I’m trying to actuate relays using a 3.3V microcontroller. To do this, I used an optocoupler to translate the signal from 3.3V to 24V, while also providing optical isolation. When I send a HIGH command from the controller, I hear the relay click, but there’s no voltage at the relay contacts when I measure them with a multimeter. The reverse is also true.
I’ve attached the schematic. Can anyone help me troubleshoot this issue?
You are measuring across the normally open contacts of the relay, when the relay is activated they are connected together so they are a short circuit. The voltage across a short circuit should be zero. When not activated the contacts will be open and they will have a voltage across them that depends on what they are connected to. It sounds like you are seeing exactly what you should see considering the circuit.
If you want to switch voltage to a device there are several ways to do it.
You can do high-side switching where the positive voltage is switched:
simulate this circuit – Schematic created using CircuitLab
Or you can do low-side switching where the negative is switched. Since you have the relay connected to GNDINT already this might be your best option.
For a two terminal device it shouldn't matter which side you switch, but if the device being switched has other connections that may have a path to ground you would want to switch the positive side.
UPDATE concerning question of using circuit for AC voltage:
For switching AC mains voltage you would need to not have any of the relay contacts connected to GNDINT or any other part of your existing circuit, they should be dry contacts, only switching the external AC circuit.
230 VAC is considered a lethal voltage. You have to be very careful with any design dealing with mains voltage, there are circuit board clearances to prevent arcing and other things you have to worry about, and there may be certifications needed if it's going to be a commercial product. If you do not have any experience working with these voltages you need to get help from someone who does.
Forgive me if I misunderstood you, but it sounds like you expect the relay to transfer something to its "output" when it activates. This is not going to happen (unless you connect the switch contacts differently), since the switch contacts are completely isolated from the coil and everything else. This is clearly depicted in the circuit symbol for a relay:
simulate this circuit – Schematic created using CircuitLab
Note: the dotted line indicates a magnetic coupling, to physically "pull" the switch mechanism. It is not a current path, and doesn't imply that the relay switch contacts have any electrical connection to the coil in any way.
In the left circuit, when the relay "activates" the COM and NO contacts will be physically connected together, but since there's no external connections to them, they will effectively connect some nothing to more nothing.
On the right I hope it's clear that when the relay activates (the transistor switches "on", allowing coil current to pass), then the NO contact will become physically connected to COM, and consequently to the battery's positive terminal.
In your schematic it's not clear what else is connected to the relay contacts. One seems to be grounded (COM), but the other (NO) is Q0/6.4C, which is presumably where you are measuring voltage.
When the upper relay is off, its NO terminal potential is ambiguous, since I don't know what Q0/6.4C is, but when the relay activates, the NO terminal becomes grounded also. That terminal will have 0V.
With the relay activated, you can expect the NO and COM terminals to have the same potential (since they are joined), and if you measure the voltage difference between them, you will measure 0V.
With the relay un-activated, the measured potential difference between NO and COM will depend entirely upon whatever else is connected to node Q0/6.4C.
It doesn't look to me that there is any voltage to measure- you should have an open circuit between COM and NO with the relay de-energized (no continuity) and a closed circuit (continuity) between COM and NO with the relay energized.
If you have no other voltage sources connected to the relay contacts, that is what I would expect to see. You an use an ohmmeter to measure the contacts, however ensure there is no voltage source connected to the relay contacts or you could damage your meter.
