Use of 6N135 opto coupler to switch circuits

Thread Starter


Joined Jul 30, 2008

We're currently trying to use a 6N135 Opto-isolater so that we can switch a 5V DC supply using a 24V DC source. We have a 24VDC source + on pin 2, and ground on pin 3. There is a 3300 Ohm resistor in line. If I meter the current between the resistor and pin 2 I get about 0.010A, which I think should be enough to switch the LED on. The measured Voltage across pins two and three drops to 1.5V.

The spec for the chip is here - IGNORE THE FRONT PICTURE and look at the PDF file that is linked to lower down. We have a single channel IC, not dual:

The output from pin 6 is intended to drive a hex-inverter 74LS00N to switch a 5V side of the circuit, the intention to act as a relay switch. However, the opto-isolater isn't behaving itself. We've tried several different ICs so I don't think its a component fault, unless we've ruined every one that went in the circuit. Pin 6 kicks out 5V when the 24V supply is not there, but when 24V is applied no change occurs. It should drop to 0V. Any idea what we are doing wrong?

I've tried to get the best photo I can of our board so you can look at that, if it helps.



Joined Apr 20, 2004
The photo of your breadboard is fuzzy. Can you post the schematic of your circuit? Does the open collector transistor in the optocoupler have a pullup resistor? How is the photodiode biased?


Joined Jul 17, 2007
Typical Vf of the input LED is 1.5V. That's giving you a current of (24V-1.5V)/3300 Ohms = 22.5/3300 = 6.818... mA. I suggest increasing the LED current to 16mA by reducing the 3300 Ohm resistor to 1400 Ohms. Use a 1W resistor.
You could use a 1/2W 2.2k resistor in parallel with a 1/4W 3.9k resistor to get 1406 Ohms.

This suggestion was derived from information provided on page 2 of Avago's datasheet:

You might find Fairchild's datasheet a bit more helpful:
Check out Page 7, Figure 3, Output Current vs Output Voltage. Note that with If=5mA, output current @ 5v is <2mA, where if If=15mA, output current is roughly 5.5mA.

You might go as high as 20mA for the LED side. (24V-1.5V)/20mA = 22.5/0.02 = 1125 Ohms. Use a 620 and a 510 Ohm 1/2W resistors in series.
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