High- and low-pass RC filters

Having generated a just-about-square wave, I’m fiddling with high-pass and low-pass RC filters (still inspired by Afrotechmods).

I’m still using the 50% duty cycle circuit from the NE555N datasheet, except that R1 is 53K and C1 is 100nF.

I’ve taken that output into a low pass filter: the signal comes through a 10K resistor into the scope, and there’s a 10nF capacitor going to ground.

As expected, with a 139.7Hz input signal, the attack and decay are slowed down a little bit:

139Hz, R1=10K, C1=10nF
138Hz, R1=10K, C1=10nF

Increasing the input signal increases the effect:

138KHz, R1=10K, C1=10nF
138KHz, R1=10K, C1=10nF

Increasing the input signal further to 26.1Khz gives a nice saw triangle wave:

26.1KHz, R1=10K, C1=10nF
26.1KHz, R1=10K, C1=10nF

And increasing it ridiculously produces a DC signal:

261Khz, R1=10K, C1=10nF

I’d like to have made a better saw triangle wave but I don’t have the right value of capacitor for the 555 circuit. However, increasing C1 to 231nF (7x33nF) and setting the input signal back to 139Hz produces a lovely one:

139Hz, R1=10K, C1=231nF
139Hz, R1=10K, C1=231nF

And a very similar wave is produced by setting C1 back to 10nF and increasing R1 to 220K:

139Hz, R1=220K, C1=10nF
139Hz, R1=220K, C1=10nF

This is explained by something called “maths“, but I’m happy enough to see that changing either one works. Afrotechmods suggested selecting the value of R1 to suit the load on the output signal, and changing C1 to get the appropriate wave. Which seems sensible.

Switching the positions of R1 and C1 gives a high pass filter, and as expected, produces the following wave:

140Hz, C1=10nF, R1=220K
140Hz, C1=10nF, R1=220K

In the process of making a silly mistake, I also found that leaving R1 out made the wave much tidier, completely getting rid of the ramping down and up that I couldn’t get eliminate in the 555 circuit! Neat:

No ramping up and down
No ramping up and down

Though, as you would expect from a high pass filter, the peaks and troughs are no longer quite flat. I’m not sure how to expect a high pass filter to behave without R1, so I don’t really know if this is meaningful or useful.

Anyway, decreasing the input signal to 78Hz produces a more pronounced change:

78Hz, C1=10nF, R1=220K
78Hz, C1=10nF, R1=220K

As does decreasing C1 to 1.4nF (3x470pF):

140Hz, C1=1.4nF, R1=220K
140Hz, C1=1.4nF, R1=220K

Decreasing C1 to 1pF gets us back to DC:

140Hz, C1=1pF, R1=220K
140Hz, C1=1pF, R1=220K

As does decreasing R1 to 10R:

140Hz, C1=10nF, R1=10R
140Hz, C1=10nF, R1=10R

However, I couldn’t quite get it back to DC by passing in a lower frequency wave. I made the filter a bit more aggressive by changing R1 to 100K, and passed in 12.5Hz. But no matter how low the frequency, there are always upwards and downwards spikes when the input signal changes from high to low, or vice versa:

12.5Hz, C1=10nF, R1=100K
12.5Hz, C1=10nF, R1=100K

I think I’ll leave that mystery for another day.