Let's Make Robots!

Running a servo through a darlington

I am wondering what's the trick to running a servo (hxt900) on a picaxe28 (pic16f886) through a darlington transistor array.

I tried many different thing sbut the only thing that works is removing the darlington and replacing it with the 330 ohms resistor array (as specified in the "start here" recipe).

The only reason I am trying to do that is so that I can use the transistor array to work with speakers. 

Would it be a better Idea to just use the resistor array and then add transistors to each of the speaker's signal?

As you can see my circuitry skills are still low but I makee up for it in enthusiasm.

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As I'm following this threat (also a ydm builder here ;) and also looking for an little amplifier. First I found the LM386 IC. with this IC and some other parts an amplifier can be build easily. A lot of info on YouTube about this. Then I looked further and have now found this little amplifier on Ebay: http://www.ebay.nl/itm/Mini-Digital-3W-3W-Amplifier-Board-USB-Power-Dual-Channel-2-CH-5V-Amplifier-/261077841129?pt=LH_DefaultDomain_0&hash=item3cc973b4e9 I'm about to buy it..

Short answer: No you can't drive your servo through the darlinton transistor array.

Long answer: The problem is the darlinton transistor is inverting your servo's control signal. A high signal from your picaxe turns the transistor on which then pulls your servo signal pin low (assuming you had a pullup resistor).

Question: Why do you need the transistor array to drive a speaker? As long as you have a 10uF - 22uF capacitor in series with your picaxe pin then it can drive a small speaker directly. If you want more volume then a set of computer speakers designed to run from USB power will solve your problem.


Thanks for the answer. It does help.


The reason I wanted to drive the speaker through the darlington is simply because I started building a variation on the yellow drum machine and managed to create good sounds out of my speaker with the signal from the Darlington. 

The design used by fritsl does not specify a Darlington but it does away without any servos. Since I was trying to be creative and didn't have any motors laying around I was trying to see what could be done with servos. I was not very happy with the result, not enough springny lifelike movement for the "head". I guess I'll go back to the drawing board and build the whole thing with the 330 ohms resistor array. Then If I want to use servos for something I can.

I thought maybe I could use some generic 2n2907 transistors directly to amplify the signal and get a better "beep" or "click" out of my little speakers. That seems doable.  Or am I confused and I need something specific to invert the signal for that too?

You will not get any amplification with the transistors as the picaxe output is a digital output. You will only get on or off. I do not know why you think you need them.

NOTE: you must have a capacitor in series otherwise you will damage either the speaker or the picaxe chip.

Now I get the feeling that I have been very lucky so far. There seems to be a lot I don't understand at all. It's a miracle I was able to make the few things I have done so far. 

On my "start here" robot, whic uses the same picaxe board and chip with a resistor array I was able (or so I thought) to amplify the signal so that it powers a very small motor. I routed the signal through a small transistor and it worked. I thought the same principle could be applied to a speaker to get enough mA to make it louder.

Just to make sure we're talking about the same thing I am using the output 1 in the F section.

The Darlington doesn't amplify your signal. It just allows you to sink a lot of current, more than the Picaxe chip itself can handle.

I have never used that board, I just used Picaxe chips in a breadboard and that was over 2 years ago. All I can say is read manual 3 very carefully. You can learn a lot from it.