Switch Confusion

You can use a momentary DPDT, which returns to center off when released, or two DPST momentary push buttons. If you are using pushbuttons, wire them as if they were single DPDT switch for reversing.

HTH

I would use two power supplies.

-12 and +12. Connect common to the motor and feed either + or 1 12 from the pushbuttons. Just be careful about pushing both buttons atthe same time.

Howard Garner

If you can come up with a +/- power supply with center ground you could do something like this. Bs are the push buttons.

-V___B_______B___+V | | Motor | | ground 0

On 2/9/2009 9:54 AM Chris spake thus:

Here's how I would do it:

Size the transformer to the voltage needed for the motor (i.e., a

12-volt transformer for a 12-volt motor). The diodes are small silicon units (1N4001, f'rinstance). You could add filter capacitors, but they're probably not needed.

Just don't push both buttons at once: you'll get n volts ac across the motor, which won't like it.

(I'm assuming you know how to wire up a simple circuit. If not, it ain't rocket surgery and information is widely available.)

Well I guess I am going to go with a dpdt momentary switch. Really did not want to, but it seems the easiest way to go.

Another question. Does any one know of a decent electronic supply house in the Norfolk, Portsmouth, Virginia area. I am in N.E. North Carolina and we do not have anything except for a "Radio Shack" and they do not have much.

Thanks Chris

Well, Chris, such places are getting to be as rare as chicken dentition. If you can't find 'em in the Yellow Pages, there aren't any. :-(

Radio Shack is a skinny ghost of its former glorious elf, but is still your best bet (at least within reasonable driving distance.)

Sigh.

Check out the local auto recyclers. It might be possible to use the momentary switches used for power windows -- I don't know if they're DPDT, but it's worth a try.

wolf k.

: >> I can not figure it out though. Since the momentary switches I have : >> are spst, I would think I would need to use a toggle of some type to : >> change polarity. : >>

: >> How do you do this? : >>

: >> Thanks : >> Chris : > Well I guess I am going to go with a dpdt momentary switch. Really did : > not want to, but it seems the easiest way to go. : >

: > Another question. Does any one know of a decent electronic supply house : > in the Norfolk, Portsmouth, Virginia area. I am in N.E. North Carolina : > and we do not have anything except for a "Radio Shack" and they do not : > have much. : >

: > Thanks : > Chris : : : Well, Chris, such places are getting to be as rare as chicken dentition. : If you can't find 'em in the Yellow Pages, there aren't any. :-( : : Radio Shack is a skinny ghost of its former glorious elf, but is still : your best bet (at least within reasonable driving distance.) : : Sigh. : : Check out the local auto recyclers. It might be possible to use the : momentary switches used for power windows -- I don't know if they're : DPDT, but it's worth a try. : : wolf k.

Or you could use a dual polarity power supply and hook one switch to the positive side and the other to the negative. It's easy enough to build one with a center tapped secondary transformer if you can find one locally.

Also, Mouser

and Digi-Key our friends when it comes to electronic stuff since Radio Shack tuned into Circuit City-mini (and look what happened to them!).

Len

I think you'd have to use two switches to energize alternate sides of a latching relay.

A better solution would be a center-off DPDT rocker switch, wired as a polarity switcher:

If you went with the relay, you would wire the contacts of the relay the same as the switch shown in the instructable. Personally I'd go with the rocker switch, but the venerable atlas snap relay will work fine if you want to do it the hard way, and they're easy to find:

*

These days, online is the best way to get small parts. Outfits like mouser.com have unbelievable supplies of odd parts, and are almost always a lot cheaper than a retail electronics distributor. In the case of radio shack, often a whole order of magnitude cheaper. I think resistors are actually TWO orders of magnitude cheaper. *

On 2/10/2009 9:06 AM Len spake thus:

My solution

is much, much simpler. Only needs a single-ended transformer and 2 diodes.

Watch out at this point ----- You will be converting from a 'Momentary On of either polarity, to Always on, polarity determined by last button pushed. [This is what is needed for Tortice type (stall) switch motors.]

Chuck D.

On 2/10/2009 9:19 AM PV spake thus:

Why on earth would you want to do either of these things? Have you looked at my solution?

Much simpler.

Besides, the O.P. said they wanted to use 2 pushbutton switches, not a single rocker switch.

Very nice. (Slaps head "Now why didn't I think of that" :-))

Hope this makes sense:

• --+---------------+ | | B B | | +-Rl-+ +-Rl-| | | | | D | | D | | | | +------------+ | \ / | +-+ \ / +-+ | C \ C | D B--+-+-/ \-+-+--B D | E | | | | E | +-+ Rh C C Rh +-+ | | | | | |

- --+--+-+-----+-+--+ Rl's are a relatively low value - sufficient to saturate the transistor - probably somewhere around 1k.

Rh's are relatively high value - sufficient to keep the transistors turned off when the relative switch is open - probably somewhere around 100k would work ok.

B are the pushbutton SPST momentary switches.

C are low value capacitors to help filter against noise. Probably somewhere around .1uF

the C-B-E are NPN power transistors - anything capable of handling the current the motor draws.

D's are diodes - the upper diodes are forward biased (Anode to the switch, cathode to the transistors). The lower diodes are reverse biased (anode to transistor emitter, cathode to transistor collector) and are there to protect the transistors from back EMF from the motor. Any typical small power diode capable of handling the motor current will suffice - 1n4002 etc.

Where wires go past others, if I don't have a + they are not joined. IE the base of each transistor is connected via Rl to the opposite pushbutton.

How it works. When you push a button it will connect the button side of the motor to positive. The opposite transistor will be turned on through Rl, connecting the other side of the motor to ground. The capacitors are there to filter out back EMF from the motor to make sure the transistors don't turn on when they aren't supposed to. NOTE: if both switches are pressed simultaneously it will be fatal to the transistors almost instantly.

This is fine and dandy IF you are looking for a constant voltage across the 'switch motor' [I.E. Stall motor switch (turnout) machines.]

It DOES NOT provide a momentary (of selectable polarity) connection to the power. Chuck D.

By "momentary (of selectable polarity)" I assume you mean the power is only applied while the momentary switch is being pushed, and that it changes depending on which button is pushed. Have a look at the circuit - that is EXACTLY what it does. As soon as a button is released, the power is disconnected from the motor, and it will turn off.

If you mean something entirely different by "momentary (of selectable polarity)", then please explain what you mean, because I would interpret it to mean power is only applied to the motor while the switch is pressed, and that the motor can operate in either direction depending on which button is pushed.

O.K. --- I was seeing a glorified 'stick' circuit, and didn't follow it through far enough. Sorry!

Chuck D.

Thanks for the info. I already put in a momentary toggle (center off). But will keep this for next time. Which will be soon.

Thanks again Chris

On 2/18/2009 2:39 PM Chris spake thus:

I still say my idea is far simpler, cheaper and easier to implement (2 switches and 2 diodes is all), but whatever.