I have a DC supply (12v, 0.1A) that I want to reverse the polarity every 3 minutes or so. The reversal needs to be done automatically as the unit is used fort 3 hours each time. Also I need to be able to manually set the reversal interval between 30 seconds and 5 minutes. I have another 12v DC supply, but where the current maximum is 2A. I need to do the same with that. This supply is in a damp environment. The other supply is in a dry environment. Anyone have any ideas of simple circuits that will do that?