In fact, it looks like that may be useful straight away.

So in Dave's situation he has a 33k ohm resistor and 5V feeding the base of the transistor Q1.

The current it was getting to operate on the V3 would have been;

I = V / R I = 5 / 33k I = 0.00015151515A or 0.1515151515mA

Now the V4 uses 3.3v we can work out the resistance required to have 0.1515151515mA at the base.

R = V / I R = 3.3/0.00015151515 R = 21780 Ohms

Since resistors don't come in such specific values we can round it up to a 22k ohm resistor which will give 0.15mA which is probably OK. Or play safe and round it down to 20k ohm for 0.165mA

Thanks @Rich, great explanation!

@Dave the value I normally use with 3.3V signals on a transistor base is 1kohm. I would suggest maybe trying to use a potentiometer and start at 33k and dial it down until 3.3V switches the transistor on.

Lol, thanks @Rich. I'm glad we arrived at similar values I guess 1.5kohm would probably be the closest standard value then.

That was for Q3 which I then realised wasn't connected to the signal and was 24v (I've edited that bit out now since it's irrelevant)

Q1 is 22k which to me seemed high, so I'm glad you said 1k ohm.

I use 1K ohm on my transistor switching circuits at 5v, I haven't tried one at 3.3v yet but I would imagine that it would work without issue.

Wow thanks Rich for doing the math for me. I guess I was getting lazy by just guessing the value based on percentages between the 5v and 3.3v drop. I think I should have a 15k Ohm resistor.

BTW, don't you sleep? Isn't it well after midnight over where you live in the "Old Country"?

I was just looking for the minimum current needed for a TIP120 to operate but I think the sun we have had today has shrivelled my brain... I think it's 1mA but don't quote me on that.

Basically, look at the datasheet for the transistor or fet you are using, find the minimum current needed on the base/gate for operation, take 3.3 and divide it by that number and that's your maximum resistor value.

So if I am right with 1mA it's 3.3/0.001 = 3k3 ohm maximum.

1k ohm will put; 3.3/1000 = 3.3mA at the base/gate which should be high enough to operate and below the maximum current for the base/gate.

Well, I did insert a 12k resistor at the base of Q1 and the circuit switches fine now. Looks like you guys are saying I can go down as far as 1.5K. I do have that value but I think I'll keep it at 12k for now. I always say if it isn't broke, don't fix it. Also the math work.

Thanks for the help on this one!

Yeah I'm the same with the whole if it isn't broke way of thinking to be honest. I know I didn't work out the resistor size for my TIP120/122 circuits, I just threw a 1k in and it worked so it was all good.

And fyi, it's 10pm here so not quite as late as you thought but no, I don't sleep much... 5 or 6 hours a day at weekends, 4 or 5 if I'm lucky during the week. That's all I need and it gives me plenty of time to play with robots