Converting Active Pulse Output to Passive

An opto coupler such as an H11D3 that comes in an 6 pin DIP package will require at least a small resistor to limit the LED current. But otherwise might be applied directly.

Depending on the frequency, a relay could be the simplest solution, single component.

If you need pulse division or multiplication, Acromag make a series of pulse translation products that also are a standalone solution. Their modules can also generate an industry standard 4-20mA signal that could be an alternate input to your pump control.

The opto coupler sounds like just what I need, could you provide a connection diagram? Thanks.

To be sure the npn output transistor is rated ok, we should know the voltage & current of the VFD input. Also the voltage & current of the flow meter output, to select the current limiting resistor for the LED input.

Do you have a method to make the VFD input scale correctly for the flow meter output? I must say, I have never run across a metering pump that accepts a pulse input, usually continuously variable voltage or current scaled to a desired pump output range.

However, it seems like it would be a handy interface to have, given the standard pulse output of typical mag flow meters.

To clarify the installation is a mag flowmeter sending a pulse to a chemical metering pump which then meters the correct amount of peroxide into the water according to the number of pulses. The flowmeter does also have an analog output, but the metering pump only has a pulse input.

The magmeter output is rated at 24vdc @ 250mA, the pulse input on the pump is also 24vdc @ 20mA.

Assuming your controller needs about 20mA of current to detect a pulse, you will need to drive the LED at about 20mA. Connect a 1200 ohm 1/2 watt resistor to Pin 1 of the IC. The resistor and Pin 2 connect to your mag meter output. If the ON duty cycle is likely to be more than 50%, make it a 1 watt resistor.

Pin 5 connects to the positive side of the VFD input module. Pin 4 connects to the return for the VFD input.

At these voltage levels, you really don’t need to worry about lead length and routing, normal industrial wiring methods will work fine.

If the output transistor doesn’t provide enough current to trigger the VFD input, you can go with a higher current into the LED, up to 60MA.

A 1000 ohm resistor will give you 24 mA, you will want a 1 watt resistor in that case.

eBay sells little 24Vdc modules on circuit boards with terminals for about $30. They had some for $2, but not 24V.

This module is what I'm going to order, that is exactly what I need. I'll let yopu know how it works out. Thanks for the help.

Your initial statement seems to indicate the flow meter pulse output could be powered directly by your VFD input power. It has a specification that would indicate a dry contact, so just hook up the wires directly.

However….

No, the flow meter pulse output cannot use an external power supply, I did ask the manufacture that directly.

The specs you show have a range, which would to me indicate the acceptable specs for an interrogating voltage, external voltage.

“High Level= 5-15vdc max 250 mA, low level <1vdc).”

It also could also be telling us at 250 mA it will only have 5 volts to drive it. So maybe we need to use the 15 volt output rating to drive the LED. Except you also mention later 24 Vdc.

Maybe the manufacturer has a load vs voltage curve, which would then confirm the active status of their output. Then size the resistor for a 20mA output at the appropriate voltage.

When designing the resistor for the LED driver, the voltage determines the current limiting resistor value.

Can you provide more specific information? An interface diagram for the flow meter and the metering pump would be nice. Better yet, can you provide the manufacturer and model numbers of the pump and meter? Knowing the approximate distance between these two devices would certainly help, too. The circuitry for a distance of just a few meters will be vastly different from the circuitry to travel the kilometers of a large farm.

One way is to use an "Open Collector" (OC) comparator such as a LM393 or LM311. The comparator open collector pulls the output low but lets the output device pull it high.

https://www.build-electronic-circuits.com/open-collector-outputs/