Project Troubleshooting

Hello Everyone!

I'm building a continuous servo controller using a 555 timer and I can't seem to get it to work. I've been following the circuit diagram on the website below, and I've attached a photo of my work as a comparison. I didn't use the exact transistor specified, a BC547, instead I'm using a 2N2222. Otherwise, I don't know what the issue could be.

Any and all help would be appreciated!

The photo was too big, cropped it down.

Check your pushbuttons are turned the right way around.


Rookie electronics it sure is!

The 1k and 47k resistors are switched (error in the circuit diagram).

Base resistor should be perhaps 10 to 22k, collector resistor should be 1k.

Check your pushbuttons are turned the right way around.

Ah, there’s a clever trick on that breadboard arrangement. By connecting to diagonal terminals, it does not matter which way round the pushbuttons are inserted. :grinning:

Check your 2N2222 is it 2N2222 or a P2N2222?

What is the tester doing, that makes you think it is not working?

Check your 100nF cap, thats 0.1uF or 104 marking.

If you have a continuous servo, it will probably turn one way continuously with one button pushed and continually the other way with the other button pushed.

What happens when neither button is pushed I don't know.

Have you used an arduino to check your servo?

Tom.... :slight_smile:

Thank you so much! I the circuit started working after I switched the resistors.

Honestly, thank you so so much. My teachers at school had no idea what they were looking at and it was incredibly frustrating.

For future reference, how did you identify what was wrong? I'm a beginner so any and all advice would be appreciated.


Oh dear - the teachers do not know everything! :roll_eyes:

I do love the irony of that website name - "rookie electronics". We often refer here to the "instructables" website where many projects are well-meaning but actually simply wrong and sometimes quite dangerously so.

OK. The transistor's purpose is to amplify a small current to control a larger one. A smaller current generally corresponds to a larger resistance (Ohm's law, but let's not get int an argument about that :astonished: ), so you expect the base resistor to be larger than the collector resistor by a minimum of ten times. The gain of the transistor is going to be at least ten (but there are further tricks there which we need not be concerned about in this application).

The problem with the swapped resistors is twofold. One is that the 47k resistor may not be sufficient in itself, to effectively "pull up" the voltage to the servo. I don't know exactly what the input impedance of the servo is (someone else may have an opinion) but I'm betting it is actually less than 47k.

Secondly, bipolar transistors suffer from "charge storage" so that if you feed a lot of current into the base, far more than necessary to perform the amplification function, this charge is stored in the base-emitter junction and it takes some time for that charge to dissipate during which time the transistor continues to conduct even thought there is no voltage on the base. This time is measured in hundreds of microseconds, but that happens to be the sort of time which is critical in control of a servo. And it is made worse by the too-high collector resistor.

So there you have it. I learned this - the hard way - some forty-odd years ago, and transistors have not changed in the meantime though we now use FETs for preference, and they have some limitations with charge storage for somewhat similar reasons.

Protocol here dictates that comments/ replies - even compliments - be on the forum itself rather than private not merely because it makes me feel better ( :grin: ) but so the nature of the problem and its solution is confirmed. You can go back to the first post and modify its title to include "(Solved)".

Where did that Quote come from?
Today? ? ?

Tom... :slight_smile:

I sort of explained that in the last paragraph.

OK, a bit subtle ...

Okay... Did you get some extra karma?

Tom.... :slight_smile:

Dunno. It’s in the “lap of the gods”. I got one karma point today (I do keep an eye on it) for something but since Caeli PM’ed me rather than on-forum, I presumed it was not from him. :roll_eyes:

Thanks for the help!

I was finalizing the project the other day, and the circuit isn't working, despite it working perfectly a few days ago. I took it out and resoldered any weak looking connections, but now the board is not responding at all.

There's a bit of motion with the servo when I first plug it in, but otherwise the switches are completely non-responsive.

Any suggestions?

Did you get some extra karma?

There was another post were a Russian guy didn't give karma even after thanking the people who helped him and after repeated asking why he finally said "Traditionally, karma is bad .." (I don't know where he got that idea. After I explained to him that here on the forum karma is good, he gave karma)