Diode clamp to ground so only the positive peaks go past the diode, single supply comparator(LM393 or LM339) with a threshold of your choice, and an open collector output pulled up to the Vcc of your choice. If you like you can amplify the signal before running it into the comparator. You can also alter the resistor network to do a Schmitt Trigger.
Remember ,"Google is your friend" and check those manufacturers application notes for circuit ideas.