Reverse engineering the Aspen Mini orange sensor

[2019-01]

The mini orange pump and its “secondary reservoir” with the sensor wire

I want to use an Aspen mini orange pump to circulate cooling water in a Liebig condenser (it may or may not work, I dont know yet). But these pumps are just made to remove AirCon/HVAC condensates, and so they are driven by a separate water level sensor (the little transparent box).

This sensor uses a buoy with a magnet, it starts the pump when the level of water rises to a limit, and stops the pump when the level returns to normal. There is an extra high level to signal an error condition.

I need to operate the pump continuously, without this sensor or reservoir. No information about the sensor was available online, so I had to disassemble it for analysis.

The sensor is fully potted in beige epoxy, itself contained in a cylindrical plastic tube. On the pump side, the wire has a RJ11 plug with 6 contacts. The potting was quite painful to remove, but @skywodd on twitter suggested heating (I used a small lighter-like torch), and it worked quite well to soften the epoxy without damaging the internal components too much.

First efforts with a wood chisel
After most of the potting was removed

So after the potting is removed, we just find 3 hall sensors with a common power supply, and no other component, not even a decoupling capacitor. The sensors are in a SIP-3 package and have a 337 marking, which suggested the AH337 from Diodes Inc. The datasheet indicates quite a old component, the marking and package matches. The AH337 is a hall switch with an open drain output, which suggests that some simple mechanical switches to the ground are enough to use the pump.

Here are some measures I did: The supply voltage seems to be quite high at 22V DC (yellow wire, brown is ground), and the two “start” wires (red and orange) have a pull-up to this voltage. The “stop” (green) signal does not (maybe the pullup appears only when the pump is running and the stop signal is usable, I did not check). A simple ground pulse on the red or orange wire starts the pump, and a ground pulse on the green wire stops the pump.

Now I can continue my other projects.

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