Defining resistors between Photon and ULN2803A

I have a problem defining which resistors i have to use between Photon and ULN2803A. Photon output is 3.3v and ULN expect 5v i think, so the resistor is necessary.

Uhm, when you say the output of a pin is 3.3V and your external device expects 5V a resistor won’t help since it only brings voltage down, but not up.
To “boost” the voltage you’d need a transistor.

On the other hand, if your ext device “sends” 5V signals out which you want to read via D0, D1 on a Photon, you’ll be fine without resistors as these pins are 5V tolerant.
But if they weren’t you’d need a voltage divider (two resistors with the pin attached to the mid point) rather than a mere in-line resistor.

BTW: The ULN2803A has a 2k7 current limiting input resistor builtin already and I’d say 3.3V might be enough to switch the darlington on. But to be sure, just test it, you won’t damage the Photon.

Thanks for answer.

I fund this in documentation of ULN2803A

The ULN2803A has a series base resistor to each Darlington pair, thus allowing
operation directly with TTL or CMOS operating at supply voltages of 5.0 V or 3.3 V

As you mention it has 2k7 resistor builtin and thats for max input 30v.

So all this means that i don't need resistors at all?

I’d say so.
But where did you see 30V as input voltage for the ULN2803A?
I’d say that does not apply to the control (input) pins but the supply for the controlled (output) pins.

I saw in documentation the max input that ULN accept is 30v. I might understand it wrong, but in my case this isn’t relevant.

Thanks for help :wink: