The "infamous" 555 servo controller

Thread Starter

abc27

Joined Dec 16, 2010
50
Is the schematic in post#1 still your current schematic, except for the pins to the servo?

If so, I will try to get it going tonight before NCIS.

John
Yes it is.

Just noticed that the wiper on your pot is not shown connected to anything. Double check how you have it connected. The center pin should be connected to one end.

John

Edit: Seems to work OK for me. Pulse range 1.02 mS to 2.90 mS. No capacitor on pin 5 seems to be needed. The problem may be as simple as not having the pot connected correctly.

John
No I am not that bad to have the pot wired incorrectly :D

What i'm thinking is the issue is my breadboard layout. I've rechecked it over and over again though. I'll try changing all of my components for fresh ones and report back.
 

jpanhalt

Joined Jan 18, 2008
11,087
I would definitely try shorter jumpers. Mine are solid 22 awg. At least that way, you can easily see what's connected to what.

John
 

Thread Starter

abc27

Joined Dec 16, 2010
50
What simulation software you using? The duty cycle on that circuit should 50Hz, can you check it out for me?

I'm absolutely stumped as to what I should do. I'm nearly positive it's my breadboard layout but I can't pinpoint where I went wrong. So I think i'm going to draw up my breadboard layout and let you guys have a look.
 

jpanhalt

Joined Jan 18, 2008
11,087
What simulation software you using? The duty cycle on that circuit should 50Hz, can you check it out for me?
Simulation? What's that? I'm talking real silicon, carbon film, polyester, and ceramic (for decoupling). Repeat rate is 20.10 Hz. That should work, but it is a little slow.

I'm absolutely stumped as to what I should do. I'm nearly positive it's my breadboard layout but I can't pinpoint where I went wrong. So I think i'm going to draw up my breadboard layout and let you guys have a look.
Bet you life on it? Check all of your component values and part numbers first. The failure rate of these electronic components is far less the failure rate of humans who assemble them.

John

Let me play with it a little.

John
 

Thread Starter

abc27

Joined Dec 16, 2010
50
Simulation? What's that? I'm talking real silicon, carbon film, polyester, and ceramic (for decoupling). Repeat rate is 20.10 Hz. That should work, but it is a little slow.
Ah so you're actually doing real life testing. I thought you were using some sort of computer simulation package. Are you doing it on a breadboard or something similar?


Bet you life on it? Check all of your component values and part numbers first. The failure rate of these electronic components is far less the failure rate of humans who assemble them.
Yeah, i'm almost 100% sure it's something to do with my layout. I can't see where i'm making the mistake. I've even tried to generate a breadboard layout using EAGLE's autorouter. Exact same issue as the previous layout.
 

jpanhalt

Joined Jan 18, 2008
11,087
Change the 3M3 to a 1M0 and see what happens. That gives me 59Hz. Oops, make that 67 Hz. (I read the wrong channel.)

John
 
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Thread Starter

abc27

Joined Dec 16, 2010
50
Change the 3M3 to a 1M0 and see what happens. That gives me 59Hz. Oops, make that 67 Hz. (I read the wrong channel.)

John
I would but I haven't actually got one available. =/

At the moment I only have 56k, 560R and 3M3 resistors around. It's strange that the duty cycle is coming out as 20Hz. I chose the 3M3 so that it would have a 40Hz duty cycle.
 

jpanhalt

Joined Jan 18, 2008
11,087
Put 2 or 3 of the 3M3 in parallel (1/r = 1/r +1/r ...). So, 2 in parallel is 3.3/2 = 1M6 and 3 in parallel = 3.3/3 = 1M1, which should be about perfect.

John
 

jpanhalt

Joined Jan 18, 2008
11,087
Edit: Ignore the following. I had the servo connections wrong. See below.


I tested it with a couple of servos, not just the oscilloscope. it is not a very stable circuit. As built, I would not consider it a useful circuit. I am using an NE555. It is extremely sensitive to the pot setting. It does sort of work with the following changes:
1) Replace 3M3 with 1M0 (i.e., 3, 3M3 in parallel)
2) Add 0.01 from pin5 to ground. Value is not critical. 0.022 will work too.
3) Add a 1K0 or thereabouts resistor from pin 3 to your positive supply. It is a pull-up resistor.

I will try some alternative 555 circuits that may be more stable. We are having a lot of snow, so there is not much else to do.

John
 
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Thread Starter

abc27

Joined Dec 16, 2010
50
I tested it with a couple of servos, not just the oscilloscope. it is not a very stable circuit. As built, I would not consider it a useful circuit. I am using an NE555. It is extremely sensitive to the pot setting. It does sort of work with the following changes:
1) Replace 3M3 with 1M0 (i.e., 3, 3M3 in parallel)
2) Add 0.01 from pin5 to ground. Value is not critical. 0.022 will work too.
3) Add a 1K0 or thereabouts resistor from pin 3 to your positive supply. It is a pull-up resistor.

I will try some alternative 555 circuits that may be more stable. We are having a lot of snow, so there is not much else to do.

John
It's not the best I know but i'm doing this for a school project where the standard expected is a simple wire-to-wire SPST, LED and DC motor circuit. I need to do well in this project so i'm going the extra mile and adding in a servo.

I'll try your suggestions but i've a few questions.

1 - Ok
2 - What do you mean by a 0.01? A 0.01 what? A capacitor?
3 - I don't really understand what you mean by connecting pin 3 to the positive supply. Surely pin 3 leads only to the servo signal pin? Do I just add a resistor between the 555 and servo?
 

jpanhalt

Joined Jan 18, 2008
11,087
CORRECTION

Excuse: I was using an old Airtronics servo and got the pin-outs wrong. (I had center signal.)

The circuit works as is. No pull-up is needed. The capacitor on pin 5 does not appear to be needed. It is still not particularly stable, but it does work.

Thus, just substitute three, 3M3's in parallel. If that is unstable, then a small resistor (100 to 300 ohm) in line with the signal may help, and the capacitor from pin 5 (0.01 to 0.1 uF) may also help.

John
 

Thread Starter

abc27

Joined Dec 16, 2010
50
Well I tried the three 3M3s yesterday without any luck. I added two 0.5k resistors in parallel to get 250ohms on the signal line and added a cap between pin 5 and negative.

Still the exact same behaviour as before. It appears as if the 555 is doing absolutely nothing as I can disconnect pin 3 and the servo still acts the same (Twitches when plugged in but does nothing else following that).
 

Thread Starter

abc27

Joined Dec 16, 2010
50
How about I just post you up a diagram of my breadboard, maybe i'm wiring it up wrong?

Perhaps, if it's not too much to ask, could you take a photo of the breadboard layout you're using to test out this circuit aswell?
 

jpanhalt

Joined Jan 18, 2008
11,087
I am convinced the circuit is OK. So trouble shooting of the construction is needed now. These may be obvious, but so was getting signal and ground reversed (as I did).

Check to be sure the diode is oriented correctly. Which 555 chip are you using (e.g., NE555, LM555, etc.)? Be absolutely sure the ground and power connections to the 555 are correct.

You might consider putting an LED on the pin3 output with an appropriate resistor and see whether there is any dimming effect. I just tested a 10 mA , red LED with a 220 ohm resistor and it worked.

John

Let's see a photo of your board.
 

Thread Starter

abc27

Joined Dec 16, 2010
50
Your breadboard looks rather neat compared to mine... To say the least :D

That said, i'm positive there aren't any short circuits on the board, i've checked a dozen times.

Here are the photos

I've uploaded them in their full resolution so i'll omit tags

[URL]http://omg.wthax.org/e_6.jpg[/URL]

Note that two wires in lower right corner lead to power source

[URL]http://omg.wthax.org/c_6.jpg[/URL]
[URL]http://omg.wthax.org/d_5.jpg[/URL]
[URL]http://omg.wthax.org/b_3.jpg[/URL]
[URL]http://omg.wthax.org/IMAG0088.jpg[/URL]

Those two resistors with their legs twisted around each other that you see in front of pin 3 are both rated at 560r to produce 280r resistance.


To clarify, the 555 is an NE555.
 

jpanhalt

Joined Jan 18, 2008
11,087
It is difficult to see all the connections well enough to trace them. I know you tried with plenty of photos. I would recommend shorter leads to make routing more obvious. Another thought is to use a marker (black?) to mark both ends of each ground wire and another marker (red?) for each power wire. That will make it easier to see that everything that is supposed to be ground or power is correctly connected.

Contacts on breadboards can also go bad, particularly inexpensive ones. That is another consideration to keep in mind. I have gotten some really cheap "bargain" boards that were worthless after one or two insertions.

Other than those suggestions, it is just a matter of tracing each wire to confirm you have them wired correctly. Another 555 might help, in case you destroyed the one you have, but 555's are pretty durable. Maybe you should remove everything and re-wire the board from scratch.

Since we have confirmed the circuit works, do you have access to any of the solder-type breadboards? One of them might actually be easier to wire.

John
 

Thread Starter

abc27

Joined Dec 16, 2010
50
Well I don't have any other breadboard but I could make a PCB? I haven't got a drill press here at home so i'll have to surface mount everything but I can do it if you really think it's necessary.

I'm quite good at soldering (And i've made a good few PCBs before for simpler projects) but i'd rather stick to the breadboard for the time being until I get it working.
 
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