Need help with a pulse/counter input circuit on hall effect flow sensor

Question

I am having trouble wiring up a flow sensor to my pulse/encoder/counter card.

I am using a hall effect flow sensor that works fine when wired to a digital counter display.

However, when I try to connect it to my high-speed counter card, it kills the whole circuit. Here is my wiring diagram:

Troubleshooting steps:

• I confirmed I get 5 VDC output from the flow sensor when the magnet rotates around to the line.
• I confirmed that the flow sensor works with just the digital counter in the loop. It counts correctly as it flows.
• I confirmed that the counter input module works by jumpering 5 VDC across the input

Problem:

• When I connect the negative from the 5 V power supply to the "A return" terminal of the counter module, the hall effect flow sensor output drops to 1.44 VDC, and also prevents the digital counter from detecting pulses.
• Removing the digital counter from the loop, the problem still remains, 1.44 VDC on the signal wire of the flow sensor.

It seems like either I'm wiring the counter wrongly (wiring it sourcing as opposed to sinking, for example,) but I don't know how else it could be wired, or perhaps the hall sensor can't output enough current to trigger the input on the module.

Any ideas?

Answer 1

According to the datasheet, the 1734-IJ has an input current specification of 19.1mA @ 5V input. This means that the hall sensor must be able to provide that much current (which is obviously not the case).

So you need a e.g. an op-amp circuit (google for "voltage follower" or "buffer amplifier") to buffer your hall effect flow sensor output. The outputs of most op-amps are able to deliver more than 19.1mA into a load.

Answer 2

So, with the help of a friend who has a mild understanding (yet still much more than me) of transistors and electronics, I have resolved this issue.

Apparently, my mistake was treating the signal line like a +5VDC source.

With only the flow sensor and digital counter in the circuit (each of which is not loop-powered,) it does indeed show 5 VDC when measured to the negative of the 5V power supply.

But as best I can understand, when you add the counter card/module into the loop (which amounts to a load in the circuit), the hall sensor reverses the current flow, sinking the sensor voltage to the power supply return.

I'm sure someone else can provide a better explanation than this, but it works now, and that's all I care about. Here is the CORRECT diagram: