Help with converting flow meter output signal to power a relay.

[1967marti]

Hi all!

I'm trying to find a cheap/easy way to convert the pulsed VDC output from a common liquid cooled PC flow meter to power a PCB mounted relay.

The board I'm working on will have a 12VDC supply to it; so the relay that needs to be powered can use any voltage below that (if it matters). The flow meters that I'm hoping to use are the cheap ones used for water cooling in home PCs. The meters have an input of 5-24VDC and an enclosed hall-effect sensor that sends out a signal that is pulsed by how fast the internal fan is spinning.
The use of this is for a coolant stream for a CNC mill. I need to have a relay triggered if the flow is stopped. The pressure of the coolant is too low to use a screw-on pressure switch, so a flow meter is my only option. Since the  flow of coolant can be throttled up or down depending on the material being cut the relay should only be triggered if it detects no pulses coming from the flow meter, AKA the speed of the pulses doesn't matter just that they occur at least 2 or 3 times every second or so. 

I figure this is something that can be done pretty easily with a simple 555 timer circuit? At least I hope so.... Any ideas on this or a circuit diagram I can use?   

Thanks!

- matt

[1967marti]

*side note*  To help grease the wheels: We have a laser cutter on-site; as a "thank you" for your help I could do some custom acrylic parts.

Thanks!

- matt

[Billy]

Hey Matt,
Not a really hard problem to solve I don't think.
...but many people that post for the first time never come back to see if there was an answer given. So effort spent to answer questions from brand new members seems a waste of effort.
Let us know if you're still looking for an answer. I'm sure we can come up with something that uses a diode, op-amp, a cap and a resistor or two.