0kelseymh 10 years ago I presume you're doing this for a display, or science fair, or something. Hopefully you don't need to build one for a friend who's waiting for a transplant :-)Visually, make a plastic or rubber model of the heart, and put a bladder (a whoopie cushion or old-style heating bag) inside it, leakily connected via hose to an electrical pump. I've got a Coleman pump for an air mattress which would probably be perfect. Put a relay (normally open) in series with the pump's power cord, and leave the pump switched on all the time. Use a 555 timer circuit or something to trigger the relay to close for half a second or so, once per second. That'll give you roughly the right inflate-deflate cycle to look sort of like a human heart. If you got really fancy, you could built a circuit around a potentiometer to change the cycle frequency (60 bpm,72 bpm, 120 bpm, etc.).By making the hose/bag connection leaky (e.g., just stick the hose into the middle of the bag, but don't seal the opening), you ensure that it'll start deflating as soon as the pump shuts off.