How to monitor an actuator's feedback line from a microcontroller?

[Skyland]

the AG-020-B electric actuator is controlled from a microcontroller. Terminal 1 is connected to the DC PSU GND, Terminal 2 is connected to the +24VDC via a high side Power MOSFET(TPS22810) as shown below. This is for power saving.



Low side MOSFET (AUIPS1021) Q5A and Q5B are used to control the opening/closing of the valve via terminal 3 and 4. The microcontroller I/O lines Valve_Ctrl_1a and Valve_Ctrl_1b provide a 3.3V signal.



- Comments are welcome regarding the power and switching circuitry.

- Any suggestion on how to monitor the feedback lines on terminal 8 and 10 from a microcontroller. A lamp is not suitable as the system will be enclosed in a box.

[David Hess]

I would use optocouplers either in place of the lamps or across the switches.

[Skyland]

I think that's exactly what I am looking for, I understand having an optocoupler in place of the lamps but how would that work across the switches?

[H.O]

...but how would that work across the switches?

When the switch is open the current passes thru the LED in the optocoupler. When the switch closes it shorts out the LED. Make sure you put the current limiting resistor for the LED in the corrent place so that you only short out / bypass the LED and instead of shorting out the supply.

[David Hess]

Just like H.O says.  With the optocoupler wired across the switch (with a series resistor), it detects when the switch is open but presumably not enough current flows to light the lamps although that does not matter.

[Skyland]

Just like H.O says.  With the optocoupler wired across the switch (with a series resistor), it detects when the switch is open but presumably not enough current flows to light the lamps although that does not matter.

Would you mind drawing a diagram?
Also, what type of optocoupler were you thinking of?

[David Hess]

Just like H.O says.  With the optocoupler wired across the switch (with a series resistor), it detects when the switch is open but presumably not enough current flows to light the lamps although that does not matter.

Would you mind drawing a diagram?

I am not really setup for that.

Also, what type of optocoupler were you thinking of?

Any transistor output optocoupler will work.  As far as the circuit goes, the optocoupler input is treated as the LED it is.

[rstofer]

Or, put an opto between +24 and pin 8 with another between +24 and 10.  In parallel with the indicators, not in series.  You need to calculate a dropping resistor to limit the LED current.

R = (24V - V_f) / I_f  where V_f is the forward voltage drop of the LED and I_f is the desired LED current.

[donotdespisethesnake]

Something like this. Repeat for 9-10. Note output to MCU is inverted.

[David Hess]

Something like this. Repeat for 9-10. Note output to MCU is inverted.

Swap the positions of the optocoupler output and load resistor for non-inverted operation.

[Skyland]

Something like this. Repeat for 9-10. Note output to MCU is inverted.

I assume you meant terminal 8 is +24V and 7 is GND?

[donotdespisethesnake]

Something like this. Repeat for 9-10. Note output to MCU is inverted.

I assume you meant terminal 8 is +24V and 7 is GND?

No, P7/P8 is a switch, it is not polarised. I'm assuming you do not have a lamp wired in, because the system is in a box, and nothing else is connected to P7/P8.

[donotdespisethesnake]

Terminal 2 is connected to the +24VDC via a high side Power MOSFET(TPS22810) as shown below.

TPS22810 is not rated for 24V operation, you will need something else.

Also AUIPS1021 is overkill in this application, but anyway is discontinued. I suggest looking for an alternative.

[Skyland]

What's overkill about the low side switch?
Also, is the 1K resistor at the gate of the low side switch necessary? I've put it there to limit the current but thinking about it the MCU current is in the uA range

[donotdespisethesnake]

What's overkill about the low side switch?

Well, how much current does it need to sink? I didn't find that info in the data sheet. Control signals are usually in the mA range, but maybe not in this case.

Also, is the 1K resistor at the gate of the low side switch necessary? I've put it there to limit the current but thinking about it the MCU current is in the uA range

It's probably not necessary but doesn't hurt.

BTW, I advise reading the datasheet for AG-020-B, since there are some bits in red you may want to watch out for...