Son of buzzin' LED
Électronique / Electronic
7400

Carefully compare the schematic for this project with the schematic for the last project Buzzin' LED. They are similar in many ways, but there's an important difference. Do you see what it is?

Better still, can you tell what effect this difference has on the project's operation?

Try to figure it out before you build this circuit. Put the power ON. You should find that LED 1 lights up, but you hear nothing in the earphone. When LED 1 turns off, then you hear a sound in the earphone.

Try to figure out why this happens. Study the schematic and when you think you have the answer, read on to check your guess.

When the output of the NAND multivibrator is 0, current can flow through LED 1 to light it but the transistor multivibrator won't work because the left transistor has a 1 signal applied to its emitter.

When the output of the NAND multivibrator is 1, LED 1 won't light but a 0 signal is applied to the emitter of the left transistor. The transistor multivibrator can then work, and you can hear the sound in your earphone.