does anyone know of a good schematic that works, i have tried some i foud on google, but they all seem not to work.

i want to control the position of the servo with a pot

i have tryed this one

http://www.geocities.com/BourbonStre...rvobasics.html

but the servo does nothing on the output of the transistor, if i directly connect it to pin 3 it will activate the servo but the servo will move slowly clockwise till it hits its end point, the pot does not seem to do much

also i have seen some with transistors on the output of pin 3 of the 555 is that needed?

any help much appreciated

 

That looks very straight forward, it sounds like you may be connecting the servo up incorrectly. The article in the link explains that there are various different connections used, you need to ensure you use the right ones!.

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I built a servo controller according to this page:
http://www.hobbyengineering.com/appServoTester.html

When hooked up, the servo immediately goes to full throw with the pot (R2) fully CCW. When turning the pot CW the servo sounds like it is being forced past its limit. Can any servo experts chime in?

I should've just bought a tester for $20. I think I'm going to anyway.
http://www2.towerhobbies.com/cgi-bin...&I=LXVK79&P=ML

 

The feedback loop in a servo is 'closed' by the geartrain connecting the driven motor to the POT. Sounds like you're turning the POT by hand? There's no feedback from the driving motors to the POT then.. There's going to only be three states, full left or full right, with increasing pulse durations (duty cycle) to the driven motor, and a very narrow dead band in the center. If the dead band doesn't exist then there isn't enough hystersis in the electronics for one, which is going to end up in oscilation near 'center' with the geartrain feedback loop connected. A servo is the most basic form of a PID loop between electrical and physical components. If the loop isn't closed physically you're going to get odd electrical behavior, and vice versa.

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Hi Lompa,

There have been reports that some of the simple servo controllers will not work with digital servos. I have an older controller that works well with all of my non-digital servos, but not with JR digitals. I don't know exactly why that is. It may have something to do with stability of the pulse or input impedance.

In any event, if you are using a newer servo version or digital servo, try using an older analog servo and see whether your 555- based controller works then. John

 

The output transistor serves to invert the signal. Using the 555 output directly tells me that something isn't right with the way you wired it up (or the transistor). A servo needs a very low duty cycle, 10 to 20% (1 to 2 mS pusle width, 50 hz frequency) though there is a lot of latitude. Without the inversion, you should have pulse width of around 17 mS which is probably why it did what you observed. With inversion, the values given in the schematic are ok. Note: the longest pulse you can have is 1.7 mS which will only get you 70% of the possible travel of the servo. A 30K pot would give you very close to 2 mS but they don't exist (I believe). The shortest pulse width is right around 1 mS. At 1.5 mS, the servo should be at mid position.

Got access to a scope? If so, that should tell you what's going on. If not, wire up an LED + small resistor (say 300 ohms) in place of R4. The LED should be fairly bright when you power up the circuit. Wire it with 2-3' (yes, feet) of wire and swing it in a darkened room. You've just created the worlds cheapest OScope... If you've wired the 555 right, you should see long streaks of light with short dark segments. The dark segments should be 10 to 17% of the overall period.

 

Here's a thread where we spoke about this (quite) a while back ...

http://www.electro-tech-online.com/e...96-servos.html

The circuit I posted worked well in my application.
Jeff

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That's a great idea for teaching! Never saw that one, but once it's pointed out, it is so obvious. Thank you for sharing it.

We used to do a similar simple demonstration of Gas Liquid Chromatography using a tube full of laundry powder and natural gas as the carrier. John

 

I figured it out when building a POV image wand. I'm sure I'm not the first, though.

 

Thanks for the tips, everyone. I ordered the servo tester from tower (actually the boss did since it was a project given to me) but I'm still going to mess with the parts I already bought with the info in this thread.