Sound-To-Servo Driver

This page describes a project that takes an audio signal and uses it to drive a RC servo. The louder the sound, the more the servo moves. The most obvious application of this is to make your own mouth animation for talking and singing creatures.

History
General Servo Theory
Circuit Theory
Circuit Modifications
Schematic
Related Pages

History

Ray Martin is a friend of ours who designs toys for a living, sometimes electronic, sometimes mechanical. We were having lunch after the TRW Amateur Radio Club swap meet, and I mentioned that some of the best hackable products with animated mouths were discontinued, and Halloween lovers were constructing singing and talking props out of alternative things like servos. Ray said, "I have a circuit that does exactly what you need on my desk right now!"

Ray went further than that. He customized the design for specific servo limits and pulse frequency, built a prototype and tested it, and sent me the details.

Since then, David has entered the circuit into our CAD package, laid out two different prototype printed circuit boards, and experimented with them.

General Servo Theory

A RC servo takes a series of pulses and uses them to direct the position of an output shaft.

General servo limits (they vary between manufacturers and models) are:

pulse width angle comment

0.6m Sec -45 degrees minimum pulse length

1.5m Sec 0 degrees center position

2.4 mSec -45 degrees maximum pulse length

pulse width	angle	comment
0.6m Sec	-45 degrees	minimum pulse length
1.5m Sec	0 degrees	center position
2.4 mSec	-45 degrees	maximum pulse length

Notes:

Increasing the pulse width by 10 µSec results in about a degree of movement on the output shaft.
These numbers are nominal, and vary slightly between manufacturers and models. For example, the HiTech HS81 likes pulses between 0.74 mSec and 2.14 mSec.
The rate at which these pulses are sent isn't terribly important - only the width of the pulse. Some typical rates are 400 Hz (2.5 mSec pulse spacing) and 50 Hz (20 mSec pulse spacing).

The driving pulse is usually specified as 3-5 Volt Peak to Peak, but I suspect that in many cases you can get by with whatever power the motor is getting. I would avoid using a drive pulse greater than the motor power.

Most servos require a power supply between 4.8V and 6.0V.

Circuit Theory

The single LM324 integrated circuit contains 4 op-amps, which perform three functions in the circuit:

Two sections of the LM324 (output pins 1 and 14) form an oscillator set to 50 Hz, the desired pulse repetition rate. The output of the oscillator is approximately a triangle wave.
One section of the LM324 (output pin 7) integrates the incoming sound, producing an envelope signal that is proportional to the average loudness of the sound.
The final section of LM324 (output pin 8) compares the output of the other two sections. This clips the triangle pulses into nice rectangles, with a pulse length depending on the envelope signal.

Circuit Modifications

The gain of the filter buffer (Pin 7 output) is +3 and the resistor network reduces the amplitude by a factor of ten and scales the offset voltage to make it compatible with the triangle wave generator to generate the 3% to 12% duty ratio as required by your servo motor. Therefore, those three resistors are the only variables one needs to modify to obtain the full gamut of duty ratios.

The pulse rate is selectable by changing only the capacitor that connects from pin 13 to pin 14 of the LM324. Some sources say, do not exceed 50 Hz for proper servo function.

Schematic

Here is Ray's original schematic:

Click for full size schematic.

David's Etch1 prototype was exactly as Ray drew it. During testing, he decided on some changes:

Power regulator on-board feeds +5 (7805) or +6 (7806) volts to controller and servo.
Potentiometers allow you to adjust servo center position and range.
Changed value for R15.

Here is David's schematic the Etch2 revision:

Click for full size schematic.

This is one of the Etch2 boards.

David is a perfectionist and wants to fiddle around with it some more, but the current results are good enough to share.

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