I’m trying to protect a Photon analog input from up to 5V input while maintaining accuracy for 0 - 3.3V range.
A 7805 voltage regulator supplies 5V to the Photon and an environmental sensor (sensor requires 5V to operate). The output from the sensor connects to an analog pin (A1). When the sensor’s probe is connected and working, the typical output range is from 0.7V - 3V which is perfect for the Photon.
HOWEVER, when the sensor’s probe is not connected (and a user may not have it connected for very long periods) the sensor’s output is 5V (and current is 3.6 mA).
I would prefer not to use a voltage divider circuit so I don’t lose resolution on the input signal.
I’ve been trying a zener protection with a 1 ohm series resistor and both a 3.3V zener (1N746A) and 3.9V zener (1N748A). For the normal <3.3V inputs it seems to work great. Problem is at 5V (or 4.65V when testing via my USB) the zener does provide protection but drops the voltage into the range of a good sensor signal. For the 3.3V zener the voltage at A1 drops from 4.65V to 2.74V and when testing the 3.9V zener it drops to 3.12V (with 1K ohm resistor it drops to 2.8V).
My concern is the 2.74V reading is in the range of the normal input when the probe and sensor are working. Ideally, I’d like the input to be as close to 3.3V as possible for voltages over 3.3V (4.65V to 5V really) so that way in software I can determine that the probe is not plugged in and in use. Suggestions?