Hi,
I’m no expert, but I think it would be quite difficult to use a POT to change the frequency of such small range. I have used the 555 timer for some time and am quite familiar with it. I have a schematic which I took of the Maplin catalogue which has worked quite nicely with me. I’ve digitized it. Have a look.
Regarding the timing capacitor and resistors, I have calculated possible values to oscillate 20Hz
Ra = 220K Ohm + 100K Ohm Pot in series (the pot should be set to about 80K Ohm).
Rb = 220K Ohm
C = 0.1uF
RL = Resistor in series with the load. 100 Ohm would be OK for an LED.
There is also a decoupling capacitor which can be attached in parallel with the power supply. 47uF 16V would be OK.
Along with the schematic came a formula to calculate the frequency
f = 1.44/[(Ra +2Rb)xC]
Resistance in Ohm and Capacitance in Farad.
To simply things so that you can use C (capacitance) in Micro Farad (uF):
f = 1440000/[(Ra +2Rb)xC]
Remember 1000000uF = 1F
Find the schematic attached. All the components are very cheap. Most likely the 555 timer would be the most expensive part.
Also remember that the resistors and capacitor have tolerance of about 10%. That’s why it’s a good practice to use a POT in oscillator circuits.
You can try and tune the POT for different frequencies.
Hope this helps. There are also many other circuits on the web.
Bye!
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