Connecting Opto Couplers
Connecting Opto-couplers (photo-couplers) to keyboards

In case of an opto-coupler connections need some more care:

An Opto-Coupler is contains a light emitting diode and a light sensitive transistor close together in one package. When you drive a current Id through the diode, the diode’s light will turn-on the transistor.

Both Opto-coupler input and output have polarity. The driver circuit needs some form of current limiting (resistor) to avoid damaging the Opto-coupler diode. Diode current around 30mA is a good value. Most opto-coupler diodes have about 1V drop when conducting 30mA current. When driving from 12V, the current limit resistor needs to be:

choose 330 Ohm standard value

When connecting the output circuit to the keyboard, the easiest way of finding the correct polarity connection, is by trial and error: Apply drive to the Opto-coupler input and connect the output to one of the keys; check with Notepad to see whether the key is activated or not. If not, reverse the Opto-coupler’s connection to the keyboard.

Opto coupler connection to keyboard

Opto-couplers are often found in switch-mode power supplies. They usually look like an IC, but have only 6 or 4 pins. Most of these couplers have a diode current rating of max 60mA DC.

In the test circuit I used 330 Ohms, which gives 33mA drive current at 12V supply.

The below picture and schematic show the test circuits and setup for relay and Opto-coupler.

(Opto-coupler package top view)

Opto-coupler: Vishay, do a search on: TCET1103 or SFH620

Toshiba, search for TLP620 or TLP421


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