3vdc motor PWM problem

Where could I find a suitable solenoid? I've never used one before. I'd love to have that type of precision with the drops.

try an auto wrecker, some vehicles have solenoid based fuel pumps, all you'd need to do is open it up and short out (or bypass) the contacts (so it doesn't run at its own rate) and feed it 12V pulses. (actually if the rate you want is low enough or the pulses narrow enough you may not need to modify it at all.

Bye. Jasen

Use 2 one-way valves like the airline valve for fish pumps, and between them a small soft rubber bulb. Around that fashion the actuator of a solenoid that will squeeze the bulb and pump the water. :) hmm maybe even adapt a small aquarium air pump to do water. I have one here I will try out and get back to you.

OK. It works very well. I removed the chamber from an aquarium air pump and with it submerged, by tapping the rubber cap - it pumps fluid just fine. All it needs now is a small magnet attached to the rubber cup and a coil over it. By pulsing the coil, you will get drops of water. The pump I used is brand Aquaculture single outlet :) JTT

...

Have you tried an accumulator and needle valve? Or maybe something like a fuel injector thingie? For making drips, that is - the talk of the astables is for the duty cycle of the motor, right?

Cheers! Rich

What about an EFI injector? Or would running water through one wreck it?

Thanks, Rich

doesn't

--
Even if it didn\'t, they spray a mist, not single drops.

--
This is interesting to me, so I\'m going to build one. :-)

I\'ll use a drop detector (either reflective or transmissive IR, I\'m
not sure which, yet) and white LEDs to strobe the drops, which I\'ll
probably leave clear right at first.  

My goal will be to generate 1/4" diameter drops that fall through
12"  with a new drop being generated every 16ms or so.  That\'ll give
the appearance of a single drop suspended in the center of the "drop
zone" with a drop rising out of the catch basin, following the
center drop when the center drop appears to rise, or a drop
appearing at the top of the drop zone, following the center drop
when it appears to fall, all depending on the detect-to-strobe
delay.

I have a submersible fountain pump which can deliver about 1.5
liters per minute with a 12" head, so that\'s enough volume VS time
to generate the quantity of drops needed.  It runs off the 120V
mains so I\'m not going to mess around with PWM\'ing it, I\'ll just
control the flow with a needle valve I had laying around waiting to
be put to use.

Anyway, that\'s the plan...

60Hz? that's a high rate for 6mm drops

they'd have to be doing 0.36 m/s to avoid the next drop behind them

at 10m/s^2 it'd take them 36 ms to achive that speed :(

I guess you could use a stream of water and just let it break into drops as it falls, or maybe modulate it somehow.

you'll probably get 120Hz modulation for free with that setup

Hmmm.

flow modulator: __ L.V. AC supply ===== [ ] / \\ ---- sub-woofer / \\ _/ \\_ valve | | T tank | .|------+------- water input |~~~~~~~~| | | `---||---' `------ hole

Bye. Jasen

Welcome aboard :)

If you get this part working, could you let me know how it's done? I'd love to try drop detection in the future... but with it you really need a PIC to get the slow motions forwards/backwards. That's why I'm gunning for constant drip :)

1/4" drops sound a little big... you should probably use a McDonalds straw for that (which I'm going to try tomorrow). I'm using 3/16" OD aquarium tube, and my drops are 3/8", which are a very nice size.

As per the 16ms... that will prove to be very difficult. The fastest drip I can get before it turns into a stream (in which case the drops are extremely erratic and tiny, but much more frequent) is such that there are 3 drops in the air at any given moment. Considering the fall is 3.5 inches.. that means there's a drop roughly every 45ms. It should be tough to cut that in third.. and if you manage somehow do let me know!

Another issue you'll run into is that 12" is pretty high. Mine was originally this height. I figured: hey, I want to watch the drop for a long time and see a nice splash at the end, too.

The problem here is that even at the 45ms rate, where the drops hit is a very chaotic area. You'll get bubbles crowding up, and splash everywhere, and since these arent periodic it will ruin the effect. I've found the best height to be a little under 4". The impact region really is the most impressive part of this piece, IMO. It gets ruined when the drops come from too high, too fast. (Since the drop rebound hits the next drop falling)

If you're rate of dripping is 16ms, and the height is 12", then you will have 16 drops in the air at any given time.

I wish I had a valve. My valve is a c-clamp tightening the rubber tube.

Anyway, best of luck. Can't wait to see how you do!

-sam

Sam, if you want me to take a picture of the pump from the aquarium air pump I will. By controlling the amplitude of the pulse on the coil that pushes the pump chamber - you control the amount of fluid it pumps per pulse. This pump is small, not the size of the whole air pump as it don't use the 120 volt coil. You said in another post that you would prefer not to make the pump yourself but is that not the fun of it? I am also going to build a few of these for Christmas presents as it would truly make unique gifts. Thanks for the inspiration :)

--- I'm planning on using an IR opto switch, with the drop breaking the beam as it falls. Basically an IRLED as the emitter and a phototransistor as the detector. The IRLED will keep the phototransistor in saturation until the drop interrupts the beam, at which time the detector will come out of saturation, its output will go high, and that high-going edge will be used to start the strobe delay.

If you substitute an LDR for the delay timing resitor then you can change its resistance by shining an LED (or an incandescent lamp) on it. That way, you can use, say, a slow ramp generator to illuminate the lamp and change the delay, so the column of drops will rise and fall slowly, automatically.

---

---

3/8" drops?

---

--- Change of plans... I want to have the 12" fall, and I want only one drop to be visible when it's halfway down, so I'll need to generate drops every 6", which is 176ms apart. That's about 5.7Hz, so there'll be a lot of flicker, but that's where I want to start. :-)

---

--- I'm more interested in manipulating the drop(s) in the column than I am in the impact, right now, so I plan to diffuse the drops when they return to the tank so they won't create pressure pulses which could affect the pump. Maybe that's part of the problen you were having originally with your pump's stability?

---

--- Right. My error, thanks.

---

--- Nothing wrong with that. Clever, as a matter of fact!

---

--- Thanks!-)

-- John Fields Professional Circuit Designer

Yea a picture would be great. Have you tested it out? I'd like how well it performs.

My best results are 22 drops/second, 6mm drop diameter.

-sam

This is over my head... I know an LDR changes resistance as a function of light received. But how will you ramp it? Seems like it could be done, but you'll need another timer to somehow set when the ramp gets reset, no?

Yea I misread my ruler... it was late at night. Turns out my drops are about the size of your hypothetical drops. I actually calculated the size using the given drops/second, mL/second, density of water, and 4/3 pi r^3... turns out they are about 6mm.

I just set mine up to that speed. It look's pretty cool... it appears to be much more stable at this drip rate.

The impact is really beautiful. The good news is that at 5Hz you should be ok, I think the rebound drop will be on its way down when the next drop comes to hit it. Should look pretty cool, actually.

More than enough! At 22Hz, you need .08L/min... so you get the idea of the small quantity of water that needs to be delivered. Let me know how it goes!

-sam

You could try a piece of scotch-brite at the landing zone - the drops wouldn't splash much, I'd think.

And slant it, so the water runs off into your reservoir.

Good Luck! Rich