Re: Switch Confusion

Doug Jewell wrote:
Chris wrote:

I would like to use two momentary switches to turn a motor clockwise or counter clockwise depending on which switch is pushed.

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


Hope this makes sense:

+ --+---------------+
| |
B B
| |
+-Rl-+ +-Rl-|
| | | |
D | | D
| | | |
+------<M>------+
| \ / |
+-+ \ / +-+
| 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.
.

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    ... Since the momentary switches I have are spst, I would think I would need to use a toggle of some type to change polarity. ... 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. ... The lower diodes are reverse biased and are there to protect the transistors from back EMF from the motor. ...
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