Transistor timer with TTL
Électronique / Electronic
7400

Here's another type of one shot circuit, but in this project you hear the effects of the multivibrator. You can see from the schematic that this project uses a combination of simple components and digital electronics.

When you press the key, the 100μF capacitor is charged and lets the NPN transistor in the left corner of the schematic operate. You can see that the collector of this transistor serves as both inputs for the first NAND gate.

The digital portion of this circuit controls the operation of the PNP transistor on the right side of the schematic. Put the power on.

When the output of the first NAND is 1, the multivibrator works, and you hear a sound from the speaker. The sound will continue until the 100μF capacitor discharges, keeping the first transistor from operating.

The output of the first NAND becomes 0, stopping the multivibrator. The sound will last about 10 seconds with the 22K ohm resistor. Try replacing the 22K ohm with the 47K ohm or the 100K ohm resistor and find out what happens.

After you've experimented with this project a bit, try this:

press the key and release it. When the sound stops, find the wire at the out of NAND gate in the left corner of the schematic. Disconnect the wire from the output. Does anything happen?

If something does happen, can you explain why?