Like I said, the circuit shown in post #74 should work with either voltage. Not quite as linear on the PWM adjust, but pretty close. All you do is connect pin 7. I may not have the right formula for the frequency on it though, it could be

F = 1.4 / ( RC)

It is odd how you are making me recheck some basic assumptions on my part.

 

Like I said, the circuit shown in post #74 should work with either voltage. Not quite as linear on the PWM adjust, but pretty close. All you do is connect pin 7. I may not have the right formula for the frequency on it though, it could be

F = 1.4 / ( RC)

It is odd how you are making me recheck some basic assumptions on my part.

when using 1u & 3500ohm (plus the 10k pot), i do seem to get accurate freq adjustment from 52-200Hz. its the 1-68Hz (by math) using 68u & 150ohm that peaked out at 30Hz on the scope.

right now i am using two freq ranges
52-200Hz using 1u & 3.5kohm
3-56Hz using 22u & 500ohm

the question is, is the 7555 output being damaged by sourcing & sinking current into a 1/3full and 2/3full capacitor across 500ohm? with an empty cap the inrush would be ~5/500=10mA, which is likely less in operation because the cap is never 100% full or empty (except for startup), etc.

so in essence, the circuit i have works the way i want it to. now i just need to convince myself that it will run forever. i have had my circuit running at different freq and pwm% for last 4 days and nothing seems to need attention, nothing hot, nothing smelling funny, etc.

the whole reason behind me using 5v is to accommodate user provided wall wart in the 7-35vdc/ac, an option for the user, less cost, etc.

 

i tried 555CN with 5v and saw the same as 7555. per Bill's request i then up'd the voltage to 12v and things look a tad differently and i am now getting better freq ranges.

1u & 3kohm yields 47-205Hz
22u & 500ohm yields 2.6-60Hz

this means with 12v the 555 output is sinking/sourcing just below 80mA as it charges and discharges the RC circuit.

 

For low voltage operation (power supply 5V) you probably want higher range resistors. At higher power supply voltages the 7555/7556 will work better than at lower power supply voltages.

The hysteretic oscillator requires good sourcing and sinking on the output. Using pin 7 (discharge) it doesn't matter as much, but even then low voltage will mean the internal CMOS (if it is a CMOS chip) will not turn on as hard. With a conventional 555 the 1.2V drop below Vcc is what is driving the irregularities.

At this point it is your call, since it is your design.

 

@11v with 500ohm (i measure 11v on output and my supply says 12.1v using 555CN), thats 0.88 watts, but 555 runs at 50% duty cycle. so this means my 500ohm is dissipating ~0.44watt and i am using 1/4watt resistor. i need to check on how hot my 500ohm is getting.

edit: using my IR temp gun, the 555 is running right around 82F and i didnt get any reading on the 500ohm resistor above what the rest of the breadboard or parts are (about 77F). ambient is 77F. the RC circuit only looks like Vout/500ohm when the cap is 100% empty.

so at sourcing (cap at 1/3 Vcc) the Vdiff is (11-(.33*12.1) = 7.007v
so at source time the 500ohm will see 7/500 = 14mA, 11v*14mA = 0.154watt

at sinking time the cap is 2/3 Vcc = ~8v
so at sink time the 500ohm will see 8/500 = 16mA, 8v*16mA = 0.128watt

well below the capability of my 1/4watt resistors.
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now, lets run the same math using 5v on 7555 (output swings 0-Vcc)

so at sourcing (cap at 1/3 Vcc) the Vdiff is (5-(.33*5) = 3.35v
so at source time the 500ohm will see 3.35/500 = 6.7mA, 3.35*6.7mA = 0.022watt
(maybe 6.7mA is too much for 7555 output to handle when Vcc=5v?)

at sinking time the cap is 2/3 Vcc = ~3.33v
so at sink time the 500ohm will see 3.33/500 = 6.6mA, 3.33v*6.6mA = 0.022watt

 

Take a look at Fig. 6 and 8. It can sink a little bit but not source much at all. Of course the current to charge and discharge the cap will change as it charges and discharges so the current won't be as high as you are calculating, but with low values of R the timing will be hard to predict.

http://www.intersil.com/content/dam/Intersil/documents/icm7/icm7555-56.pdf