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Opto isolated. I cannot figure out how I will trigger it. Instructions are vague to say the least. Thanks. wendeekun Low Voltage MOSFET Switch Module 12V 24V 36V Field Effect Transistor Module Mosfet Kit Uses Optical Isolation https://amzn.eu/d/7X0lZdg


Added by RM:
Copied from comments by Mikmikmikmik on Spehro's answer.
This demonstrates that the real requirement is not what was stated: Previously unmentioned related devices in bold.

  • So, here's what I am trying to do, only now do I realise the output of this sound module needs to be inverted to go low when sound happens. Sound Detection Sensor Module Suitable for Arduino and Microcontrollers amzn.eu/d/iM3DgYp.
    This goes into pin 2 of a delay off 555 monostable. build-electronic-circuits.com/555-timer

  • I was able to get pin3 of 555 +ve, on trigger of +ve into pin2 from the sound board when sound occurred, don't ask me how. Maybe it is the low pulse after the sound board realises there is no more sound. BUT, when pin 3 of the 555 was connected to the + in of that MOSFET board, and -in of the MOSFET board was connected to ground, the MOSFET stayed energised and wouldn't turn off. I am missing the small cap at pin5 of the 555 CCT though. –

  • Do I connect +in of that MOSFET board to 5V and invert pin3 of the 555 output and connect that to the -in of that MOSFET board? If so, do I need a R from +5V to +in to restrict current once inverted 555 pin 3 is active? Thank you all. –

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  • \$\begingroup\$ If you buy a module without good specifications, then you either need to trace the circuit, or run some tests. The input is on the two male header pins. The amazon page says the input works from 3.3V to 12V. I see an LED and resistor that are probably in series with the opto input. If that is true then it probably won't work at 3.3V. On a similar module, I had to short out the LED to get it to work with my MCU GPIO output level (3.3V). \$\endgroup\$
    – Mattman944
    Apr 30, 2023 at 16:23
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    \$\begingroup\$ This isn't a support site for e-waste sold on Amazon without instructions how to use them. \$\endgroup\$
    – Justme
    Apr 30, 2023 at 18:27

2 Answers 2

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This looks pretty dubious. There is an Everlight (Taiwan) version of the original Sharp PC817 and a clone (I think) version of the IRF5305 P-channel MOSFET. Here is a schematic of what looks like the same thing from this video.

enter image description here

Note that the PCB lacks the flyback diode on the PCB (not optional for loads which are inductive or even if wiring is long) and also there is nothing limiting the Gate-Source voltage- anything over about 20V on the supply will exceed the absolute maximum MOSFET Vgs when the module is "on". So 24 and 36V will both do that.

In any case to turn it on you simply apply 5V across the two header pins marked 0 and +. The IRF5305 is a logic-level MOSFET so supply voltage of 5V to approaching 20V is okay. I would guess it won't die with a regulated 24/36V but it's very much not good engineering practice.

If you need it to work with a 3.3V drive, you can short out the visible LED. The IR LED in the '817 has about 1.2V drop and the visible LED will have 2-3V drop, depending on type and color, so the current will likely be insufficient or marginal with 3.3V drive.

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  • \$\begingroup\$ So, here's what I am trying to do, only now do I realise the output of this sound module needs to be inverted to go low when sound happens. Sound Detection Sensor Module Suitable for Arduino and Microcontrollers amzn.eu/d/iM3DgYp. This goes into pin 2 of a delay off 555 monostable. build-electronic-circuits.com/555-timer \$\endgroup\$ Apr 30, 2023 at 21:08
  • \$\begingroup\$ I was able to get pin3 of 555 +ve, on trigger of +ve into pin2 from the sound board when sound occurred, don't ask me how. Maybe it is the low pulse afte the sound board realises there is no more sound. BUT, when pin 3 of the 555 was connected to the + in of that MOSFET board, and -in of the MOSFET board was connected to ground, the MOSFET stayed energised and wouldn't turn off. I am missing the small cap at pin5 of the 555 CCT though. \$\endgroup\$ Apr 30, 2023 at 21:16
  • \$\begingroup\$ Do I connect +in of that MOSFET board to 5V and invert pin3 of the 555 output and connect that to the -in of that MOSFET board? If so, do I need a R from +5V to +in to restrict current once inverted 555 pin 3 is active? Thank you all. \$\endgroup\$ Apr 30, 2023 at 21:21
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There is a jumper (2 pins), with + and - printed on the PCB, with this you can switch the IRF5305 P-channel MOSFET on.

The current flow goes from + over the 1 kΩ resistor, over the LED and then over the internal IR-LED of the 817 optocoupler.

They use a 10 kΩ pullup to source (possibly +12 V) to gate, this means you need a current of 1.2mA (if the transmission fac tor is 100%), if it is only 50%, you could need 2.4 mA on the input. But it could be 600% too and then you would need only 0.2 mA. There is no special marking on the 817, so you have to try it.

If the voltage drop at the red (?) LED is around 1.8 V, on the IR-LED around 1.1 V and the 1k resistor (U=R*I=1 kΩ * 2.4 mA = 2.4 V), then you need a voltage of 1.8+1.1+2.4=5.3 V to switch the MOSFET on.

I would say, try it with 5 V if you want to switch 12 V and measure the gate-source voltage to verify the result.

I you have soldering-skills, then you could change R3 to 100 kΩ, it does not make any sense to use such a low resistor value.

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  • \$\begingroup\$ @Nedd - R2 is simply a gate resistor. The gate has a capacity of around 1nF and with the transistor inside of the 817 you can let the current flow to Ground. The resistor is reducing the current. Because of the LED in series to the optocoupler LED, the necessary voltage is possibly relatively high, but with this configuration (this red LED in series) you can directly see the amount of current flow over the internal IR-LED. \$\endgroup\$
    – MikroPower
    Apr 30, 2023 at 18:59
  • \$\begingroup\$ MikroPower - As a follow-up to the prior comment, your assessment does seem correct, and per the info from Spehro. So the circuit is ill spec'ed, and prone to other problems, again per Spehro. Isolating the indicator LED (or shorting it) seems to be an option for operation below 5V. \$\endgroup\$
    – Nedd
    Apr 30, 2023 at 20:07

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