Electronics > Beginners
Simplest circuit to connect/diconnect a wire.
<< < (3/6) > >>
KMoffett:

--- Quote from: hgg on April 20, 2019, 12:35:53 pm ---But generally speaking can you connect the MOSFET drain/source or source/drain and achieve the same result?

Assume that you do not know where the two connections are going and you want it to behave like a relay
which connects two wires no matter the polarity.

--- End quote ---

No, MOSFETs are polarity sensitive. They need to be connected correctly.  The connections that I outlined are for a N-channel logic level MOSFET.

Ken
SilverSolder:

I would just use a reed relay,  they are cheap and cheerful.

For example,  https://www.digikey.com/product-detail/en/littelfuse-inc/HE3621A0500/HE206-ND/133255
hgg:
Just tried with a IRLB8743 MOSFET and didn't work.
Drain to Vcc, Source to blue wire and PIR signal to gate together with a 10k to ground.
Green wire goes to Vcc, and I want to bridge blue and green together.



SilverSolder, I thought of that as well, but I don't have any available at the moment.
It also needs 10mA.

RES is that an optocoupler at the control side?
Thanks
Ian.M:
RES showed an internal diagram of a PhotoMOS optocoupler which is basically a miniature solid state relay.  Here's a typical one's datasheet: http://www.ixysic.com/home/pdfs.nsf/www/CPC1017N.pdf/$file/CPC1017N.pdf

Note that its load side is only good for 100mA, and its control side is just like driving a LED - i.e it needs a series resistor.

*IF* the wireless doorbell draws less than 100mA through its switch,  the one I linked would do the job.   If it draws more current, it would need something beefier.

While a reed relay could also work, they don't like large load currents, especially surge currents while switching, which tends to result in rapid failure.   If its more than just a few mA for a logic signal, you'd need a normal relay, and the PIR may not have enough 3.3V output current to drive one.

You cant do it with a single MOSFET - if you use a
NMOS one like your IRLB8743, with the source to the load, you need to drive its gate well above the Vcc its drain is connected to to turn it on and pull the gate right down to 0V to turn it off.

Use a PMOS one, source to Vcc and drain to the load and its more practical, but you then need a driver for it as its off with its gate at Vcc (use a pullup resistor to Vcc) and turns on when the gate is pulled down to a lower voltage.  If Vcc>3.3V you cant drive the gate direct from a 3.3V control signal.  A low Vgs threshold NMOS one could be used to drive the PMOS gate.  See https://www.eevblog.com/forum/beginners/mosfet-175797/ from last month.
hgg:
I see.
I just checked and the current that passes between the two wires when shorted is
around 10uA.

So in order to achieve the same result as thePhotoMOS optocoupler,
I need an optocoupler and two mosfets, correct?
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod