Howdy
I have a solid state relay I would like to drive with and audio signal. I found a relay with a 5vdc input voltage for the control, and so I need to amp my line level source up to this amount (from about 1V to 5V).

I have been experimenting with inverters and op-amps, but cant seem to pull enough voltage.

I was under the assumption that with an op-amp wired as inverting amplifier, the gain would be RF/RI (where RF = feedback resistor and RI = input resistor). Well I'm trying it with RF = 10k and RI = 100, but I'm still not reading the kind of voltage gain I would expect (definitely not 1000 times).

Am I misunderstanding the concept here? Does anyone know a very simple circuit that would give me 5-6v to trigger my relay? Maybe an lm386 would work, can anyone confirm this?

I have done this before using a power amp driving a 9 and 12v relay, but Id like to make it battery powered.

Thanks yall.

Well, 10K / 100 is only a gain of 100, but there is another factor. Your line out probably has a source impedance in the hundreds of ohms, which lowers the gain even further.

You should probably change to a non-inverting configuration, with the output going to the 10K resistor, the other end of this resistor connects to the inverting input of the op amp and the 100 ohm resistor. The other end of the 100 ohm resistor then goes to ground. Apply your signal to the non-inverting input.

CONCERNS...
- Audio is AC. Your relay is DC. You can get AC relays, but they are really designed to run from 50/60Hz line frequency, not audio frequencies.

- You also need a constant voltage to drive a relay. You might be able to get a fairly constant voltage by “driving the heck” out of the amplifier (using high gain and driving it into heavy clipping). But, this is a “messy” design, and the relay might “chatter” when the audio level is low.

- A standard Op-Amp can’t supply enough current to drive a standard relay. (Check the specs on both.)

- I also wonder about your battery power. Most Op-Amps require positive and negative supplies, so that's either two (or more) batteries or some sort of power-inverter circuit. And, the current required by a relay coil might drain the batteries too quickly

SUGGESTIONS...
I have an Op-Amp applications book. If you send me a private message with your email address, I’ll scan some sample-circuit schematics and email them to you (tomorrow). The circuits I describe below are all simple.

- You can make an active rectifier (to convert AC to DC) with an Op-Amp. (A diode alone won’t work, because you need about ½ volt to “switch it on”, and your average line-level audio will be too low.)

- You can then use a capacitor to “hold” the DC voltage. (I think the combined circuit is called a “peak detector”.)

- Next you need a “comparator” to “switch-on” the relay whenever the DC level reaches a preset threshold. You can use an Op-Amp as a comparator, but a comparator is designed to put-out a fixed DC voltage (or zero), and I think most comparator chips have “open collector” outputs which might interface better with a relay driver.

- You’ll probably need a transistor, FET, or driver chip (ULN2003 ?) to drive the relay. (Most of these circuits will invert, so you'll have to check the polarity of the "logic" to make sure the audio signal doesn't turn the relay off!)

I think your block diagram looks like this:
Audio Signal --> [PEAK DETECTOR] --> [COMPARATOR] -> [RELAY DRIVER] -> [RELAY]

And, the 100 ohm input resistor is a "load" on your line-level signal, and it might be reducing the signal level. 10K - 100K is more typical for a line-level input.

Thanks for the replies so far.

DVDoug, I really appreciate the explanation behind everything you say, and it all makes perfect sense.
I am currently using a two-9volt battery supply.
Wasn't sure if input /control current was a huge concern...I have not seen it mentioned much when searching for relays.

pdq, thanks, I am going to try out your suggestions because that also sounds like it makes sense...and after all I'm in this for the experimenting.

Also in regards to your last post DVDoug, that makes sense...most things I read said to keep those values around 1k-100k, but I just chose 100 (naively) trying to get as much gain as possible.