This takes place very early, before the Reverse Flash first appears, but with spoilers up to the finale.
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"Well, all I have to do now is turn on the power, it's as simple as that, and the first ever meta-human detector will be fully functional!"
Flipping a switch on the side of a small piece of machinery that looked suspiciously like an old cell phone, Cisco pointed his newly completed device at Barry to demonstrate, and a single small light bulb on top started to glow green. Dr. Wells entered the opposite side of the room, trying to figure out what the two young people were up to.
"Woah, hey, that's pretty cool. How does it work?" Barry asked him, taking a step back.
"Well, I think it detects trace energy from the explosion, but I was really just playing around with an idea," Cisco responded as he turned the device away from Barry. The green light bulb flickered out but turned back on when Cisco moved it to face Barry again. "I think it might be able to detect any abnormal powers in people, but I need to test it still."
Cisco turned the device to face Dr. Wells, who had given up watching them and moved his wheelchair to look at a nearby computer screen. The green bulb stopped glowing for a second when it left Barry, but all of a sudden, started burning brightly when Dr. Wells came into view.
"Huh, that's odd." Cisco gave the device a quick shake, rebooted it by flipping the switch a few times, and moved it to face Dr. Wells again. The light turned on the moment he came into view.
"Maybe something's wrong with the sensor?" Barry offered up as help. Cisco nodded to himself before turning the contraption off. "Give me a chance to tinker around with it, and I'll show you again when I'm done," he told Barry. He left the room to gather a few tools needed to open up the back of the device.
Ten minutes later, Cisco booted up the machine again, pointing it at Barry as he did so. The light turned on as it should. He pointed it at a wall, and it turned off. Back at Barry, and it flickered on again. Then Cisco turned the sensor towards himself, and it started glowing green when he came into view.
At this point, Cisco put the device down on the table and turned it off. He turned to Barry and shrugged. "I thought it might be able to help us, but the sensor must be picking up body heat or something rather than meta-human energy." Barry told him not to worry about it, and Cisco nodded. "It was just an idea. Every now and then something I build has to not work." The two laughed for a moment before moving on to get other work done. That night, Cisco packed up the broken device into a shoe box and stored it in one of Star Lab's many storage closets. He shrugged again as he left it behind.
Many months later, after everything that had gone down with Dr. Wells/the Reverse Flash was old news, Cisco ran into a familiar looking box one evening while sorting through some of Star Lab's old inventory. He picked it up, read the label on the old shoe box, stared at it for a few seconds, and then took off to find Barry.
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This was supposed to be under 300 words, but too bad. Don't you just love the irony, though? I'm too tired to think of a creative disclaimer, so disclaimer. There, done.