# 555 Tone Generator (8 ohm speaker)

This is a basic 555 squarewave oscillator used to produce a 1 Khz tone from an 8 ohm speaker. In the circuit, the speaker is isolated from the oscillator by the NPN medium power transistor which also provides more current than can be obtained directly from the 555 (limit = 200 mA). A small capacitor is used at the transistor base to slow the switching times which reduces the inductive voltage produced by the speaker. Frequency is about 1.44/(R1 + 2*R2)C where R1 (1K) is much smaller than R2 (6.2K) to produce a near squarewave. Lower frequencies can be obtained by increasing the 6.2K value, higher frequencies will probably require a smaller capacitor as R1 cannot be reduced much below 1K. Lower volume levels can be obtained by adding a small resistor in series with the speaker (10-100 ohms). In the circuit on the right, the speaker is directly driven from the 555 timer output. The series capacitor (100 uF) increases the output by supplying an AC current to the speaker and driving it in both directions rather than just a pulsating DC current which would be the case without the capacitor. The 51 ohm resistor limits the current to less than 200 mA to prevent overloading the timer output at 9 volts. At 4.5 volts, a smaller resistor can be used.

Parts:

• 1x 555 timer IC
• Misc condensators, resistors, etc.

## 3 thoughts on “555 Tone Generator (8 ohm speaker)”

1. Norman

Hi,

What if you changed the value of the speaker to 4 ohms? How would that affect the value of the 51 ohm resistor and 100 uF capacitor?

Thanks!

1. Norman

forgot to mention…. 5 watt speaker and 8 volts VCC.

Thanks!

2. Russ - K1HOP

The 4-ohm speaker, being lower impedance, will want to draw more current if driven by the same voltage as the 8-ohm speaker. The circuit on the left should handle it since that 2n3053 is quite a hefty power transistor. But if your 9v power source is a 9v “transistor battery” that’s going to suck all the energy out of it pretty quickly. If it’s a wall-wart power pack or other supply, it still will be called upon to supply up to 2 amps or so!

You can lessen the current drain (and hence the speaker volume as well) by increasing the value of the resistor between the 555 output and the base of the transistor to drive it less hard. In fact, that 330 ohm resistor could be made variable and become a volume control of sorts.

The circuit on the right has less flexibility, and the 4 ohm speaker (and the 8-ohm as well) will attempt to overload the poor li’l 555 oscillator. That’s why he inserted the 51 ohm resistor in series to limit the maximum current that the 555 will see. That same resistor should do the same job when a 4 ohm speaker is used, as both 4 and 8 ohms are quite small (and of the same order of magnitude) in comparison to the 51 ohms. If there are problems, increase that resistor value a bit… even going up to 100 ohms and see if you and the 555 are both happy 🙂

Enjoy!