Wide voltage range input regulation (24V - 3.3V)

Question

TL; DR: I need a circuit capable of handling input voltages from 24V to 3.3V (with a 10mA/5mA maximum current) and delivering 3.3V to the microcontroller (ESP32) input.

Recently, I've been working with the EPS32 microcontroller. And this project in specific requires signals from different sources, with different maximum output voltages: 5V, 24V and 3.3V. The frequency wouldn't be more than 100KHz.

My signal sources can provide up to 10mA, but a 5mA limit would be more confortable for me to work with. Galvanic isolation isn't a requirement, but it would be interesting!

I searched on the web for circuits to implement in the project, but almost every one that I could find, could only handle a single specific voltage. The majority of the designs I found were based on the classical optocoupler circuit, with a current limiting resistor, calculated for a certain voltage.

In fact, I found an Electronics Stackexchange question that was almost exactly what I needed (How is this 5-24V input circuit working?), but I don't have access to any JFETs in my local store, also, I couldn't find any MOSFET to effectively replace this JFET.

I was thinking if OPAMPs could do this job, but I am not sure if this is a good practice.

Any help will be very appreciated!

PS.: Sorry if my question doesn't express my current effort in the project, as it doesn't contain any circuit/design attempt made by me, but I am really lost with this part of my project! So I couldn't figure out any decent circuit.

Answer 1

My interpretation of your question is:

• You want to drive an opto-isolator LED from a digital on/off signal.
• High level is 3.3 to 24 V and a low of 0 V.
• LED current of 5 mA when on.
• Use readily available components.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. A constant current sink.

Opto-couplers use infrared LEDs with a forward voltage of 1.4 V approx. This leaves a couple of volts to work with for our current limiter.

• R2 biases Q2 on.
• Current flows through D1, Q2 and R1.
• When the current reaches 5 mA the voltage at the base of Q1 will reach 0.6 V and it will begin to turn on. This will steal the base current from Q2 preventing further rise in D1 current.

Figure 2. A quick run of the CircuitLab DC sweep indicates that this should work as required.