Beginner question about switching. Please help.

[John Bobson]

Hello all.

This is a repost of my previous request. I am really stuck. Simply put,
I need a transistorised circuit that can reverse the direction of a
motor when two switches are struck alternatively.

Truth Table

Switch 1 X X X X
Switch 2 X X X X
Motor Direction CACACACA ... etc.

C = clockwise , A = anticlockwise

Some research after the posting of the original query has shown that I
really need a transistor set/reset flip-flop (bistable) with
complementary outputs.

http://ourworld.compuserve.com/homepages/Bill_Bowden/bistable.gif

but apparently I cannot simply wire the motor across the collectors.

I must emphasise that I am not using a relay, because my application
would require that the relay be switched rapidly and continously.

Original query:
I am building a machine that involves a trolley running up and down a
short
track. A microswitch is located at either end of the trolley (that's
two, plus one for detecting derailings). I want the trolley to change
direction when one of the 'bumper switches' hits a fixed stopper at
either end of the track.

I only want to use transistorized mechanisms, so this rules out the use
of a relay. With only rudimentary knowledge in electronics, I can't
design something that actually works. I had no luck in modifying a
'multivibrating oscillator' for this purpose.

I have on hand a number of power transistors (All NPN) and countless
scavaged resistors, capacitors, diodes, small signal tranistors, etc.,
so there should be no problem with components.

The motor used in this scheme is a 7.5V model, drawing approximately
500ma.

Any helpful pointers, schematics or suggestions will be greatly
appreciated.

If this explaination is not clear enough, please indicate so.

[Vincent Houston]

Looks like what you are looking for is a H switching network,

This consists of 4 transistor, 2 pnp 2 npn.

_______________________VCC
| |
pnp1 pnp2
| |
|------ load ------|
| |
npn1 npn2
| |
-------------------------Gnd

Transistors are turned on in pairs pnp1 & npn2 or pnp2 & npn1 .

You could look at using TIP31(NPN) and TIP32(PNP).

Allow a dead time between turning one pair off and turning the next pair
on
or else the blue smoke my get out.

hope this is helpful

[Matt Jones]

In article <3641C2...@iinet.net.au> Vincent Houston,

vin...@iinet.net.au writes:
>> This is a repost of my previous request. I am really stuck. Simply put,
>> I need a transistorised circuit that can reverse the direction of a
>> motor when two switches are struck alternatively.
>>

Here's a dorky looking ascii schematic of the H-Bridge circuit I use.
All transistors are NPN. Resistor values may differ depending on your
application.
I use it for switching 200V through a piezo device, so I don't have to
worry so much about inductive spikes when I turn of the motor or change
directions, but that's something you'll want to look out for.

You should also check out Doug Jones' page on Stepper Motors for other
H-Bridge circuit versions. This one is a modification of one I got from
that site (thanks, Doug).

Cheers,

Matt jones

Please view in a fixed-width font such as Monaco
or Courier.

H-Bridge Circuit Some sort of diode
All xters are NPN protection may be
V+ necessary also.
***********************************************
* * * *
* * * *
2.1Mohm * * 2.1Mohm
* *
* c c *
**** b b ****
* e e *
* * * *
* * * *
* * * *
* |< ***** motor ******>| *
c c
***b b***
* e*****************************e *
* * *
* * *
* gnd *
470ohm********* ************470ohm
* * * *
* ***** * *
* * * *
470ohm * * 470ohm V+
* * * * *
* * * * *
* * * * *
* * * * ********
gnd * **
** *
* *
* *
* *
S1***not***S2

[John Bobson]

Thanks for your suggestions.

Please excuse my idioticy, but how is S1 and S2 wired (in series with
each other?), and what is the not in between.

[Matt Jones]

In article <36427BE2...@reply.via.usenet> John Bobson,

ple...@reply.via.usenet writes:
>Thanks for your suggestions.
>
>Please excuse my idioticy, but how is S1 and S2 wired (in series with
>each other?), and what is the not in between.
>

The idioticy was all mine. I should have drawn it better. S1 and S2 are
wired directly to V+, so that they send current into the base of a
transistor when closed. The "not" in between them refers to making sure
that they aren't both on or off at the same time (actually, I don't think
that matters in this circuit, if they're both the same then the current
through the motor shuts off, so this can actually be used as part of the
control design). I'm driving the switching with TTL inputs, so I
literally have an inverter (a "not" gate) between the two switches, but
not exactly as it's drawn here obviously.

My apologies for the lack of clarity. Haven't been getting enough sleep
lately.

Cheers,

Matt

[John Bobson]

Thanks for your clarification.