Need help opamp or optocoupler?

Question

What is the best route? Optocoupler or opamp (if opamp, which one?)

I need to reduce this 5.8V pulse generator to 0.84V

Have an automotive application I am trying to find a solution. Basically I want to add an additional distance counter to my car that utilizes the speedo.

The speedo utilizes a switch that creates the below signal. The new distance counter pulse generator creates the second signal. I want the external counter to utilize the same signal as the speedo. However the car's speedo has a voltage of 5.8V on the signal terminal and the external counter has a voltage of 0.84V on its terminal. What is the best route optocoupler or opamp?

Answer 1

If only level needs to be changed and no isolation is needed, you can just use a voltage divider (2x resistor). The signal is slow enough for that.

You can also use opamp with gain lower then 1 to reduce the voltage, this option would be used if there is a very low impedance system or if decent currents are required.

Optocoupler would be used if you need isolation. In deed you will normally have an option to reduce voltage by simply putting lower resistor on the secondary side.

Answer 2

If you trust the 5.8V signal to never exceed +5.8V, and to never fall below 0V, then you could employ a simple resistor potential divider:

^{simulate this circuit – Schematic created using CircuitLab}

Such trust is hard to justify in automotive applications, so you should probably employ clamping, using a zener diode:

^{simulate this circuit}

This will prevent OUT from exceeding +0.84V, or falling below -0.1V, even with a grossly exaggerated input:

An opto-coupler is a great way to protect your circuit from errant inputs. Here's a design that combines an opto-coupler with diodes, for robustness:

^{simulate this circuit}

D2 protects the LED from inputs that may go negative, and won't be necessary if you are sure that can never happen. The +12V supply is your vehicle battery, and Zener diode D3 holds the collector at a reasonably well regulated +4.7V, regardless of noise on the supply.

This will produce 0V or a little over +0.8V at OUT, even for input signals that fall well outside the 0V and +5.8V limits you expect.