Hello again. I have another tricky problem. I'm trying to build an automated minecart unloader that will detect when the minecart has been completly emptied. Obviously, I need a section of track with a hopper directly underneath to unload the items from the cart. I tried it with plain track and everything worked just fine. Unfortunately, in order to use a comparator to monitor the cart, I had to replace the standard track with a detector rail. Now when the cart stops at the hopper, it powers the comparator, but it also powers the hopper underneath. As a result, only one item gets unloaded before the hopper stopps working. Does anyone have a clever solution for working around this issue?
P.S. I know could probably make the track a circuit so it unloads one item each pass, but it would take forever to fully empty the cart that way. I need something a little more elegant.
I notice you are reading the hoppers, rather than the cart. I had a problem similar to the OP's -- I was trying to read a storage minecart, and discovered it has to be a detector rail under it -- but that meant I couldn't get the cart going again when it was empty -- a piston would scrape up the rail, and move the rails ahead of it out of line. Also, given a "lack of signal" from the comparator, my circuit can't distinguish between empty cart and no cart. And the "stop block" for the cart is above a dispenser, which makes running that signal back to the powered track problematic.
I'm currently using buttons to send it back and forth (it's for a chicken farm), and just switched the upper end from powered rail to no-rail (with fences) and piston.
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I made a construction where I have downwards ramps with powerrails. Two rails earlier is a detector rail that switches off the powerrail, just as long as the hopper needs to start sucking items out of the minecart. As long as the hopper is working, a comparator linked to it keeps the powerrail off and a second hopper below it on, so it can't completely empty the upper hopper, which works as a filter. It's three wide and tilable.
Load the cart, send it on it's way. As it makes the circuit it will pass over 4 hoppers (# of hoppers can vary of course to suit your size or speed of unloading preferences). On the back stretch it passes over detector rail with comparator. If it still has something in it, it will power the switch and send the cart for another circuit. If the cart is empty it will not power the switch therefore sending the cart back to the start (or wherever you want it to go). Of course, adapt and adjust as needed. I will use this to unload my carts into a sorter. Hope this helps or kick starts an idea or solution to your problem!
Thats for all the suggestions so far. I'll have to try a few of these when i get time and see what works. For the time being, I changed the comparator to detect the dispenser (which is ejecting the unloaded items into an elevator) and that seems to work.