PWM with a 555 Timer...

Got a quick question.

I want to use a 555 timer chip as a pulse-width modulator, in
order to drive an R/C servomotor. The pulse width will need to be
varied between 1ms and 2ms for this application.

What I don't know how to calculate, is how the varying voltage at
Pin 5 (Control V) of the chip affects the timing. The external RC
circuit changes the timing by t=1.1RC - that much I know... but how
much does differences in ControlV alter this?

Any advice would be helpful.

 

Take a look at http://notrocketscience.mysite.wanadoo-members.co.uk/ibmport.htm
I never built it, it was only simulated but should work. From memory,
it takes a variation of around 1/3 of the supply, relative to the
positive rail to get the variation required. Only vary the voltage
between 1/3 and the full positive supply.

 

Thanks. I'll keep that in mind, and look at the website later.


Oh - and just FYI... the project I have in mind *is* rocket
science. I'm planning to use the servos to drive the fins on a
vertical-stabilization system for my camera rockets.

 

Have you considered using a Picaxe-08M?
It is ideal for PWM motor control and as can be software or hardware controlled.

 

Never heard of that particular device. I *have* been considering
using a Basic-Stamp... my only problem would be how to write a
non-multitasking program to vary one of the PWM outputs after a
trigger event.

 

Have a look at http://www.rev-ed.co.uk/picaxe/

You would not consider using a Basic Stamp after trying one of these.
Extremely easy to use and program.

 

Looks to me like it's a slightly beefed-up Basic Stamp. Many of
the same commands and everything.

No Canadian or US distributors yet, either... according to the
website.

 

I've been looking at the online manuals... it's got potential. The
'servo' command in particular would be useful - but I can't find
anything that will tell me if two servo commands for different pins
would be mutually exclusive. If I can put (say...) a 1.5ms pulse on
one pin, and a 1.95ms pulse on another at the same time. If it came
right down to it, no doubt I could use two chips - each running a
different servo channel.

 

I use picaxes all the time, andthey're so relatively dirt cheap that
using two would be ideal, as long as you can tolerate the weight.

Iv'e got a question for you though...would not making a stabalisation
capacity with movable fins turn your rocket into a guided projectile,
which falls under all kinds of new rules that are quite different from
jsut a rocket that you fire and hope goes where you want it to. I have
a feeling that the FAA would not appreciate it.