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Opto Isolator 5v -->opto-->24v Relay

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New Member
Opto Isolator 3.3v -->opto-->24v Relay


My electronics knowledge is.. limited.. at best, until i google'd it i didn't know what an opto-isolator was.

Anyway my problem. I have the following components;

control board, 1 PIN output @ 3.3v (dont know mA)
24v timer relay Link here

I can not fire the Relay from the 3.3v PIN, but it does work from a 5v supply, from the same board. I've been googling for 2 days but just can't find/understand an opto that would provide me with the voltage-shift I require to fire the relay. (The opto output could be anywhere between 5v-24v higher the better!)

P.S the circuit needs to be protected, hence 'using' an opto.
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New Member
To add some more information;

We've had 2 batches of these relays.

The first batch of relays would fire from the 3.3v supply.
The second batch wouldn't.

(Batch 2) I've tested if the relay would fire by connecting a wire to a 5v output on the control board and touching B1, which fires activates the relay. This works.

What opto would provide me with a voltage shift from 3.3v to 5v/24v?
How would I wire said circuit?
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Most Helpful Member
I suspect your Opto isolator has an LED that has a voltage drop greater than 3.3 volts, its probably 3.5 -3.8 volts.
As far as the output side just make sure its designed for the type of current, AC or DC, and has a voltage rating of at least 2x you r relay voltage. Sam with the current capacity.
All of the big electronics components dealer carry them on-line so finding one with the right specs and operating ranges and with built in snubber is not hard.

Digikey, Mouser and often times eBay have loads of SSR devices.


New Member
When using opto-couplers you will need two different power supplies. One is for the control circuit on the LED side of the opto, the other for the relay circuit on the transistor side. This is how you get the voltage shift.

Hook the output pin of the controller up to the LED just like any other LED. Make sure you have a current limiting resistor if needed. The transistor side will need to handle 1.8VA to drive the coil.

It's just tolerances (luck) that are allowing you to turn on the relays with 3.3 and 5v. From the link you provided, the relays coil voltage is from 17 to 265V AC or DC.


New Member

"so finding one with the right specs and operating ranges and with built in snubber is not hard."

This is where one of my main problems lies. My main expertise is a software engineer but as part of my job i have to delve into the scary world of electronics! :confused:


"It's just tolerances (luck)"

Completely correct. I have to drop the power to some GPRS hardware, so its wired through previous mentioned relay, I then use a GPIO PIN @ 3.3v from the controlling board (the only software-controllable voltage i can get out of it) to try and fire the relay. I couldn't find any components that would work using 3.3v so am currently relying on pure luck!
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New Member
So I have my opto/resistor and am trying to wire it to achieve the above. A guy I know told me to wire the following circuit to see how the opto operates but since I don't have an LED i cant!

Can someone draw me up a schematic of how this circuit should look with the previously mentioned relay attached to the transistor side? it would be most appreciated!!!
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