In this Photo Instructable you'll install and LED in my tone generator circuit to help you visualize the square wave tone output of the 555 Timer IC in astable mode.
Please review my step-by-step Instructable, "Build a Tone Generator--Improve Your Snap Circuits by Adding a 555 Timer IC" (http://www.instructables.com/id/Build-a-Tone-Generator-Improve-Your-Snap-Circuits/).
Begin this build with the 555 timer IC tone generator circuit. See picture 2.
Parts Needed for build:
1 10uf Capacitor # 6SC C3
1 100 ohm Resistor # 6SC R1
1 1 Red LED # 6SC D1
Step 1. Replace 0.02uf capacitor C1 with 10uf capacitor and make sure the "+" is toward the whistle chip (see picture 3)
Step 2. Replace 470uf capacitor C5 with 100 ohm resistor R1 (see picture 3)
Step 3. Install LED D1 with the "+" toward the 100 ohm resistor R1
Here's a video to demonstrate the LED synchronized with the squarewave output of the tone generator: