The past few days were about generating a linear ramp, in the form of a triangular wave, and as you saw, it was quite easy to generate – despite the lack of a function generator.

The result was a voltage alternating between about 0.6V and 3.0V in a linear fashion. Here’s why…

I want to see how the MCT62 optocoupler passes a signal through it. More specifically, how a linearly increasing voltage would come out. Let’s look at that chip schematic again:

So the idea is to apply that linear ramp through a current-limiting resistor into the opto’s LED. Then we put the photo-transistor in a simple 5V circuit, with again a current limiting resistor between collector and 5V – like this:

From left to right:

• apply a triangle wave to the LED, which varies from 0.6 to 3.0V
• there’s a 1 kΩ rsistor in series, so the maximum current will stay well under 3 mA
• the phototransistor is hooked up as a normal DC amplifier
• there’s another 1 kΩ pullup, so this too cannot draw more than 5 mA current

Prediction:

• when the LED is off, the output will stay at 5V, i.e. transistor stays off
• until the input rises above the 1.2V threshold of the (IR) LED, not much happens
• as the voltage rises linearly, so will the current through the LED
• depending on the transfer function the transistor current will rise accordingly
• and as a consequence, the output voltage will drop

So if that behavior is linear, then the output voltage should drop linearly. Let’s have a look:

• the YELLOW line is the triangle wave, as generated earlier
• the PURPLE line is the voltage over the leftmost resistor
• the BLUE line is the voltage on the transistor’s collector output
• the RED line is the derivative of the BLUE line
• the zero origin for all these lines in the image is at two divisions from the bottom

First of all, the purple line indeed rises slowly once we start rising above 1.0V, and it stays roughly 1.2V under the input signal (yellow line).

The blue line is the interesting one: it takes a bit of input current (i.e. LED light) for the transistor to start conducting, but once it does, the output voltage drops indeed. Once we’re above 2.0V, the blue line becomes quite linear. As indicated by the fact that the red line is fairly flat between horizontal divisions 5 and 7.

So in this range (and probably quite a bit above), we have a linear transfer from input current to output current. Or voltage … it’s all the same with resistors.

In terms of current, we can use the purple line: it’s flat with a diode current between 0.7 and 1.7 mA (and probably beyond).

The output voltage only drops to just over 2V, so the phototransistor is still far from reaching saturation (“conducting all out”).

So what’s the point of all this, eh?

Well, one thing this illustrates is that you can get a pretty clean signal across such an optocoupler, as long as you stay in the linear range of it all. There is no real speed limitation, so even audio signals could be sent across reasonably well – without making any electrical connection, just a little light beam!

It’s not hard to imagine how this could be done with discrete components even, sending the light to a glass fiber over a longer distance.

You can call it wireless signal transmission, albeit of a different type: optical!