An amateur radio repeater used to be a complex assemblage of equipment that would easily fill a 19″ rack. There would be a receiver and a separate transmitter, usually repurposed from commercial units, a home-made logic unit with a microprocessor to keep an eye on everything, and a hefty set of filters to stop the transmitter output swamping the receiver. Then there would have been an array of power supply units to provide continued working during power outages, probably with an associated bank of lead-acid cells.
More recent repeaters have been commercial repeater units. The big radio manufacturers have spotted a market in amateur radio, and particularly as they have each pursued their own digital standards there has been something of an effort to provide repeater equipment to drive sales of digital transceivers.
But what if you fancy setting up a simple repeater and you have neither a shed full of old radios or a hotline to the sales department of a large Japanese manufacturer? If you are [Anton Janovsky, ZR6AIC], you make your own low-powered repeater using an RTL-SDR, a low-pass filter, and a Raspberry Pi.