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I would like to make the jaw of a plastic skull open/close based on words spoken by a Raspberry Pi. I can make the Pi send the audio to the headphone jack, and would like to use this output to run 5v dc motor which will open the jaw, and the jaw will close with springs when there is no sound out the audio jack. I already have the jaw setup with motor and springs and it works when 5v applied, now I need the circuit to switch the voltage to the motor. I tried the following suggestion from another question:

"You want an audio level detector".

schematic

This worked marginally - the motor was slow to respond and very low speed/power. In another post it was suggested to use a FET, but looking at specs for FETs, I was not able to determine which might work with 5v power, input of less than 2v on gate to turn it fully on, and be able to power the motor which draws up to 350ma.

Can you suggest changes to the existing circuit or provide a circuit/ parts information to use FET for this?

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  • \$\begingroup\$ Perhaps you should consider an external power source and control the supply to the motor with the audio level. It sounds like the audio power output is too low. \$\endgroup\$ – Solar Mike Apr 30 '18 at 3:32
  • \$\begingroup\$ This is not a very wise idea. Have the pi control the motor using its GPIO pins to signal a motor driver; have it also produce synchronized audio. But don't try to control the motor with the audio - to do that, you need a more sophisticated detection scheme. \$\endgroup\$ – Chris Stratton Apr 30 '18 at 5:11
  • \$\begingroup\$ What op amp are you using? \$\endgroup\$ – Bruce Abbott Apr 30 '18 at 5:16
  • \$\begingroup\$ Solar Mike, I am restricted to a 5vdc battery pack to run this. Chris, are you aware of a way to produce synchronized output on GPIO pins and audio? I have just started looking into PulseAudio but dont have a clue how to use it yet. \$\endgroup\$ – LeRoyLee13 Apr 30 '18 at 14:35
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schematic

simulate this circuit – Schematic created using CircuitLab

change the output to a tip122 transistor, connect the motor + to 5V the negative lead to the collector. also, try varying the size of c3 after reworking the output if it seems to sluggish. Also, you can add a voltage doubler in there as well to give it some more drive level. I'll draw you the detector and output with a voltage doubler:

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  • \$\begingroup\$ Thanks for the answer. I will give it a try. Is the "signal" input the output of the op amp from my diagram, or directly from the audio jack? Are the characteristics of TIP122 such to allow full conduction with much less current draw from the c2 capacitor charge? Yes, lowering the size of my c3 did improve response time but seemed to reduce transistor conduction even more. I like the suggestion of voltage doubler - I'll keep an eye out for your update. \$\endgroup\$ – LeRoyLee13 Apr 30 '18 at 14:33
  • \$\begingroup\$ that should work for you. but if you think the op amp is going under heavy loading during its negative cycle of the signal, insert a 100 ohm resistor between the output of the op amp and its coupling capacitor. It takes just very small currents in the base circuit to move a darlington driver. These transistors have been out in the industry for years driving things from lamps, pinball coils, to little motors inside ticket dispensers. They are intentionally designed to saturate and take inductive loads. \$\endgroup\$ – drtechno Apr 30 '18 at 15:49
  • \$\begingroup\$ btw I did something like this a long time ago when I built up a anima-tronic robot that went around a track in a pizzeria that one of his servers would operate. Their robot was an RC tricycle with a big platter mounted on the front. It had a wireless audio monitor pack and a wireless lapel microphone. the audio I split and buffered, one way went to a small audio amp, the other way was rectified then it drove a tip122 for the solenoid in the robots mouth, and another one drove lights in the eyes so the robots eye lit up and the jaw moved when the server talked through it. \$\endgroup\$ – drtechno Apr 30 '18 at 16:12

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