Buzzin' LED
7400

Here's another circuit that uses both transistor and NAND type multivibrators. It allows you to see LED 1 light up at the same time you hear a sound through the earphone. When you finish building this project, put the power ON. You'll hear a pulse in the earphone each time the LED lights up. Do you know why this happens?

Assume the output of the NAND multivibrator is 0, and trace that output from the NAND multivibrator to the transistor multivibrator.

Do you think the operation of the transistor multivibrator is affected by the NAND multivibrator?

If you say yes, how is it affected?

Try using other electrolytic capacitors in place of the 10OuF capacitor in the NAND multivibrator to see what effects they have on the circuit. Try changing the transistor multivibrator and see how you can alter its operation.

You can use the speaker instead of the earphone with this project, by connecting the NPN transistor, the output transformer and maybe a resistor or two. Try adding the speaker-but be sure to write down the new circuit you design.