Our computer's clock is built using several 555 timers. The first is configured as an astable oscillator. See https://eater.net/bbcpu8-clock-module for more.

Part 2: https://youtu.be/81BgFhm2vz8
Part 3: https://youtu.be/WCwJNnx36Rk
Part 4: https://youtu.be/SmQ5K7UQPMM

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You can get all the components used in this video from any online electronic components distributor for a few dollars.

Complete parts list (everything in this video):
1x 555 timer IC
1x 1MΩ potentiometer
2x 1kΩ resistors
1x 100kΩ resistor
1x 330Ω resistor
1x 2µF capacitor
1x 0.1µF capacitor
1x 0.01µF capacitor
1x LED
Solderless breadboard
22 gauge wire
USB charger and cable or some other 5v power source