Since I have started adding fertilizers to my aquarium using the Estimative Index, I’ve found myself dosing 10ml of liquid into the tank 3 times a week. This gets old very soon, so I started thinking about a way to automatically add the correct dosage of liquid. Since all problems look like a nail if you have a hammer, I started thinking about syringes actuated by motor-driven threaded rod. When looking in my junk bin for a suitable timer to drive the motor, I found a simple 230V timer and realized that it already contained a small but heavily geared motor which turned on a daily cycle. So I decided to try and build a dosing pump out of it. This is the result.

Oh, and while I was at it, I decided to make a double dose version which allows me to dose 2 separate liquids on a 24h regimen.

First I removed the center cover from the timing ring, and then took off all of the timing notches.

Then I opened the case and removed the microswitch and indicator LED, along with its resistor.

While I was in there I drilled a hole and installed a protruding bolt which will form a pivot point for the syringes.

I then constructed a mounting bracket for the syringes out of angled aluminium channel, which conveniently has the correct inner dimensions to snugly hold 10ml syringes. I also made a clamp arrangement to hold the ends of the plungers. An aluminium plate was shaped roughly oval and screwed onto the timer’s revolving ring.

Lastly I drilled a hole into the oval alu plate and into the syringe holder, in such a way that the plungers actuated correctly. This is the critical step; get the location of these wrong and either the plungers will hit the end of the syringe (thus stalling the timer motor), or the plungers won’t extend far enough to fill to a correct dose. You’ll find that the travel of the plunger is equal to the diameter of the circle made by the mounting hole in the oval timer disc, and the mounting bracket hole should be positioned so that the plungers can fully retract.

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