Audio/Servo Driver Circuit

by Scary Terry

As long as there is sound present, the servo will drive to its "max" position. If the sound is short in duration, the servo will not have time to drive to "max" but will drive part way and return to "min" position. While this method of moving a mouth is not perfect, it's pretty good and I'm very happy with the effect. It's important to remember that any sound will drive the servo, voice, music or noise, so if you're trying to make a Bucky mouth move to a voice track, you shouldn't have music in the background of that particular track. There is a way to include music or other sounds and still have the appropriate jaw movement. Jeff Stevens came up with a solution and I include his email at the bottom of this page.

If you have questions or comments, please contact me.

Cowlacious Designs offers this circuit as a kit (or completely assembled). Believe me, this is a much easier way to build this circuit than trying to do it from scratch and the kit is only a few bucks more than buying the individual parts. (Note: I am not affiliated with Cowlacious Designs. I have given them permission to use my design, but I make no money from the sale of the product).

Circuit Description From the output of your audio source, the center conductor goes to "Audio In", the outer shield goes to ground. The signal is amplified by U1a whose gain is controlled by R3. You should be able to use audio sources from a line level to a "reasonable" speaker level source and adjust the gain using R3. U1b compares the audio from U1a to a reference voltage set by R6 and sends a switched output voltage through diode D1 to CMOS switch, U2. R6 should be adjusted so that with no signal, LED D2 just turns off. Once a signal is present, R6 can be used to fine tune the servo action. Capacitor C2 is used to smooth the servo operation. I show a 10uf capacitor but have used values as low as 2.2uf depending on need. Lower values make things jerky, higher values slow down the servo action. I recommend experimenting to find out what's best for your situation. You may want to consider just putting in a 0.1" spacing socket in place of C2 so it will be easy to substitute different value capacitors. That's how Cowlacious is shipping their kits now.

U2 is simply used as a switch. In this case, a high on pin 12 closes a switch between pins 10 and 11, triggering the servo driver (actually, it's not a full closure but about 90 ohms of resistance). A high on pin 13 closes the switch between pins 1 and 2, which is what lights the LED. (The LED is optional, but it really makes it easier to make adjustments and insure the circuit is working correctly. If the LED is not used, pin 13 should go to ground). There are two additional switches that may be used in this IC, pin 5 controls the switch between pins 3 and 4, and pin 6 controls the switch between pins 8 and 9.

The circuitry surrounding U3 is the servo driver. In its original form, the circuit was a "servo tester". I've made some modifications to make it suitable for this application. Variable resistor R10 sets the starting point of the servo, so at one extreme, the servo will start at 0 % and go all the way to 100% while at the other extreme, the servo will start at 99% and go to 100% of its travel. Capacitor C5 helps to keep the power supply stable during servo operation. J1 is the connector for the servo.

This circuit, without a servo attached, only draws about 2 ma, but actively driving a servo, draws around 600 ma. I'd recommend a 5 volt power supply of at least 1 amp. I've found good sources for this type of supply to be MPJA and All Electronics.

My favorite place to purchase servos is ServoCity. They've got good prices and I've never had any problems with their service. My current favorite standard servo is the Hitec HS-425BB. With dual ball bearings and nylon gears, it's a good compromise between performance and price.

I've had a report of one guy driving seven servos off the one circuit and it worked fine. The important thing to keep in mind when driving multiple servos is that you need a power supply that will handle the combined current of the servos.