I want to make an adjustable circuit in order switch between 3 or 4 solid state relays in a row, similar to those which are used to control running light bulbs.

I have thought about using a counter circuit on a multiplexer, but are there any easier ways to do this?

 

Check out the 4017 decade counter.

 

How do you want to switch the relays on/off ,one at a time ,or will there be more than one on at any one period, can you give us your diagram?

 

How do you want to switch the relays on/off ,one at a time ,or will there be more than one on at any one period, can you give us your diagram?

I want to switch one at a time and control the frequency by a potmeter.

The ultimate choice will be to also switch the dc polarity on the output on each relay. Are there any solid state relays which are capable to do that?

 

Any reason you're using relays? I mean, whenever I hear "frequency" and "relay" in the same discussion, I have to wonder if a mechanical relay is the right choice. A SSR may be better, or a transistor might be best of all. It just depends what you want to switch (AC vs. DC, how much current) and how fast you want to switch it.

 

Any reason you're using relays? I mean, whenever I hear "frequency" and "relay" in the same discussion, I have to wonder if a mechanical relay is the right choice. A SSR may be better, or a transistor might be best of all. It just depends what you want to switch (AC vs. DC, how much current) and how fast you want to switch it.

A SSR is what I intended to use. It is the same as solid state relay as I wrote above, but maybe I should use power transistors instead.

Which transistors will be the best and cheapest choice for this purpose? I´m planning to use 12V on output and maybe 1 - 2 Amps.

 

Does your application similar like this?
NE555 → CD4017 → MOSFET → light bulbs

 

I´m planning to use 12V on output and maybe 1 - 2 Amps.

If that's 12V DC (not AC), then MOSFETs are the way to go. I have a bunch of IRF540N that I use as my "default" MOSFET. I believe Radio Shack (a U.S. retailer - you haven't noted where you live) stocks the IR510 which is similar but has a lower amperage rating. It would still be enough for your application, I believe. Just about any n-type "power MOSFET" you can find will work for your low speed, "low" amperage project. If it's DC.

 

Don't understand why you want to reverse the supply as its just a set of lights your switching on?

can you post your circuit idea, so we can assist further.

 

Don't understand why you want to reverse the supply as its just a set of lights your switching on?

can you post your circuit idea, so we can assist further.

I wrote "similar to running light bulbs". I can´t reveal what I make plans for due to the fact that it is intended for the commercial market.

However I can tell that I need to switch the polarities of small electro magnets.

 

Does your application similar like this?
NE555 → CD4017 → MOSFET → light bulbs

Yes that can work if the solution allows slower frequencies down to a few Hz. I know from earlier that not all IC´s can operate at that slow levels.

 

Yes that can work if the solution allows slower frequencies down to a few Hz. I know from earlier that not all IC´s can operate at that slow levels.

There is no any problem for NE555 to generate the frequency only a few Hz, and you just need the current less than 2A, so it's easy to get the MOSFETs.

There is a sample at the 555/556 Astable section, you can easily to adjust the frequency to match your need.

 

... I need to switch the polarities ...

By this you mean true current reversal, where the current through the coil changes direction, right?

The common solution for that - often used for reversible DC motors - is an H-bridge. If search for H-bridge motor controllers, you'll find a lot of options including many pre-built modules.