How it Works / How it Fails : Touchless Soap Dispenser

Today’s subject is a little machine that automatically dispenses soap.

Touchless Soap Dispenser

Touchless Soap Dispenser

This particular unit had been working in our kitchen for a while but, stopped working after a few months. The machine would sense my hand and the motor would run but, no soap would dispense from the nozzle.

Today, I took it apart to see how it works and, hopefully, come up with a fix for it. (I really like having a touchless dispenser). Here’s how it went:

First tried the obvious thing that any end-user of this machine would be expected to do. I added some soap. I replaced the batteries. This didn’t work. Got exactly the same result as before: motor could be heard whirring and the blue LED on the nozzle housing would illuminate but, no soap coming out of the nozzle!

At this point, I began dis-assembly, looking for any hints as to where the problem was.

Before removing even one screw, I confirmed one thing. In the photo, you can see metal disk, about 10mm in diameter that, to my eyes, looks like the reversing sensors on the back bumpers of many cars. Turning the unit upside down, I could see two small holes on the underside of the nozzle housing that seemed to be portals for some kind of emitter-detector pair. So, waving your hands down by that disk won’t accomplish much! That disk it turns out is only a switch to turn the unit on.

Inside the base, I could see a DC motor attached to a plastic assembly which had lengths of flexible tubing exiting from one side. Later, I would take this all apart and confirm the assembly was indeed a rotary peristaltic pump. I ran the pump while watching the transparent tubing. I could see bubbles moving in the right direction but not far enough before the motor’s duty cycle expired. To satisfy my curiosity, I took the pump apart and saw three planetary rollers that, during operation, would squeeze the contents of the tubing toward the nozzle.I couldn’t really find anything that might be causing this in the base area so, I proceeded to take apart the tank and the nozzle housing thinking there must be some kind of restriction further downstream. The nozzle  itself is kind of a 90 degree elbow with one arm of the elbow receiving the flexible tube, the other arm being the nozzle itself. I blew air through the elbow and, I could not detect any blockage as I could feel the stream of air exiting. I measured the inner diameter of both ends of the elbow and found that the exit diameter was much smaller than where the soap enters the elbow. So, I decided to bore out the exit hole of the nozzle. Maybe the tubing material gets tired over time and absorbs too much pressure in the long run going up to the nozzle to the point where there is insufficient pressure to push the soap all they way out? Boring out the nozzle would reduce the pressure drop.

After re-assembly, the machine started to work perfectly. Only unfortunate thing really is that I probably really didn’t need to disassemble the whole dispenser: I could have just drilled the nozzle in place! Oh well. My hope is, somebody might find this and go directly to this solution.