trahanc Posted January 9, 2006 Report Share Posted January 9, 2006 Hello All,I write here because I am just about to give up. My project was as follows: I want to control a hobby DC motor (with a stall current draw of about 3A) with an audio signal coming from a pair of powered computer speakers. I am a super Halloween fan, and my hopes are to operate a jaw mechanism in this manner. I know servo controllers are normally used here, but I want to hack into Walmart's toys and use the already constructed jaw mechanism. Recently, I found this website http://home.rica.net/jimk/projects/servo/index.htm , which got me really excited. SO, I build the circuit and tried it out. It did, in fact, work, but only for one of my walmart toys ... the one with the lightest jaw (less load - it draws only 0.5A at stall). The others only hummed with an intensity proportional to the music. So, I emailed this guy, but recieved no reply.So, I write here in hopes that someone will help me in either altering the above circuit (and confirming my diagnosis), or just as easy, constructing a completely new circuit.Any help is greatly appreciated.Thanks,Corey Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 10, 2006 Report Share Posted January 10, 2006 Hi Corey,Welcome to our forum. ;DA TIP31 transistor has a wide range of spec's. Weak ones can't drive a 3A motor. Also the MPS2907 transistor isn't strong enough to feed a more powerful output transistor.The output from a computer speaker's amplifier is too weak to feed the circuit.The circuit has a single rectifier diode, so the motor is powered for a max of only half the total time. The circuit doesn't have a filter, so the motor hums with the music.It's a crappy circuit. :'( Quote Link to comment Share on other sites More sharing options...
trahanc Posted January 10, 2006 Author Report Share Posted January 10, 2006 Wow, thanks so much for the warm invite and quick reply. And thankyou for going to the site and checking out this particular circuit. So, if I understand your reply well enough (I am an electronic novice), you mean to say that (1) I first need a stronger feed transistor (as opposed to the MPS2907), one that (2) can feed a more powerful transistor (as opposed to the TIP31). I need (1) because the output from the computer speaker is too weak. SO, any suggestions as to a pair of transistors that may fit my bill?>The circuit has a single rectifier diode, so the motor is powered for a max of >only half the total time. As far as the single diode rectifier, I'm not sure that I really need something more than this, as this circuit need only be a crude driver. It would be something interesting to test though.>The circuit doesn't have a filter, so the motor hums with the music.wow, I thought the hum was from the motor trying to push against the jaw resistance, but not having enough current to move it. Thanks for the tidbit. :)Thanks Again,Corey Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 10, 2006 Report Share Posted January 10, 2006 (1) I first need a stronger feed transistor (as opposed to the MPS2907), one that (2) can feed a more powerful transistor (as opposed to the TIP31).Yes. Your motor draws more current. I need (1) because the output from the computer speaker is too weak.No. Replacing the transistors with more powerful transistors increases their current to match your motor. The voltage of the signal from a little computer speaker's amplifier is too low for this circuit. This circuit should have its own amplifier and volume (motion) control. Quote Link to comment Share on other sites More sharing options...
trahanc Posted January 10, 2006 Author Report Share Posted January 10, 2006 It really sucks that this was such a crappy circuit. Especially since it is the only one for this purpose I have found online after hours of searching. I will replace the transistors with your advised.I found this full wave rectifier at Radio shackhttp://www.radioshack.com/product/index.jsp?productId=2062581&cp=&kw=full+wave+rectifier&parentPage=searchI suppose a capacitor would work as a filter. I assume I should use one with a voltage rating of at least the circuit power supply. Any clue as to what capacitance I should use?I also assume the rectifyer and capacitor should be in circuit before the transistors.Thankyou infinitely for your support here, ANDif you ever have any 'stupid' quantum physics questions, please write me ... I owe you lol ;D-C Much appreciation for your help. Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 10, 2006 Report Share Posted January 10, 2006 I wouldn't use a puny 1.4A rectifier with a 3A motor.Try a 220uF or 470uF capacitor between the rectifier and 100 ohm resistor. The larger value reduces hum more but causes the jaw to slowly return to home. The positive wire of the capacitor and rectifier connects to the positive supply. Quote Link to comment Share on other sites More sharing options...
trahanc Posted January 10, 2006 Author Report Share Posted January 10, 2006 Got it. This is my last set of questions, promise. (1) What purpose does the audio amplifier serve here? I went to Radio Shack and could not find it's specs. It doesnt appear to be a step-up or step-down.(2) How do I know the voltage/current output of my computer speakers. I know the speakers are 3 Watt RMS and 6 Watt System Power, and I think the two speakers are 8 OHM. Ok, I'm done. Thanks audioguru for all your help.-C Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 10, 2006 Report Share Posted January 10, 2006 (1) What purpose does the audio amplifier serve here? Quote Link to comment Share on other sites More sharing options...
indulis Posted January 10, 2006 Report Share Posted January 10, 2006 Without taking into consideration the current needed to drive the motor, I don't see this circuit as being that bad, "for what it is", a cheap way to periodically activate a motor. Sure, with a servo you could get "movement" proportional to input amplitude, but the circuit isn't so simple anymore. Obviously this circuit is for unidirectional operation, and there is some other force applied to return the mechanism back to some predetermined position by either gravity or spring force. Is significant filtering of the rectified signal, or full wave rectification for that matter, from the transformer really needed? Wouldn't that just keep the motor "running" longer, thus defeating its periodic movement for the illusion of speech? I could see some "light" filtering with a time constant that is around the natural period of the motor being used, but not anymore than that. Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 10, 2006 Report Share Posted January 10, 2006 Hi Indulis,The main complaint was that the motor was vibrating like a speaker. The full-wave rectifier and filter capacitor takes care of it. I mentioned that if the capacitor's value is too big then the return motion would be slowed. ;D Quote Link to comment Share on other sites More sharing options...
indulis Posted January 10, 2006 Report Share Posted January 10, 2006 OK... say you go full wave. All that that will do is make the motor turn for as long as there is a signal in either polarity, provided it's "low" enough to turn on the PNP. In other words, signal = mouth open and, no signal = mouth closed. I would have thought the intent was to have motion to "loosely" mimic speech, and full wave WILL NOT ALLOW THAT!!!My guess would have been that motor was vibrating because it was being driven at to high a frequency, at which point, even if you had 100 A available, the current has very little to do with anything!!! You Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 10, 2006 Report Share Posted January 10, 2006 Hi Indulis,The signals that are rectified to activate the motor in the jaw are probably speech.Half-wave rectified or full-wave rectified speech would both be similar, except full-wave would have less vibration and give full output volage.I have my new Sound Level Indicator project displaying right now. Its LM3915 drives its LEDs with a voltage that fluctuates exactly like speech. It has half-wave rectification and looks exactly like the full-wave rectified one that I had before.Its filter has a 100ms decay time constant so it is pretty quick. If a motor tried to follow its speed then the mechanism would probably make quite a racket. A longer time constant would work best for the jaw's motor.Here's my project:http://www.electronics-lab.com/projects/motor_light/009/index.html Quote Link to comment Share on other sites More sharing options...
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