Poor Man's Hi Torque Servo, circuit

[[email protected]]

Hello Electronically Superior Beings,

I am electron challenged and I need a simple circuit to control a
wiper motor that I'm using as a high torque servo.
The motor has hi and low speed circuits and a "park " position.
What I want to happen is this:
S1 (NO) is closed and sends signal to some kind of device(D) that
powers up the wiper motor. When motor cycles, S2 (NC) in series
downstream of D, opens and stops wiper motor at desired location for
30 secs. When S2 closes again, the wiper motor continues rotation back
to "Park" and shuts off. D re-sets for new signal.

I can't use S1 as switch for motor, because it necessarily moves away
from activation force. I located a timer for the 30 second pause.

What is D called and how do I get one?

Thanks for your help,
John

 

[Lostgallifreyan]

[email protected] wrote in

Hello Electronically Superior Beings,

I am electron challenged and I need a simple circuit to control a
wiper motor that I'm using as a high torque servo.
The motor has hi and low speed circuits and a "park " position.
What I want to happen is this:
S1 (NO) is closed and sends signal to some kind of device(D) that
powers up the wiper motor. When motor cycles, S2 (NC) in series
downstream of D, opens and stops wiper motor at desired location for
30 secs. When S2 closes again, the wiper motor continues rotation back
to "Park" and shuts off. D re-sets for new signal.

I can't use S1 as switch for motor, because it necessarily moves away
from activation force. I located a timer for the 30 second pause.

What is D called and how do I get one?

Thanks for your help,
John

For the active switch in 'D' you could use a power MOSFET to let a positive
signal voltage switch its resistance from several hundred megohms to a few
milliohms. They are often used as motor switches for that reason. A good
example is the IRF630, good for 9 amps, more for very short surges, and
found cheaply on eBay, often in quantity.

That leaves your switching able to work with small signals, which increases
your options and lowers costs and raises reliability.

 

[Lostgallifreyan]

Thanks for your quick response, but could you speak a bit more slowly.
My electronics vocabulary is in its infancy.

John

Read slowly...
Ò^O

Ok, here's a basic circuit:
http://www.picotech.com/applications/pwm_drivers/graphics/mosfet.gif

That was meant to be driven by a fast pulse train for speed control, but
it's still just a switch, as you can work out from that diagram. Part
numbers and voltages can vary but the idea is right.

This doesn't solve your switching logic but it will make it easier to apply
it.

(I answer at the bottom so it's easier for others to follow. Top posting
makes sense in private emails but not so well here).

I'm too tired to go further but they're not all in bed in the US and
Canada yet.

 

[John B]

Thanks for your quick response, but could you speak a bit more slowly. My
electronics vocabulary is in its infancy.

John

 

[ehsjr]

Hello Electronically Superior Beings,

I am electron challenged and I need a simple circuit to control a
wiper motor that I'm using as a high torque servo.
The motor has hi and low speed circuits and a "park " position.
What I want to happen is this:
S1 (NO) is closed and sends signal to some kind of device(D) that
powers up the wiper motor. When motor cycles, S2 (NC) in series
downstream of D, opens and stops wiper motor at desired location for
30 secs. When S2 closes again, the wiper motor continues rotation back
to "Park" and shuts off. D re-sets for new signal.

I can't use S1 as switch for motor, because it necessarily moves away
from activation force. I located a timer for the 30 second pause.

What is D called and how do I get one?

Thanks for your help,
John

I'm assuming S1 is a normally open momentary switch.
Device D is the relay (Relay1) shown below:

/
+ ---+---o o---+------+-------+
| S1 | | |
| | | |> | <-Relay
| / | | | Contact
+---o o---+ [Relay1] [Motor]
Park | |
| S2 |
Gnd -------------------+---o->o---+



You have not stated how S2 moves back to the closed
condition, but in a reply stated that you have a
30 second timer. If we assume that S2 is cam operated
and stays in the open position until the motor is
energized again, you can use a second relay that is
held energized for 30 seconds after S2 opens. The
normally closed contact of the second relay would be
wired in parallel with S2. Here's the diagram, modified
to show that:


/
+ ---+---o o---+------+----------+
| S1 | | |
| | [Relay1] |> | <-Relay
| | | |
| | | +-------+
| / | | | | Contact
+---o o---+--|<--+ | [Motor]
Park D1 | | |
| +---|<--+
| D2 |
| S2 |
Gnd -------------------+---o->o---+--+---> Trigger
| | Signal
Timer Relay Contact-> '-> | | to timer
| |
+------+

How it works: When in the park position, the park switch
and S1 are both open. Pressing S1 energizes Relay 1, its
contact closes, the motor moves and the park switch closes,
keeping Relay1 energized until the mechanism moves back
to park. When the mechanism reaches the S2 position, S2
opens and removes the (-) trigger signal to the timer.
After 30 seconds, the timer relay de-energizes and the
relay contact closes. That causes the motor to run again,
moving away from the S2 position and closing S2. That
sends the (-) trigger signal to the timer which energizes
the timer relay. The mechanism returns to the park position,
which opens the park switch and de-energizes Relay1, and the
motor stops. Diode D1 protects the switches against the
inductive spike created when power is removed from Relay1;
D2 protects against the inductive spike from the motor
when power is removed from it. The timer relay coil should
also have a diode placed across it, wired the same polarity
as the other diodes (banded end connected to the + side).

Ed

 

[John B]

Read slowly...
Ò^O

Ok, here's a basic circuit:
http://www.picotech.com/applications/pwm_drivers/graphics/mosfet.gif

That was meant to be driven by a fast pulse train for speed control, but
it's still just a switch, as you can work out from that diagram. Part
numbers and voltages can vary but the idea is right.

This doesn't solve your switching logic but it will make it easier to
apply
it.

(I answer at the bottom so it's easier for others to follow. Top posting
makes sense in private emails but not so well here).

I'm too tired to go further but they're not all in bed in the US and
Canada yet.

OK, a picture worth a 1000 words and good advise for forum posting/replys.
But I have some homework to do. Thanks for your help.
John

 

[[email protected]]

Hello Electronically Superior Beings,

I am electron challenged and I need a simple circuit to control a
wiper motor that I'm using as a high torque servo.
The motor has hi and low speed circuits and a "park " position.
What I want to happen is this:
S1 (NO) is closed and sends signal to some kind of device(D) that
powers up the wiper motor. When motor cycles, S2 (NC) in series
downstream of D, opens and stops wiper motor at desired location for
30 secs. When S2 closes again, the wiper motor continues rotation back
to "Park" and shuts off. D re-sets for new signal.

I can't use S1 as switch for motor, because it necessarily moves away
from activation force. I located a timer for the 30 second pause.

What is D called and how do I get one?

Thanks for your help,
John

I'm assuming S1 is a normally open momentary switch.
Device D is the relay (Relay1) shown below:

/
+ ---+---o o---+------+-------+
| S1 | | |
| | | |> | <-Relay
| / | | | Contact
+---o o---+ [Relay1] [Motor]
Park | |
| S2 |
Gnd -------------------+---o->o---+

You have not stated how S2 moves back to the closed
condition, but in a reply stated that you have a
30 second timer. If we assume that S2 is cam operated
and stays in the open position until the motor is
energized again, you can use a second relay that is
held energized for 30 seconds after S2 opens. The
normally closed contact of the second relay would be
wired in parallel with S2. Here's the diagram, modified
to show that:

/
+ ---+---o o---+------+----------+
| S1 | | |
| | [Relay1] |> | <-Relay
| | | |
| | | +-------+
| / | | | | Contact
+---o o---+--|<--+ | [Motor]
Park D1 | | |
| +---|<--+
| D2 |
| S2 |
Gnd -------------------+---o->o---+--+---> Trigger
| | Signal
Timer Relay Contact-> '-> | | to timer
| |
+------+

How it works: When in the park position, the park switch
and S1 are both open. Pressing S1 energizes Relay 1, its
contact closes, the motor moves and the park switch closes,
keeping Relay1 energized until the mechanism moves back
to park. When the mechanism reaches the S2 position, S2
opens and removes the (-) trigger signal to the timer.
After 30 seconds, the timer relay de-energizes and the
relay contact closes. That causes the motor to run again,
moving away from the S2 position and closing S2. That
sends the (-) trigger signal to the timer which energizes
the timer relay. The mechanism returns to the park position,
which opens the park switch and de-energizes Relay1, and the
motor stops. Diode D1 protects the switches against the
inductive spike created when power is removed from Relay1;
D2 protects against the inductive spike from the motor
when power is removed from it. The timer relay coil should
also have a diode placed across it, wired the same polarity
as the other diodes (banded end connected to the + side).

Ed

Ed,
I can't thank you enough for the hand-holding and spelling it out for
me. I understand
your clearly described circuit perfectly. I assume that there are some
values for the diodes
that I need to figure out, but I'm sure that I can handle that. Again,
1000 thanks. You've made my week.
JOhn

 

[ehsjr]

Ed,
I can't thank you enough for the hand-holding and spelling it out for
me. I understand
your clearly described circuit perfectly. I assume that there are some
values for the diodes
that I need to figure out, but I'm sure that I can handle that. Again,
1000 thanks. You've made my week.
JOhn

The diodes can be anything in the 1N400x "family".
If your timer triggers on (+) instead of (-), you
can move S2 to the + side. I would use a delayed
dropout circuit instead of a timer (assuming the
30 seconds does not have to be exact) like this:

+12 ----------------+---[Relay]---+
| |
+-----|<------+
D3 |
/c
Trigger(+) ---[1K]---+-----+----| TIP120
| |+ \e
[POT] [470uF] |
| | |
Gnd -----------------+-----+------+

The TIP120 is an NPN darlington power transistor. You set how
long it keeps the relay energized with the 100K potientiometer
(which you configure as a variable resistor by shorting one
outside contact to the center contact).

The + trigger signal from the circuit is interrupted when
S2 (which is now on the + side of the motor) opens. The
charge on the 470uF capacitor keeps the transistor conducting
until the cap discharges through the pot. How long that takes
depends on the resistance the pot is set to. As long as the
transistor conducts, the relay remains energized, and its
normally closed contact remains open, so the mechanism does not
move. Once the cap discharges sufficiently, the relay drops out,
the normally closed contact shorts out S2 and the motor moves
again. The 1K resistor limits the current to the base of the
transistor.

Ed