# How to wire-up a flow sensor with Hall effect to operate a 12V relay

#### sghioto

Joined Dec 31, 2017
3,951
but my guess is around
15 liters/minute at 1500 rpm.
OK, if that is accurate then the sensor output is 112.5hz. Why don't you just measure the frequency from the sensor at 1500rpm?
SG

Joined Jan 15, 2015
7,148
There you have it. The magic part is:
"In a 1L column flow is 7.5 * 1 * 60 is equivalent to the number of pulses flow through a premium output Voltage: 3.5-24VDC, current can not exceed 10mA, Ends stream output 450 pulses a liter of water"

Sghioto had it figured and there is the 7.5 K Factor. Sensors like this are "close enough" even though they aren't linear they are good enough. Now you have a frequency to work with. It would be nice, as suggested, to check the output for a given RPM or two.

Ron

#### bowsprit

Joined Oct 2, 2018
51
I can't run the engine, as I am waiting for parts from Germany., which should have arrived by now, but due to covid induced transportation problems, they have not....
But I could built the circuit with the LM2907 ic and change values at a later date to suit.??

#### sghioto

Joined Dec 31, 2017
3,951
But I could built the circuit with the LM2907 ic and change values at a later date to suit.??
Sure why not. Change C to .1uf for relay closure below 50Hz.
SG

#### bowsprit

Joined Oct 2, 2018
51
SG,
According to the diagram you sent me, the Relay closes with c=1uF when f reaches 5Hz and in your last reply "Change C to .1uf for relay closure below 50Hz." --> What do i change?

#### bowsprit

Joined Oct 2, 2018
51
According to Johnson USA, Their Flexible impeller pumps,(my saltwater pump) put out a minimum of 4 US gallons/min (14 l/min) at 0.3Bar at 700 rpm and 11 Gallons/min (38.5 l/min) at 1750 rpm.
Hence the relay must become energized at a minimum of 12 l/min.

#### sghioto

Joined Dec 31, 2017
3,951
Hence the relay must become energized at a minimum of 12 l/min.
Change R to 56K.... leave C at .1uf
SG

#### Ian0

Joined Aug 7, 2020
7,032
using the LM2907 speed switch. From the data sheet:
10.2.2 Other Application Circuits This section shows application circuit examples using the LM2907-N and LM2917-N devices. Customers must fully validate and test these circuits before implementing a design based on these examples.
View attachment 216262
The output will drive 50mA, so that’s most 12V relays and quite a lot of buzzers,. As you need the output energised when the speed is lower than the threshold, use the n/c contacts on the relay, saving a resistor and a transistor; but you do need the 16-pin version, with pin 11 biassed to half supply.

#### bowsprit

Joined Oct 2, 2018
51
Thanks for that info Ian0

#### sghioto

Joined Dec 31, 2017
3,951
use the n/c contacts on the relay, saving a resistor and a transistor; but you do need the 16-pin version, with pin 11 biassed to half supply.
I disagree with the above. Never been a fan of having a relay normally energized and don't see the need for the 16 pin version.
SG

#### Ian0

Joined Aug 7, 2020
7,032
I disagree with the above. Never been a fan of having a relay normally energized and don't see the need for the 16 pin version.
SG
For the 8-pin device, the non-inverting input to the comparator is connected to pin 1, and the inverting input is connected to ground.
The hall-effect device output will go from just above ground to just below +5V.
The non-inverting input will never go below the inverting input, so the comparator output will be permanently stuck high.
It won't work.
The 8-pin device is designed for magnet/coil based sensors which generate their own signal.

Permanently energised relays are a fact of life - every safety circuit in the world works that way.
Presumably, the relay is required to isolate two supplies. Otherwise it wouldn't be used.
Using the n/c contact confers an extra advantage, if the supply to the sensor and IC fails the relay will close and sound the buzzer.

If there is no supply to isolate, then there is no need for a relay. Use the same circuit as you drew to drive the buzzer instead of the relay coil.
A lot cheaper than a relay.

Last edited:

Joined Jan 15, 2015
7,148
Regardless of the relay configuration you choose and alarm be it audible, light or both something to consider is when this is powered, until you have an engine started and drawing sea water you will have an alarm state.

Ron

#### bowsprit

Joined Oct 2, 2018
51
I notice there is confusion using n/c or not contacts of the relay . My original diagram presumes the 'engine running' state ( e.g the hall sensor outputting & no alarm)

My engine start sequence, is as follows: On turning the key to 'contact', the Main Positive and Negative relays for the starter motor come in, power flows to the oil pressure sensor, water temp sensor and hall sensor relay, (which is currently sounding the alarm)
Turn the key 1 step further, the engine starts: ........The start relays disengage, both oil pressure & temperature sensors remain powered up and the hall sensor relay de-energizes (killing the alarm) , conversely when the water flow fails, the hall effect pulses decrease to near zero, the relay energizes and the alarm should sound.

A lot of yachts have just a 'high water temperature' alarm. The problem is that by the time that alarm goes off, the saltwater impeller has been running dry for some time and has disintegrated. (from experience).

The water 'flow' monitoring idea is a excellent solution, PROVIDED you gentlemen agree with each other on components use and design.
I can get the 8-dip type easy, but not the 16-dip
Here's hoping....................

A BIG thank you, so far, to all contributors.

#### bowsprit

Joined Oct 2, 2018
51
Currently marooned in New Zealand due to Covic19

#### sghioto

Joined Dec 31, 2017
3,951
Wow, is that a good thing?
SG

#### bowsprit

Joined Oct 2, 2018
51
Well........depends.........for purchasing a LM2907 16pin, it's no good here.................. but on the other hand , very good, as there are barely any covid problems, and what ever there is under quarantine and under strict control, unlike the wild west covic probs in US.
Either way, I can't leave, as ALL ports of entree are closed around the S. Pacific.
--------------------- I am digressing...........
How about using a 555 timer (got plenty of these in stock) as drc_567 suggested ??

#### drc_567

Joined Dec 29, 2008
1,156
... 555 chip is definitely a possibility, but it will be necessary to locate a missing pulse circuit diagram that actually works. The circuit that was previously posted does not seem to function properly, at least in a LTSpice simulation. There is another missing pulse circuit on a TI 555 datasheet, but I will have to confirm that it is a good one.

#### bowsprit

Joined Oct 2, 2018
51
I just (in 5 min ago) used a bridge rectifier and held the hall sensor under a running tap. It generated 5 vdc

#### sghioto

Joined Dec 31, 2017
3,951
But does the voltage change when the flow is slower?
SG