The laws for electronic surveillance are different for every state. Since I do not know which state you're in, I can not tell if it's legal. Here is a link to the Electronic Surveillance Laws for each State in the USA
. Please scroll down to your state, and click on the link under "Cite" to actually see the laws.
In a summary though, the majority of states say that recording a conversation is legal as long as ONE person gives their consent. If you are a person in the conversation, you can be the one consenting, so you can record any conversation to which you are a party. To cover the states that require TWO people, you would hold out the recording device for them to see and say "I am recording our conversation. If you continue to speak to me, that will count as your consent to record. <Then proceed with the talk or question here.>"
In the instance of a voice activated recorder in the car to catch him and the OW talking, here you are not a party of that conversation--and chances are slim to none that either of them would give consent to being recorded! Thus the VAR is not usually admissible in court as evidence of adultery...but it would be proof to you that you are not nuts, it's not all in your mind, it's not just you being jealous or controlling. It would be the smoking gun to prove TO YOU that it is right and reasonable for you to move forward with protecting yourself financially and legally.