Limiting Amplitude of 555 Oscillator Output The Cheap N Easy Way

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The two diode signal clipper is fine, if you add a series resistor (that can be upstream of the capacitor or downstream). Without this resistor, there is a large current spike on each edge that will heat the 555 and bounce the battery voltage around.

You can also connect the ends of a pot between the DC blocking capacitor and ground and take the signal from the wiper, so you can adjust the amplitude anywhere between zero and full output. A 10k pot should work fine for most signal injection applications. If you want to protect the wiper contact and 555 from a short to ground at full amplitude, you can either add a series resistor to the capacitor end of the pot or in series with output from the the wiper.

Thanks for the reply, John.

What value resistor would be a good starting point?

I thought about the potentiometer to add a 'volume' control, but without really knowing the output amplitude (my DVOM doesn't respond fast enough to read the voltage) I figured I'd just fix it at 0.6V so that I could always know it was 'safe'.

Most amplifiers that I work with (guitar, bass, etc) expect something on the order of a 100mV signal directly from the instrument. They are often used with a boost or distortion effect that (on average) has a gain of about 100, so a 1V signal should be ok. I don't know if the 7-8V signal directly from the 555 would damage anything or not, so I decided to take the safe route and cut it way down.

-phaeton

The 555 has a pretty high peak current rating, but to keep the battery voltage from dipping each time a positive swing takes place, I think limiting the current to a few milliamps is good enough. A 1k would work.

Sounds like a plan. Small Schottky signal diodes also make good low voltage clamps similar to germanium diodes, if you don't have those to spare.

Excellent! Thanks for the tips. I have a bunch of Ge diodes, but they're a little costlier than some others. That, and i don't want to use them frivolously since someday they will all be gone. :-(

So this technique with the diodes is called 'clamping'?

-phaeton

Clipping or clamping, depending on who you ask.

If you can handle a little surface mount device, these have two diodes in one package for about $0.13 each from Digikey:

You may want to consider using a voltage divider through a padded RC network other wise, compressing the signal is going to place a load on the circuit which will drain a battery faster than expected. I guess you could pass a series resistor from the output of high value before the clamps. In either case how ever, you'll still get square wave as a results. I think you did say something about injecting it into an amp? If that being the case, you'll want to use a simple RC network that can shape the wave form and reduce level drive (attenuate).

Just to be devil's advocate, the way I learned it, they're two different things. A clipper limits the output excursion - a clamp changes the DC bias such that one signal excursion never exceeds a certain voltage, more like a level shifter:

Input signal, 4Vpp __ __ __ +2V | | | | | | | | | | | | 0V | | | | | | etc. | | | | | | -2V _| |__| |__| |__

Clipped to 2Vpp: __ __ __ +1V | | | | | | 0V | | | | | | etc. -1V _| |__| |__| |__

Clamped at 0V: __ __ __ +4V | | | | | | | | | | | | | | | | | | etc. | | | | | | 0V _| |__| |__| |__

The clamp voltage can be anything you want, and can be on the top or the bottom of the waveform. They do this to TV video, (clamp the sync pulses to some reference) but call it a "DC restorer".

Cheers! Rich

I agree that clipper or limiter are the better choices for reducing the peak-to-peak swing.