us engineers like to overcomplicate things
 

rmn_tech

Member
Dr P.

I thought that either there was a misunderstanding or like myself on many occasions trying to use a sledgehammer to crack a nut. :banghead:;):)
 

davets

New Member
I'll try the two switches w/o relay and see how they work. I have no idea what this motor will do to this car for speed. I could try to gear it down if I have to but may just lower the voltage if needed. Would a smal pancake style battery run this small motor?
 

dr pepper

Well-Known Member
Most Helpful Member
Depends on the max current the battery will provide, if it spins the motor well it'll probably do something.
Sounds like your talking about a memory battery like pc's used to have, there should be enough go in one of those to shift something small, leave yourself room in the car for something bigger if needed.
You might be able to gear it down by using an o ring or elastic band from the motor to a wood pulley driving the rear axle.
If you can get a dead video recorder the motor that laces up the tape is usually a worm drive type affair, I dont know if this would be the right kind of speed you want, depends on the size of the wheels.
MFA do a range of dc geared motors, and I think they do one called a 'clearbox', sort of a plastic gearbox where you can change out gears to get different speeds, I seem to remember the shaft can be pushed right through, ok for wheels to go on.
Here: https://www.ebay.co.uk/itm/1-5V-6V-...ertechnik-370161-/121013780678#ht_2473wt_1003
 

davets

New Member
That Clearbox looks like the ticket if the shaft ran the other direction. It looks like its 4" wide or so. I've got 1 3/4" width to work with.

The batteries I was thinking of were 3.0v like the ones in watches or electronic testing tools.
 

audioguru

Well-Known Member
Most Helpful Member
A 3V coin cell battery is made to supply a very low current.
Don't guess.
Look up how much current the motor uses in the motor manufacturer's datasheet.
Then find a battery that can supply the current in a battery manufacturer's catalog.
 

davets

New Member
I'll try to find it but this is a motor from a bin at a surplus place. I don't see any numbers on it. By current do you mean a Mah rating? Thank you all. I'm learning what I'm sure is basic stuff for you all.

I've also found a mini 12v motor for a slot car that runs 25,350 rpm. Would this give me more options for power supply?
 

alec_t

Well-Known Member
Most Helpful Member
If space is very restricted a 12V supply able to provide the current you are likely to need is out of the question. A 12V motor is unlikely to perform well at less than 6V or so.
 

audioguru

Well-Known Member
Most Helpful Member
Since you do not have specifications for the weight of the car, the power of the motor or the ratings of the battery then your project will require a lot of testing.
Build a car, try it with a dozen different unknown motors then try the motors with a dozen different batteries. Try it with and without gears or pulleys.
Then do it again with another car, over and over.

In engineering, we obtain or we research then we make a detailed specification of the required performance of a product (your car).
Then we select parts from a catalog and with detailed datasheets we design the product. We build the product one time.
We test the product thoroughly to make sure it meets the spec's and is reliable then we are finished with this design and go on to the next project.
 
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