on/off tube filler using 2 switches to run motor

[blown fuse]

I'm hoping someone can help me with this. I will say this machine feeds ball bearings for simplicity.

I have a dc motor that feeds ball bearings into a tube about 4 feet in the air. Right now there is a micro switch near the top to stop the motor when the tube is full of ball bearings. The problem with this is it keeps turning on and off when just one ball drops, it's very loud and annoying.

I would like to use two switches to control the on/off of the motor. The second switch being about a foot from the bottom of the tube. I would like to use some kind of metal detecting inductor micro switch instead of the arm type.

So it would work like this. Tube is empty, motor turns on. Tube fills all the way up and top switch turns off motor. Motor stays off until ball bearings drop below bottom switch and motor turns on to fill tube again. There would also need to be a small delay so the motor wouldn't stutter as a ball fell past the sensor.

I'm looking for the simplest and cheapest way to do this. Thanks for your help.

 

[MikeMl]

Read this recent thread. That circuit up to U1 (without U2 and U3) will do what you need.

 

[blown fuse]

Wouldn't the first ball bearing down the tube trigger both S1 & S2? I'm thinking of using induction proximity switches.

Would I replace D3 with the motor?

 

[MikeMl]

Ok, then we need a latch that gets set when the level of balls in the tube drops below the bottom switch, and gets reset only when the top switch detects that a ball has been present for longer than ~1sec. This way, the momentary switch closure caused by a ball falling down the tube past the top switch is ignored. The latch is reset only by the "continuous" presense of a ball at the top...

I will work something up.

 

[dougy83]

Either of the attached circuits may do the trick.

One shows the implementation when two microswitches are used. Here, if both are off for a small period, then the relay turns off; if both are on for a small period, then the relay turns off. Any other state and the relay just holds its state. The second contact of K1 (not drawn) provides the switch output.

The second shows a similar function using schmitt triggers, which could be used if the inputs are from a logic proximity switch. The output is logic level.

Both circuits will have an issue if dropping a number of ball bearings in the top results in the top switch duty cycle being > 50%.

 

[MikeMl]

Here is my design. Look at the upper plot pane, where the simulation control signals for the two micro switches are shown. LTSpice uses a switch model that is controlled by a made-up voltage. The switch is closed when the control voltage >0.5V.

Initially, the tube has at least one ball which is holding the bottom switch in the open position, as depicted by the LightBlue trace V(bot). At three seconds, the ball is removed causing the bottom switch to close. That cause the motor to start, dropping a series of balls past the top switch as shown by the Green trace V(top). Eventually, at 12.5s, the tube fills with balls so the last ball holds the top switch closed, shutting off the motor.

I'm using a 555 as a Schmitt trigger with predictable trip points of 4V and 8V. The purpose of the network consisting of R1-4 and C1 is to discriminate between a low state (<4V), an intermediate state, and high state (>8V) of V(rc) Red trace V(rc) in the lower plot pane. When the bottom switch closes (empty stack) this immediately pulls V(rc) low, triggering the 555, causing its output pin 3 to go high, thereby pulling the motor relay. The current through the relay coil is shown by I(L1) purple trace.

To reset the 555 to turn off the motor relay, V(rc) must get above 8V, which happens only when the top micro switch is closed for more than 90% of the time. The pulses caused by the falling balls do not get V(rc) high enough; only when a ball keeps the top switch closed does V(rc) rise high enough to reset the 555. The motor relay should be 12Vdc with a coil resistance of >80Ω.

Note that it takes a while after the top switch comes on steady for the circuit to shut off the motor. You should mount the top switch a little down the tube because an extra ball or two may drop in after the top switch comes on steady.

 

[blown fuse]

no wonder why the factory only supplies one switch. lol more complicated than I thought.

Mike, only one problem.. there won't be a ball bearing in the tube to start with. The lower switch has to be up at least 10 ball bearings so that the bearing feeding machine doesn't run dry before the feeder motor kicks on to refill the tube and hand feeding them isn't practical.

thanks for your help. What you gave me is very close to what I need.

 

[MikeMl]

...Mike, only one problem.. there won't be a ball bearing in the tube to start with. The lower switch has to be up at least 10 ball bearings so that the bearing feeding machine doesn't run dry before the feeder motor kicks on to refill the tube and hand feeding them isn't practical.

No. I just happened to depict the initial condition such that there where balls in the stack. The design doesn't care. It works just fine if the stack is empty when the power is turned on. In that case, the bottom switch is initially closed, that triggers the 555, and the motor starts running. The first ball down the stack creates a pulse at the top switch (which is ignored) and then falls to the bottom and opens the bottom switch. The motor keeps feeding balls just as before until the top switch stays closed.

I though about a version with only one switch. In that case, I would locate the switch at the bottom, and use a timer to run the motor for x seconds after the first ball trips the switch. I would still use a 555 to drive a relay...