I'm experimenting with a standard 555 low-duty-cycle oscillator using a 555 (e.g. figure 5.34 of the Art of Electronics, or here). But I want to make the pulse frequency variable, but without changing the pulse width.

To me the obvious thing to try was to replace the discharge resistor with a series of resistors connected to a selector switch, but that doesn't seem to work - the pulse frequency is much higher than one would expect from simple calculation.

I'm guessing this has something to do with the stray capacitance of the wires and the selector switch.

Question 1: Is there a way to modify this to make it work?

Question 2: What is/was the simplest way to accomplish what I was trying to do?

(To be more precise, I want something that outputs ~20 us pulses at several different frequencies ranging from 20 to 200 Hz. It doesn't have to be very accurate or stable, ~5% accuracy is perfectly acceptable.)

 

Hello,

So you want 20 μSec pulses at a variable frequency.
Use an 555 as oscillator and a second one that is triggered by the oscillator to generate the 20 μSec pulses as a monostabile.

See Tony's page for more info:
http://www.uoguelph.ca/~antoon/gadgets/555/555.html

Greetings,
Bertus

 

Ah, thanks, I'd considered using a one-shot triggered by a timer, but hadn't realized I could just use a second 555 for this purpose.

So the first 555 now only needs to be a variable square-wave generator. What is the best way to make the frequency variable? Selector switch on the front panel connected to different capacitors?

 

Have you seen this?

555 Hysteretic Oscillator