I want to replace the reed switch with something that will close/open the circuit continually, adjustable to anywhere from perhaps once per second to perhaps 4 times per second.
I imagine that the heart of this would be a 555 chip, but that's about all I (think) that I know. I have never constructed anything like this before.
Are there any simple plans anywhere to accomplish something like this?
I take it the reed switch is activated by a magnet normally.
In that case you would need to wind a coil around the switch. Any small tube (straw) that just fits over the reed should work as a coil form. Then you need to wind many turns of as fine a wire as you can find (such as 30Ga or finer). You will have to experimentally determine how many turns and how much current it takes to activate the switch. From that you can calculate the voltage and resistance in series with the coil will provide the required current.
Otherwise just substitue a reed relay for the reed switch. They have the coil already wound.