Electronics Forum / Basics / October 2006

Hi David,

What do you really want to do?

You can reverse the motor with the switches and/or the relay, what do you
want to do with the timing?

BTW: Get yourself a 3 volt supply and forget about the resistors.

If you are looking for something more complicated Google PWM Motor control
and "H" Bridge.

Tom

---
Do you want the switches to be manually activated or do you want the
motor to actuate them alternately?
---

---
I can see using a DPDT relay to do the polarity switching, but what
purpose would the timer serve?
---

---
Dunno.

It's not really clear what you want to do.
---

---
That depends on what you want the circuit to do,  so you need to be
a little clearer about that.
---

---
Almost anything is possible but, again, your description of what you
want needs to be more detailed.
---

---
There are other options but, for the moment, let's hold off on that
until you explain, in greater detail, what you want.

Hi, David.  Your description is a little sketchy.  I'm going to make a
few assumptions here, and you tell me where I'm wrong.

You've got a low voltage DC motor which you want to run at two speeds.
To do that, you want to apply two voltages: 1.5V and 3VDC.

Now, you want to run the motor both clockwise and counterclockwise.  To
do this, you have two pushbuttons labelled CW and CCW.  When you press
the CW button, you want the motor to go clockwise for 5 seconds.  When
you press the CCW button, you want the motor to go counterclockwise for
5 seconds.

You forsee needing at least one 555 for this project and a DPDT relay,
but you're not sure what's going to happen if someone, say, presses the
CW button while the motor is going counterclockwise -- you don't want
to smoke anything.  OK.  Let's see if we can simplify this and make it
safe from accidental short circuits.

The first thing you could use is a specialty relay called an impulse or
latching relay.  This is a relay that's got two coils and two internal
magnets.  When you energize one of the relay coils, it moves the
armature toward that direction.  A weak magnet helps keep the armature
on that side, whether the coil is energized or not, until the other
coil is energized.

These relays can be a little expensive, but if you're not switching too
much current in your motor (you're using a 9V battery, that indicates
your motor doesn't draw more than a few tenths of an amp), a surplus
telecom relay should do the job.  You'll have to suppress any switching
arcing, but it is still an inexpensive solution.  Jameco sells one that
can switch up to 2 amps as their P/N 206543CM for $3.79 ea.

For the rest of this, the circuit should be pretty simple.  You can use
three "C" or "D" batteries in series to give you a 4.5V power supply
(you'll need the higher supply for the surplus relay) and an LM555.
Cut and paste this wiring diagram to M$ Notepad if your newsreader
doesn't have fixed font:

(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

The value of R is equal to the DC resistance of the DC motor. D is any
diode like the 1N4001 or 1N4002.

When you press either SW1 or SW2, you energize the appropriate relay
coil, and also trigger the 555 one shot (you seem to have R and C
values down).  The motor will be on for your 5 seconds.

I'd guess this is pretty much complete.  Now tell me how I got it
wrong.

Cheers
Chris

There are some simple setups at the bottom of the below page.

http://www.geocities.com/zoomkat/switch.htm

Why? Explain what you want it to do and why.

View in courier font:

 +3 ----+ +------+-----------------------------+
        | |      |                             |
        |>|      |                         +---+
       Rrly-1    |                         |
        n/o      +---v n/o             n/c v  Drly-2
                     --------[Motor]--------
             Drly-1  ^ n/c             n/o ^---+
                     |                         |
                 +---+                         |
                 |                             |
Gnd -------------+-----------------------------+

Rrly = Run relay - energize it when you want the motor to
run. It is a single pole relay

Drly = direction relay. Energize or de-energize to change
direction. It is a double pole, double throw relay and
contains 2 sets of contacts, labeled Drly-1 and Drly-2.

Both relays are shown in the enrgized position. +3 gets
to the top of the motor through the energized Rrly-1 and
Drly-1 n/o and common points. Gnd gets to the bottom of
the motor through the Drly-2 n/o and common point. When
you de-energize Drly, Gnd gets to the top of the motor
through the Drly-1 n/c and common point. + 3 gets to the
bottom of the motor through the Drly-2 n/c and common
point, and the Rrly-1 n/o and common point.

That is a basic circuit to give you independant control
of direction and whether the motor runs or not.  It was not
clear what you have in mind as to timing and changing
direction.  However, whatever you want to do is achievable
by controlling how/when/how long you energize the relays.

Ed

Pretty much, but I think you'll need more than  just a 556 and one
relay. You'll need at least two - one for "on/off" and one for direction.
And each of them will need a driver transistor, snubber diode, and so on.

Actually, it depends on what kind of circuit you build. What I envision
is the two sections of the 556, each being triggered by its own
microswitch, with the output of one going to the "reverse" relay and
the outputs of both "OR"ed together for the "ON" relay. In that case,
if it's going in reverse and you push the forward button, it will
start the forward timing cycle, so when the other timer times out,
the reversing relay will drop out, and try to instantly reverse the
motor. If it's going forward and you press the "reverse" button, it
will immediately try to reverse the motor.

So you'd need to come up with some kind of trigger lock-out while
the motor is running.

Good Luck!
Rich