Does this switch exist?

It may just be my google-fu failing me, but I can't find switch I am sure must exist:

A sliding switch with reversing (DC) polarity.

At one end of the slider, power is full "forward".  Slide the controller, and power decreases.  Reach the centre, and power is zero/off.  Slide past the centre, and power increases again, but with reversed polarity, ie in reverse.

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kelseymh5 years ago
I know you marked Canucksgirl's answer already, but I didn't see this idea suggested.

Use a "volume control" linear potentiometer. Mouser has a whole range of slide potentiometers. The first one on that list even has a center-detent option, to give you a tactile "center off" condition.

Wire it up with, e.g., +5V at one end and -5V at the other. Make sure the housing is grounded to the chassis, and choose one with an insulated lever, so your users don't get a little zap each time.
Kiteman (author)  kelseymh5 years ago
That looks right - will it do the same thing as the joined sliders in the circuit Canucksgirl found?
Yes, with one difference: The short block of insulator put between the slider housings gives you a whole "chunk" of OFF, rather than just a transient position.

Mouser may have better options for this, already built. +/0/- is very common for audiovisual controls.
Kiteman (author)  kelseymh5 years ago
Sounds ideal.
I know I'm kinda late getting here to comment on this one, but I'd like to offer my suggestion, which is just to use a slider type potentiometer wired as a voltage divider, and then follow that with something that looks like a push-pull amplifier.  That way you'll have something that can source (or sink) enough current to drive your motor. 

The basic  shape of this thing is shown in this picture:
from this article:

and I have taken that basic circuit and sort of adapted to your power supply and motor, choosing/guessing some components that might work for this.

Guessing the transistors have to beefy enough to handle a few tenths of an ampere, or 100s of mA, of current, and to dissipate a few watts of power.  Also it would be nice to make them a complementary pair; i.e. the PNP transistor has the same specs, but like reversed, as the NPN one.

The op-amp I picked,  TLC272, is one I've used before that seems to work well from a low supply voltage, in this case just 6V (from the +3V rail to the -3V rail)

Note this kind of amplifier (well follower actually, amplifier with gain = 1, Vout=Vin) necessarily wastes power unless the transistor is completely on, or completely off. Also note that only one transistor turns on at a time.  When supplying Vout>0, the NPN transistor will be part way on, with the PNP completely off, and when supplying Vout<0, the PNP transistor is part way on, and the NPN is completely off.

As others have noted, the efficient way to drive a motor is with pulses, so that the transistors are always completely on, or completely off, and little power is dissipated by transistor this way.  But of course the more efficient way tends to be more complicated.

Anyway, the circuit I have presented here is the most simple thing I can think of, that I think might actually work, and do what you want it to do.
What power and what budget are you thinking about? Most potentiometers will only handle low currents.

An old fashioned mechanical speed controller as used in RC does the trick for larger currents. For example:

You could use an electronic speed control with a servo tester as input.
Kiteman (author)  masynmachien5 years ago
Low current is all it is - 3V motors running from (probably) AA batteries.
OK, I gues you are talking abut some kind of "toy motors". These easily draw a current around a couple of 100 mA. At 333 mA the power is 1 W.

This also means the resistor value of the regulator should be in the range of 100 Ohm. At 3 V this 100 Ohm + the resistance of the motor gives less than 30 mA, which is rather low for many DC motors to start running. Unless we are talking of really low current motors

High wattage and low resistance is not common for potentiometers. Most start at 1 kOhm, and supposing they can handle the current, this means the motor will only run near in about 10% of their range near the "full on" end point.

I would advise to go for switches/controllers meant for motors, like in model trains or RC cars/boats.
Kiteman (author)  masynmachien5 years ago

(This whole question has been a learning experience)
canucksgirl5 years ago
Do you mean something like a model train controller?
This one has a dial control (versus a slider), so I don't know if that helps you or not... (but I know I've seen them with a "T" slider operating in the same manner as you describe).
Kiteman (author)  canucksgirl5 years ago
That's the kind of functionality I'm after, but I was after something smaller, and way cheaper.

#2 son & I need 4 for a project we're planning.

Kiteman (author)  Kiteman5 years ago
Oh, and we need it to be a linear slider, so that the four side-by-side can operated sort-of-at-once on a controller.
Ok (disregard that last one then...) ;-)

I also saw this one, but I'm not sure how this one works?
Alright... one last idea.

On this site, the OP was looking for the same thing (more or less), and the last person replied with an idea (that I think would work), along with the schematic. So if you want to build it yourself, it'll likely be a lot cheaper.
Kiteman (author)  canucksgirl5 years ago
Oh, that looks like a logical step.

I think we've settled on simple control first, like this, then your idea as long as the first stage shows promise.
I'm glad I could help.

Looking forward to the Ible when its all done. ;-)
How about this? It's £4.99 on U.K. eBay, and never used...
Centre must be definitely "off " ?

Kiteman (author)  steveastrouk5 years ago
That was the plan - this was supposed to be a simple idea to control a few motors.

Push the slider to one end, motor goes forward quickly, slide it back, motor goes slower, keep sliding and the motor goes into reverse. It seemed logical that centre would be "off", but it's not vital that there is an actual "off" position on the slider.

I'm starting to think that the prototyping stage will simply use something like this to just go full-on in either direction.
frollard5 years ago
It could be done with a center off linear pot (not sure if it exists, but if not, a person could fudge it with a linear pot)...

if you can come up with a double pot then you could use the input from that slider to drive a 556 or 2 555's in a pwm configuration - each with reversed polarity.

Otherwise, it would involve using (in my opinion) a microcontroller such as arduino or your favourite flavour -- and using linear pots and have the duino drive a motor controller h-bridge with the analog outputs. You could use standard cheap sliders and add manual detent do the middle - and do all the processing in software - not too expensive if you build it yourself.
rickharris5 years ago
your looking for a double pole double throw - possibly with centre off.

example DPDT
Kiteman (author)  rickharris5 years ago
No, I knew about DPDT switches - I was hoping that there was a switch that combined the functions of a DPDT with that of a variable resistor - a near-continuous slide from max forward to max reverse.