Curcuit that acts like a switch/relay but do it with a transistor?

i have this motor controller (220AC) and it works super!
Now i have phisical switches to turn it on and change it's rotation.
Now i want to be abel to control this with a MCU and a transistor
This is how the wiering looks like (see atacced file)

Now i want to be able to control number 9 and 10 with a transistor.
Have tryed NPN but it seems to me i don't understand something...

Some basic understanding or any idea how can i make this thing work?


24V is a little high, verify your transistors are rated for that much voltage in their datasheets.

I don't see a reason why it wouldn't work, if you connect the 24V ground and Arduino ground together. When using transistors as switches, you pretty much always need a common ground.

The transistor is not the problem... the problem is how to wire the transistor so this will work :smiley:
The problem is that the "load" have it's own ground so i cannot wire it like
put 24V+ to collector and between the 24V+ and collector put the load... I cannot do this, becouse the load have
its ground seperate. So if i connect 24V+ to collector and pin X to collector this means 24V+ is always there so the switch is always on reagarding the transistor...
Get it?
Any idea?

If you cannot have a common ground, then you probably want to use a relay instead of a transistor.

I know this but the thing is i don't want to use relay.
Shure there is a way to to this with a transistor

What makes you think it can actually be done? Why do you believe relais (which are slower, prone to fail and expensier) are still produced and sold in such large volumes?

I cannot do this, becouse the load haveits ground seperate.

An opto-isolator can be used if you need isolated grounds. I don't see that you have a ground available on the terminal strip unless the "reference" is ground, so you might have to tap-into ground somewhere else.

I haven't got the circuit details figured-out, but there should be a way to use an opto-isolator.

When you want to replace a switch in an "unknown" circuit, a relay is often the best solution. And of course, a relay doesn't need a common ground. In fact, a relay doesn't need a ground on the contact-side.

What you are looking for is a "solid state relay" and there is a plethora of choices. It's usually best to pick one that has around the same abilities as the load you are trying to switch. That is, I wouldn't put a 5 Amp SSR on something that is only switching 10-20 milliamps. This device: Catalogs/Datasheets Download | Automation Controls | Industrial Devices | Panasonic although expensive ($7.50) is a robust solution. You can of course roll your own, but you may fail a few times before you get it right. I would do it with a transistor optocoupler connected to another NPN in "darlington" mode. I doubt a raw optocoupler will have enough CTR to reliably turn on the input. If trying to drive from the Arduino you are limited to 20mA on the LED side, so the opto without another transistor is "iffy".

I would do it with a transistor optocoupler connected to another NPN in “darlington” mode.

Just for my own education here – I assume this will introduce some voltage drop in the switched load? The Vce of the opto plus what, the Vbe or Vce of the NPN?

You're right, you introduce a voltage drop but not any more so than that of a commercial prox switch. Usually 24V inputs are designed to switch on at around half voltage, at least may PLCs' I/O cards do. The absolute best you can do is a drop of 1.4V if both are saturated.

The link I posted takes you to a MOSFET solid state relay good for several hundred milliamps and an "on" resistance of 0.37 ohms.

I think either method will work reliably and its a 6-pin DIP, not so tiny it will drop on a breadboard.

Sample circuit using PC817 optoisolator (datasheet ) (Digi-Key):