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Beginner's Guide (Inputs)

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DaOld Man:
You can control speed on this setup, however it is limited.
You can use a variable resistor called a rheostat to vary the voltage on the motor.
You must turn the knob of the rheostat to change it's resistance.
This cuts back the voltage on the motor by adding more resistance in the circuit.
With lower voltage, the motor will turn slower.
At one time, many large DC motors were controlled in this same fashion (yeah I remember them).
But this method has some very serious drawbacks.
The rheostat absorbs a lot of energy, which means it produces a lot of heat.
Because of this, the rheostat has to be large, and is usually open to allow cooling.
The size (or wattage) of the rheostat is based on the maximum amount of current the motor can draw.
A DC motor draws the most current when it is stalled, or the shaft held so it cant turn.
A DC motor also draws a large amount of current on startup, but it is usually only a few milliseconds unless something is jammed and the motor cant turn. If the motor hangs on something, the rheostat could possibly get extremely hot.

Another possible alternative for speed control would be to use a variable voltage power supply for the motor's power.

DNA Dan:
Hmmm. Certainly not as simple as I thought, but I completely understand what you've laid out. So far my setup will need a GM2 motor with bracket and wheel, secret motor driver, two limit switches, a DPDT switch, some diodes and either a rheostat or a PWM controller. A few questions however:

1) What sort of limit switches should I use? Leafs? Also once they are tripped, the rotation needs to be stopped in this position correct? Otherwise I see the switch being tripped, than the switch pushing back an closing the circuit again. What do people use for this? A weak magnet? A bump in the travel? Whatever it is the amount of power in the rotation needs to be able to overcome this in the opposite direction.

2) What about the diodes? Where are they connected on the switch? Just between the red and black? Also which ones should I get for the setup with the GM2 motor?

3) For the rheostat or the PWM controller, this needs to go between the secret motor driver and the motor leads correct? What about a voltage down regulator? I assume that secret motor drive needs 5v, so can't I just bump the voltage through a second regulator at say 3v and slow the speed?

Sorry for all the questions. Your help has clarified a lot already.

DaOld Man:
OK, instead of relays, lets study a very common and cheap integrated circuit called a "Quad And Gate"
A Quad And Gate has 4 And Gates in one package.

Each AND gate has two inputs and one output.

For the output to be high (or + 5vdc), both inputs must be high. (Input 1 AND input 2 = output).
If either input is low (ground) then the output is low.

On the drawing below, the H push button is connected to the input of one gate (pin #2 on the chip).

The Normally Closed limit switch is connected to the other input (pin #1). Both button and switch connect back to +5vdc power supply.

Now notice the output of the same gate (pin #3) is connected to D1 of the drive and also to input pin #2 , through resistor R3.

Also there is a resistor connected between ground and pin #1 (the limit switch input).

Now here's what happens:
With both limit switch and H PB open both inputs are low. How? Well pin1 is low because the resistor R1 is pulling it to ground.
Pin 2 is low because the output pin 3 is low and they are connected together through resistor R3. So the secret drive receives a low on D1. Motor does not run in that direction.

If H PB is pressed and the H LS is still open, input pin 1 is still low (because of R1 to ground), pin 2 goes high, but output pin 3 remains low because pin 1 is still low.
Resistor R3 prevents a short circuit when the PB is pressed, due to output pin 3 being low and pin 2 being high.

The motor still does not run in that direction.

Now if the H LS is closed due to the monitor not being in horizontal position, pin 1 is made high through that limit switch. Pin 3 remains low due to pin 2 still being made low by pin 3 being low.

Now if H PB is pressed, input pin 2 goes high. both inputs are high so output pin 3 goes high.
Drive turns on because D1 is now high.
When H PB is released, output pin 3 feeds back through R3 to keep pin 2 high. Pin 3 stays high until H LS opens.
Then pin 1 goes low through R1 thus setting pin 3 low, thus turning off the drive. Pin 2 once again is made low by pin 3. The circuit is reset and waits for both H LS and H PB to be made high. One or the other alone does not turn on the drive.

The same is repeated on the second gate for vertical. This gate has pins 4 and 5 as inputs and pin 6 as output.

The other two gates need their inputs tied to ground, to keep them from erratically switching on and off, which could induce noise into the circuit. The outputs of these gates (pins 8 and 11) connect to nothing.

Clear as mud?

DaOld Man:
Just for comparison, here is a relay latching circuit which basically does the same thing.
This drawing only shows the circuit for horizontal. You would actually need two relays and repeat this circuit for vertical.
The relay has a coil and two normally open contacts. the coil has to be rated the same voltage as the power supply.
RA and RB are the contacts in this drawing.
Pressing H PB energizes the relay coil, but only if the limit switch is closed (not being made by the monitor disc).
RA latches the circuit by supplying an alternate path for the current to flow when H PB is released.
RB turns on D1 of the drive.
The relay and drive stay on until the monitor reaches end of travel, making the limit switch, which opens and turns off the relay. If monitor bounces and limit switch closes again, circuit remains off until H PB is pressed again.

This circuit has advantages over the AND gates. Most probably being simplicity. fewer parts (no resistors needed.)

But there are some distinct disadvantages:
Size. One relay may be several times larger than the quad and gate, and this circuit requires two relays.
Price. Quad And gates can be had for under a dollar. A relay may cost several dollars each.
Power consumption. The relay circuit will consume much more power than the AND gate one. (This probably wont be a concern because the relays draw no current at all when turned off.)
Noise. Click each time you press the button and when the limit makes.
Power must be constant. Cant vary the power supply to control speed. Relays dont like that. (The AND gate may not be variable either, depends on what type chip you choose.)
Life span. Relays have moving parts. They will some day wear out, but it may be years. Sockets can be used with plug in relays that make the relays very easy to swap out if one fails. (more expense though).

DaOld Man:
Brain teaser:
There is a possible flaw in both the above circuits.
What would happen if the Vertical push button is pressed while the monitor is turning towards horizontal? (or vice versa).

And, if something did happen, how would you fix it?

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