Rapid led display switching
This project is a control circuit to produce pulses. When you turn power ON and press S1, the LED display lights for short period then goes off, even if you keep pressing the key.
You can make up a game. Have a number or letter displayed on the LED, and let the players see what's on display.
The circuit uses the capacitor charging current to turn the transistor ON. The transistor then completes the circuit to the LED anodes. When the key is open the capacitor is discharged by the two 10K resistors.
When you press S1 key, the capacitor is quickly charged up to 9V through the transistor. The charging current through the B-E junction turns the transistor fully on to turn the LED on, but this last only up to the capacitor becomes fully charged.
After the capacitor is charged to the 9V, no more current can flow to the transistor so the transistor is turned off. You might want to try different values of capacitors and see their effect.
However, don't use capacitor values higher than 10µF or the transistor can be burnt out by excessive current! Of course you can have many different numbers and letters on the LED, especially if you play the persistence-of-vision game, by changing the wiring on the LED.