Here's a trivial little project which uses a tiny85 (it'd fit in a tiny25 but I have stacks of cheap '85s) to monitor a flow-sensor.
The flow sensor (YF-S402C) provides a stream of pulses when the water is moving. The AVR has a single output which is enabled after a few pulses, and kept on while there is flow occurring, swtiching off with a 1s timeout.
It's used to protect a water-cooled CNC router spindle. The coolant pump is pretty cheap and they're infamous for failing, so this interlock is used to cut coil-current to the main contactors that power the machine if/when flow in the coolant-return line ceases.
Code is here (github, GPL3). Not much going on there except that it uses a pin-change interrupt.
Picture of the assembled prototype here.
The schematic is too simple to bother drawing and it runs on the internal oscillator at 1MHz. The usual 10k/100n RC reset circuit, 100n decoupling cap and a 10k pullup on the input pin. Output pin goes to a 5V ebay relay module that's totally not UL compliant so this will need to be sealed up and safety-grounded.