B737-800 pilot crew reported a takeoff configuration micro switch malfunctioned causing a warning and a rejected takeoff.

Narrative

This report was recommended for the events that happened on the morning of Day 0. On taxi out to the runway; everything was normal and the aircraft was properly configured for takeoff - the flaps were set to 5; the parking brake released; the spoiler handle down and the trim set in the green band. When TOGA was pressed we immediately got a takeoff configuration horn and we subsequently rejected the takeoff around 40 knots. We then got a Master Caution Proximity Electronics Unit (PSEU) light on the overhead panel. After we exited the runway we double checked to make sure the plane was properly configured and we both agreed it was. We called Maintenance Control and they told us to return to the gate and have the plane looked at. While investigating the issue maintenance determined the speed brake lever caused the takeoff configuration horn. It was passing the tests and the plane was going to be returned to service. While looking at the speed brake handle closer at the gate the Captain noticed a tiny part hanging from the base of the speed brake handle. We showed maintenance and they determined it was part of the micro switch for the speed brake handle that had broken away and likely caused our erroneous takeoff configuration warning. From our understanding; maintenance fixed this issue and was later returned to service. The likely cause of our erroneous takeoff configuration warning was a broken micro switch on the speed brake handle. Following SOPs; verifying configuration as a crew and making sure both crew members are fit to fly are the best mitigation for takeoff configuration warnings.