Attn: All LM555 IC Wizards

Btw... in the above I was referring to Astable Operation Mode in the Lm555. When calculating. What is the units of resistance and capacitance For example. I have the value

Ra=10K Rb=54K C=0.1 uF

Formula: f= 1.44/[(Ra+2Rb)C]

for the above values...

f= 1.44/[(10,000 ohms + 2(54,000))x 0.0000001 F] = 1.44/[0.0118] = 122 Hz

Right?

log

log wrote:

--- Unless you have some overriding reason for wanting to use a bipolar

555, you should use a CMOS 555 in this configuration: 7555 +-------+ +--|TH | | |__ OUT|--+---->OUT +-O|TR | | | +-------+ | | | +----[R]------+ | [C] | GND

That way can get about a 50% duty cycle output, get rid of one of the resistors, use smaller valued capacitors and larger valued resistors, and generally make your life simpler since for an astable you can write, for the period:

T = 1.4RC

or, for the frequency,

1 f = ------- 1.4RC

You state that your frequency range is 0.1 to 100kHz, which is 100Hz to 100kHz. That's a frequency range of 1000:1, so the change in resistance will also have to be 1000:1.

One problem you will have is trying to get 100kHz with the pot at the low resistance end of its travel, so the thing to do there is to assume that the pot's end resistance is zero ohms and insert a fixed resistor in series with it so that when the pot is up against the stops you'll still have that fixed resistance in there. Like this:

+-------+ +--|TH | | |__ OUT|--+---->OUT +-O|TR | | | +-------+ | | | +---[R1]-+->[POT] | | | [C] +----+ | GND

Now, if you consider some reasonable value for R1, like 1000 ohms, then the pot needs to be 1 megohm in order to get the 1000:1 frequency spread you need.

So, if you've got 1000 ohms in there with the pot cranked all the way down to to zero ohms, your capacitor has to have a capacitance of:

1 1 C = ------- = --------------------- = 7.14E-9F ~ 7nF 1.4Rf 1.4 * 1000R * 1E5Hz

7nF isn't a standard value, but off-the-shelf pots are available in resistance steps of 1-2-5, so for half the resistance (500k) the capacitance would increase by a factor of two to 14nF. 15nF is available and it's pretty close, so with R1 equal to 499 ohms and a

500k pot that might work for you. If it didn't, then you'd have to fiddle with series and parallel combinations of caps to shift the frequency range to where you wanted it to be.

-- John Fields Professional Circuit Designer