Water flow switch

For low flow setups, like to K40 tube cooling water, you really want a flow switch that disables the laser when the water isn't flowing. A visual indicator is fine but you may not be watching it all the time.

Flow switches are available online but don't operate at low flowrates (under 1 l/min) or cause a severe restriction that an aquarium pump can't overcome or will reduce the flowrate. Float switches are available online but don't indicate flow is happening.

Other flow switches use a paddle wheel and need to be powered for the detector that detects pulses.

This Thing uses a float to indicate that water is flowing by monitoring the level in a leaky bowl. When flow stops, the water level decreases as it leaks out and the flow switch (reed relay) opens.

This Thing is attached to the side of the cooling water container and water flows through it before spilling back into the main volume. It won't work on the inlet side of the flow system.

It needs no power or additional circuitry

Water flows into the bowl from the bottom inlet and some leaks out the bottom of the bowl where there is a small hole. Because the water coming into the bowl is more than the amount leaking out through the small hole, the bowl fills up and the float rises. The bowl will overflow through the upper outlet. When the float reaches the top it closes a reed relay because it contains a small magnet.

The impedance to water to flow is small.

The float contains 2 small magnets (for balance) although only one, or even only half a thin disc magnet may be enough to close a reed relay in the centre of the tube running up the centre of the bowl. Make sure it switches reliably. My magnets are thin discs, about 8mm x 1mm

The round float piece is printed as one piece with supports and simply broken off the centre tube afterwards. The lid is also printed with supports.

The only part that needs to be waterproof is the reed relay and its connection.

I've made 3 Bowls, all need supports to print because the barb sticks out into mid-air but use the setting for support placement "Touching Buildplate" for the Inline version so that it doesn't fill up any of the water passages, it will only put supports under the two barbed tube couplings. You can of course use this settings for the other Bowls, it should work for them too.

I suggest the easiest one (no out Barb) if you don't mind the water running out the sensor directly.

Of course you don't need to print this, a tube, a pill bottle, a styrofoam disc for the float and the reed switch and magnet can be assembled into the same system with some hotmelt.

You can buy a small float switch online, all it will need is the leaky bowl, even just a plastic cup with a hole in the bottom attached to the side of the water container will suffice, no need to get complicated. The discharge goes into the container which overflows when water is running. Just make sure the float turns on when at the top, you may need to mount your float upside down.

I use telephone cables to connect the reed relay to the K40 switching circuit. This is a nice cheap, thin flexible cable with a connector attached. It also has the advantage that you can install a telephone interference filter to prevent noise pickup - those ADSL filters for landlines that (used to) carry your internet.

I use all 4 wires in the telephone cord (if it has 4) to reduce the resistance, I connect pairs in parallel. The line filter's common mode choke will add about 24 ohms impedance if you use it (you will only be able to use the inner 2 conductors, red and green if you want the filter to work, the outer wires don't get filtered and so have lower electrical resistance)