5v tolerant pin

At the last moment, I realized a breadboard layout was going to connect Photon pin A3 to the output of a digital FF. I need to read the state of that FF. Unfortunately A3 is NOT 5v tolerant.

Question: Is there a simple, standard trick with a resistor or two to allow me to get this to work easily?

Maybe a resistor in series, or a couple resistors to drop the voltage?

I figure someone has done this already and learned what works and what doesn’t.

Hi @jim_hahn

Can you just us a simple voltage divider to drop the 5VDC signal down to 3.3VDC?

I think that works. I just thought it’s such a common problem (?) somebody would have worked it out. And made all the mistakes with respect to VIH,VIL and current draw.

TTL puts out max 5v HIGH and we want, say, 3v to appear on the Photon A3 input.

TTL output goes to resistor B, then to Photon A3, then through resistor B to ground.

A/(A+B) = 3/5

A = (3/2) B

If B is 10K (a reasonable choice?) then A = 15K

Well by my calculations that would take your 5VDC line and convert it to 3VDC so that should be good if you ask me. As long as you wire it like this(Vin being the TTL signal and Vout being A3:

Of course I would check the Vout voltage with a meter before connecting it to A3 on the Photon.