servos

[danielsmusic]

i have some futarba servos and need to controll them with electronics.

do i need a driver chip or can i do it some other way?

 

[jbeng]

I have used two 555 timer chips to make a driver. The first 555 is connected as an astable (free-running) oscillator operating at 50 Hz. The output feeds to the second 555 which is operating in monostable (one-shot) mode. The output pulse width of the one-shot is controlled by a variable dc voltage on the control voltage pin of the second 555. This analog voltage can come from a pot or from an D-to-A circuit, I've had good results with both.
Here's a link for a page which might help you:

It's a short tutorial on how the servos operate. They're actually rather easy to work with.
JB

 

[danielsmusic]

do you have any drawings?

 

[jbeng]

Here's a circuit I've used. It works for me, but as I often say, your mileage may vary.
JB

 

[danielsmusic]

do you have to have to use the con voltage on the second 555 or can i put a cap on it instead? thanks for the drawing!

 

[jbeng]

The control voltage applied to pin 5 of the second 555 dictates the pulse width of the output, which controls the position of the servo (ie. 2.5v= full left, 3v=center, 3.5v=full right, if I remember correctly). If you put a cap on pin 5 (like the first 555), you'll likely find that it [the servo] sits there and does nothing.
JB

 

[danielsmusic]

thanks, sorry i relizesd just after posting just one more thing. when i built it into to livewire a circuit simulater it doesnt work could it be the simulater?

 

[jbeng]

... when i built it into to livewire a circuit simulater it doesnt work could it be the simulater?

Don't know, never used the simulator. Maybe I (you) should let my friend know that your simulator says the servo drive circuit I built for him doesn't work ... :lol:

Here's a circuit I've used. It works for me, but as I often say, your mileage may vary.

JB

 

[The Question]

I know this thread is OLD, but I would like to try to build this board. I've already built a simpler one and it work, but doesn't work with digital servos.

Well. My question would be what caps do I use? polarized, np, ceramic, electrolytic? And what do I use to get a variable 2.5-3.5v input?

 

[ElecTroy]

One could use a fairly simple adjustable voltage regulator for the control voltage, as seen on this page . Or I imagine a pot at the right value from this circuits +5v to ground, and the wiper on the wire expecting the control voltage might work. Just some ideas

 

[ElecTroy]

Ok lets try actually getting the address to the page on it this time! Sorry about that.https://diyaudioprojects.com/Technical/Voltage-Regulator/]LM317 / LM338 / LM350 Voltage and Current Regulator Calculators[/url]

 

[The Question]

digital servos work on pulse width modulation. wouldn't work

 

[ElecTroy]

Sorry I assumed you were speaking of the circuit mentioned earlier in the thread, that produces the control pulse modulation, varied by the control input voltage between 2 and 3.5v. No you are correct direct voltage on the signal wire of the servo wont work. I havent tried it yet myself, I hope to be able to
control brushless motor ESCs without the receiver. I am a new member so is telling me my post will be in a delay que.
voltage ragulator page is LM317 / LM338 / LM350 Voltage and Current Regulator Calculators
servo control ciricuit mentioned above is
https://www.electro-tech-online.com/attachments/servodrv-gif.5798/

Hope that helps.
Troy

 

[madhippiescientist]

I tried this circuit just today, and remember pin 8
needs GND (not in schematic). I couldn't get the control
voltage range indicated to keep my servo from far CCW
(I couldn't get it to move beyond "10 o'clock" at all). I
even tried the V all the way up to my supply (regulated
5v, so maybe that's why?). I altered the monostable's
capacitor, to no ideal, until I changed R4 to less resistance,
but I still couldn't get beyond "2 o'clock". It also made
"bottom out" noise at full CCW. All of this via diodes for
cutting the voltage down & testing 5V all the way to 2.6V.
BTW, I did get it to work however, by leaving in 3 diodes
in series to pin 5 of the monostable, and using a 4.7K
resistor in series with a 20K potentioneter for R4. In case
this is of any use and would like to control the servo by
resistance instead of V.

 

[jpanhalt]

I tried this circuit just today, and remember pin 8
needs GND (not in schematic).

To which schematic does "this" refer?

If you are referring to the schematic using a 555, pin 8 is Vcc; pin 1 is ground.

John

 

[ElecTroy]

Hmm yes according to this data sheet it does show pin 8 is VCC.
If it worked for him it must feed through pins 2 (trigger) and 4 (reset)
somehow while being held in a continuous trigger reset condition.
I cant claim I am familiar enough with these ICs to know, however
I would have thought shorting pin 8 to ground would kill the whole thing,
Schematic does seem incomplete considering there is no ground indication
at all direct to the ICs.

for Madhippiescientist, after looking at the datasheet it would appear the
controlvoltage can be as high as 15V, perhaps the output range you need
could be achieved this way? though I am sure changing some other
component is a more practical choice.

Thanks to you and Jpanhalt for the added input.
Troy

 

[ElecTroy]

Maddhippiescientist, perhaps moving your grounds to pin 1 will correct the the probs you are having? Best hopes, Troy

 

[madhippiescientist]

https://www.electro-tech-online.com/attachments/servodrv-gif.5798/
Is the circuit I was referring to, and you guyz are
atop of the ball. I did mean pin 8 to vcc and pin1
to ground (this is how I wired it, but working the
nocturnal shift kinda drains the brain)...