yep same for the output aswell,
http://www.ti.com/lit/ds/symlink/lm555.pdf
Page 5, Output Low,
Page 6, Figure 4 Vs Page 7, Figure 5.
Thanks for the pointers. From what I gather from the datasheet, the lower the supply voltage (i.e. Vcc = 5V) and the greater the output current, the worse the rail-to-rail performance of the 555. The trigger will switch the state of the flip flop at 2/3Vcc, but the output voltage may only be 2.5V, so it will take much longer for C to charge and thus the frequency of oscillation drops.
To test this circuit, I was driving an 8ohm speaker directly from the output of the 555. This was probably causing significant voltage drop and making matters worse. (It might even exceed the 555s max source/sink current.)
Also what type of capacitor are you using for C1? SMD ceramic caps can be surprisingly sensitive to voltage, even running them at half their rated voltage can give a significant reduction in capacitance.
I'm using a through hole 0.1uF 50V X7R ceramic. At least, I believe that's what I have in my stock.
You probably used the TTL NE555/LM555, rather than the CMOS TS555/ICM7555.
The TTL 555 has an asymmetrical output voltage swing. If drops more voltage when the output is high and less when low. The duty cycle will not be 50% and the frequency will vary considerably, as the supply voltage changes because the voltage drop is not proportional to the supply voltage.
Replace the 555 with the CMOS version and don't load the output. Take the output signal from pin 7 and a suitable pull-up resistor.
I'm not so worried about the 50% duty cycle as I am about keeping frequency independent of supply voltage. Is the voltage drop on the output
inversely proportional to the supply voltage? A higher Vcc means a smaller drop?
Why do you suggest using pin 7 as the output instead of pin 3? It makes sense that it would work, but why is it a better choice?
I only had the TTL 555 in stock so I used it. I'm going to try the CMOS 555 and buffer the output and/or design a small transistor amplifier or use an LM386 (or similar) to drive the 8 ohm speaker. Hopefully that will do the trick.
Thank you everyone! Really appreciate it.