A good five+ card suit in a hand of a wide­ranging point-count, anything from six or seven points (with a very good suit) to 17 or 18 points: that's what a suit overcall looks like. It's the strength of the suit that matters, not the strength of the hand. 

Use as a guide the Suit Quality Overcall Test: Add up the number of cards in the suit (five+) to the number of honours in the suit. The total should get to (at least) the number of tricks bid for. In marginal situations, be more cautious when vulnerable.

Hand (i): Bid 1 ♠. Just six points, but a suit quality count of eight; that ten of spades really helps to bolster the suit. 1 ♠ over 1 ♣. consumes plenty of bidding space - maximising the nuisance value.

Hand (ii): Pass. 11 points, but what a poor suit, poor shape (you'd much prefer Hand (i)'s 5431) and defensive looking 'soft' honours (picture cards as opposed to aces).

Hand (iii): Bid lNT. Yes, you could bid 1 ♦, but, bidding lNT as an overcall shows 15-19, balanced with a stopper in the opposing suit. Perfect.

Hand (iv): Double. You can't bid l♥ for the lack of a fifth card. This double for take-out shows a three-suited opening hand with short clubs  and asks partner to choose a suit. Ideal.

Our deal emphasises a key overcalling point: if you really want your suit to be led, bid (even with a poor hand). If you don't particularly want your suit to be led, don't bid (even with a fair hand):