why I cant get short duty cycles ? resistors that I used are 20% tolerant but each measured with multimeter before putting on the board to be sure they all have right values.
calculations for duty cycle from that website.
The formula used is as follow:
t= 1.1RaC
Ra is the total of R4 and R3. So, the minimum pulse time when R4 is set to 0, is:
t= 1.1 x 8200 x 0.0000001
t= 0.902 ms
Note that this minimum pulse width time is longer than the trigger pulse to ensure that the pulse width generator doesn't constantly generate0.65ms pulses one after the other, but at a steady +- 50Hz interval.
WhenR4 is set to maximum, the time is
t= 1.1 x 18200x 0.0000001
t= 2.002 ms
To get a stable output frequency, use film capacitors (not polarised). Ceramic capacitors are not good for timing, very unstable, the numbers in your screen shot (which you should simply have posted as text) show it's drifting. Polyester film type is normally good enough.
It it doesn't react.to the pot I suspect your wiring is wrong there. Take it out, measure the resistance with your multimeter, see if it behaves the way you expect.
That would do the job - but doesn't explain the very high duty cycles. That shows an unexpectedly high resistance in the pot, and why I suspect something is wrong with that pot.
Wvmarle you are right its pot/board connection. I have to press it down to get low values when released its giving very high resistance. Pot is working when outside the board.
Second as i mebtioned i installed 2x220nf on each 555. But schematic asks for second 555 only 100nf. Cant explain how i come up with 220 on board instead of 100 :-).
Will change the pot to resistor anyway i need 3 positions 1ms 1.5 and 2ms.
tolerances of my resistors are 20% so I assume you can round values if using silver or gold stripe(band) resistors.
I have not seen silver and gold bands on resistors for about 40 years. The widest tolerance you can get these days is 5% ( old gold band ) with 1% resistors being a fraction of a cent more expensive.
Anyway because there is a pot in the circuit it doesn't matter. Only with R1 might it matter a bit, but not much.
surepic:
Well, if this idea wont work then i will put arduino nano or micro and will forget about 555 forever :-))
Mike why you were driving servo via fet instead of mosfet? Isnt it more efficient in this shcematic to use mosfet? Something like 2n7000?
I'll take a stab: first I'll assume you mean bi-polar when you wrote fet. If so, then:
Based on the 4k7 pull-up, that servo doesn't require much current to drive it, thus bipolar vs MOSFET is trivial, since power loss is trivial.
BTW: a 2N7000 is not a good choice for 5V switching. More like 2N7002, or even VN2222LLG
surepic:
Now i need to switch whole circuit from 3.3v 20ma gpio. Current consumption for the circuit with motor is approximately 550ma-560ma 5v.
Any suggestions what transistor/mosfet to use?
When you say 550mA-560mA, are you talking average current, peak current, or ???
And, when you say "motor", are you still talking about a Servo, or is it an actual morot? And if so, what kind of motor? Typical brushes included DC motor, or something more exotic?
And will there be any PWM involved? Or, is it just a strait ON for awhile, OFF for awhile?
And, I guess the ultimate, qualifying question is: when you say "whole circuit", do you mean that same circuit, but controlled from a 3.3V port, rather than a 5.0V port. And, will the rest of the "whole circuit" be powered at 5V? In which case, is this a question of voltage translation?
Why am I confused? Because there are two opposing hints:
"whole circuit" might imply, "original circuit"
yet, "what transistor/mosfet to use" implies a disregard for the "circuitry" that comes before the transistor/mosfet.
In other words, are we disregarding that apparent settled upon circuitry, or is this a question about how to drive the settled upon circuitry? Or, is this a whole new question?
Servo motor with peak current 550ma. No pwm 555s are generating pwm with help of your link.
I want straight on and off. Whole circuit i mean the last image i attached i.e. pwm generating circuit from 5v plus servo motor attached. All together 560ma peak powered from 5v.
Control of switching circuit on and off should be done via 30ma max 3.3v input.