I made a NE555 circuit with speaker. It works fine, but when I connect its output to the oscilloscope, I get this view:

NE555 circuit output view

How could I reduce these voltage spikes? How can I get nice square waves? Or maybe this is normal and I can't do anything?

My circuit:

enter image description here

Component values are:

\$ C_1 = 100nF \$

\$ C_2 = 10\mu F \$

  • \$\begingroup\$ Where in the circuit are you measuring that? You didn't spec the SP1 impedance. I think the spikes would be even higher if it wasn't for the IC's internal protective diodes OR you are simply looking at the capacitor's charge curve. \$\endgroup\$
    – jippie
    Oct 16, 2015 at 19:42
  • 6
    \$\begingroup\$ 555 is not designed to drive 8 ohms. The spike you see (just to the 9V supply) comes from the inductive component, the "square" portion is about right for +/-100mA into 8 ohms. \$\endgroup\$
    – user16324
    Oct 16, 2015 at 19:50
  • \$\begingroup\$ @Brian Drummond: I don't know why you didn't put that in the answer box. \$\endgroup\$ Oct 16, 2015 at 22:55
  • \$\begingroup\$ @Respawned Fluff : too short. Sometimes a quick comment can point in the right direction, until someone else fleshes out a more complete answer. \$\endgroup\$
    – user16324
    Oct 17, 2015 at 11:13

1 Answer 1


The speaker has inductance and so does any power supply rail .Bypass the supply rail at pin 1 and pin 8 of the IC with a electrolytic of say 1000 microfarad .Remember that tha 555 output stage takes power spikes due to crowbarring ,This is stated in the www.badbeetles.com website .The 555 isnt designed for 8 ohm in fact the output current capability depends greatly on the manufacturer.Placing 47 ohm in series with speaker will help but reduces available volts for speaker.If you want anything like full output voltage swing then use a complementry emmiter follower after the 47 ohm and before the cap . Your 10 microfarad cap should be inreased in value because i am assuming it is electrolytic .Reasoning is ripple current so go for 470 microfarad which is a very typical audio output coupling cap value .The electro will last a long time ...


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