The way you build your devices depends much on what kinds of signals you have, and which you want to generate. If you want to send a 'constant' signal (actually not constant, just a single 'on' not accompanied by an 'off'), you need to hold the pressure plate down. If you want to send a 'pulse' signal (an 'on', followed by an 'off'), you must make sure that the cart passes over the plate only once (or at least doesn't keep it constantly activated).
For a device that sends a signal as long as it's provided with power, the power-to-signal converters offer several options.
I'd recommend either the ramp-based one, the one with two opposing rollers or my improved circular design:
There are two converters in the picture, below only the paths are shown, above the basic installation with roller (pushing north) and pressure plate. It has the advantage that it can be extremely space-economical, because gear assemblies can be placed on top of the track without hindering the cart.
If you want to e.g. convert a lever flip into a short 'on' signal, the easiest approach would be to use a pair of alternating rollers:
#R-R#
.G.G.
The 'R'ollers push against each other and the 'G'ears powering them are linked to the same lever, but one is pre-toggled, so that one activates when the lever is in the 'on', the other when it's in the 'off' position. Each lever flip will thus send the cart to the other roller _once_, and a pressure plate between the two rollers will send a signal. A handy trick when you want to start a process that actually needs devices to 'cycle' and not set to a single position (which is what levers normally do).
Using the activation delay of some buildings as a timing mechanism for minecart processes is a pretty neat idea. You can forgo the second minecart by putting the pressure plate _before_ the roller pushing against the door/floodgate etc., the 'open' signal will still process; although this will slightly increase your repeat period. You can deliberately lengthen the period by using the different behaviour of bridges:
Conservative power-based bridge repeater:
A raising bridge, roller pushing west, two pressure plates: the one directly west of the bridge is linked to the bridge, the one north of the roller is linked to the gear providing power. The passing cart switches the roller off and comes to rest on the roller. ~100 steps later, the roller reactivates, but by now the bridge has processed its 'on' signal and is raised, blocking passage. Another ~100 steps later, the bridge lowers again and lets the cart through, which passes over the pressure plate sending another 'on' signal, ordering the bridge to raise 100 steps later, and cycles around over the roller-deactivating plate. It's probably possible to run it off a single pressure plate, but getting build/activation order wrong could mean the cart gets its push off the roller before the bridge has raised, causing the cart to either pass through early _or_ get crushed by the bridge raising while the cart's trying to pass.