Chestshop admiting signals help

I have a donation chest that has a chestshop sign on it and I'm trying to find out how to make it so everytime someone buys something the chest emits a signal so it can launch a firework.

I'd search on YouTube for it. I know someone's done a video on it. It uses comparators, I know that much.

i think that uses trapped chests which i think aikar broke

Hopefully this helps: http://empireminecraft.com/threads/buying-use-access-1-5-redstone-edition.23207/

hmmm, let me put some thought into that.

But imagining that the chest is full (and thus giving out a full signal through a comparator) how would it be able to detect that there is one less item (slightly less signal) and then shoot the firework?

By setting the comparator up to a hopper that will only put one item in the chest. You can extend the signal with a repeater that hooks up to a dispenser shooting a firework. Sorry but logic.

For one item in the chest yes, but that wasn't really mentioned.

ikik!!!! you make a.... nvm... imma gotta think bout this

I found this;
https://www.youtube.com/watch?v=B9fDbKVTB4U
With a bit of tweaking it could fit the need maybe

Ok, took me a sec but i got a working version. Its on smp4 8272. I was back feeding the chest with hoppers but that doesnt work. The key was the dropper under the chest to refill it. I will be on later if you need me to break it down for you.

Do this:

Have a comparator coming out the back of the chest.
The comparator powers a block with a torch on it.
The torch powers the redstone.
The redstone powers the dropper (NOT dispenser).
And the dropper puts another dirt into the chest when someone buys one (donates).
Now all you have to do is hook up a dispenser with a firework.
Also, make sure you keep the dropper full of dirt and that there is always exactly one dirt in the chest.