Automatic Volume Control Circuit
Automatic Volume Control and Limiter
Transform the usability of communications equipment with this automatic volume control. Never again will you have to max the volume just to hear something and then get blasted out of your seat by a sudden loud noise. Save your ears and improve your comfort with this simple but effective circuit.
This circuit is based around a bootstrapped amplifier comprised of Q1 and Q2. Q1 is arranged as a common emitter amplifier whose load resistance is R1. Q2 is a simple emitter follower. The output of Q2 is used to bootstrap R1 via the 10uf coupling capacitor C4. This has the effect of boosting the apparent resistance of R1 to a very high value, thereby boosting the voltage gain to in excess of 500.
Q1 is biased from the emitter of Q2 through R7,C5 and R8. R7 and or R8 may need to be adjusted depending on the Hfe of Q1. With the correct values the voltage at the emitter of Q2 should be 2 to 3 volts.
C5 is necessary to prevent undesirable negative feedback and should be a ceramic or film capacitor.
C2 is a supply filtering capacitor. R3 and C1 also filter the supply voltage going to Q1 providing a reasonable amount of supply line rejection for the amplifier. Without this filtering action the amplifier would be liable to audio frequency oscillation or excess noise.
Variable negative feedback is applied to the amplifier using the Light Dependant Resistor (LDR) R10.
The dark resistance of the LDR is about 10Meg. The resistance can drop to a few hundred ohms when the LDR is exposed to bright light. The gain of the amplifier is determined by the ratio of R3 to the LDR resistance.
The lower the resistance of the LDR the greater the amount of negative feedback and the lower the amplifier gain is. The gain can be varied from more than 500 to less than 1 by controlling the amount of light falling on the LDR.
The audio output of the amplifier is feed via the current limiting resistor R11 and capacitor C6 to the diode D2 and the base emitter junction of Q3. The 2 semiconductor junctions form an anti-parallel diode pair that will effectively clamp any voltage in excess of +.0.7 volts or -0.7 volts. This clamping will protect your ears from loud impulse noise such as static or lights being switched on or off.
Also when the peak input signal begins to exceed about 0.5 volts some current will begin to flow through the base emitter junction of Q3. This current causes a much larger current to flow between the collector and emitter of Q3. The maximum value of this current is limited by R5 and causes the LED to illuminate. C3 (100uf) smooths out the variations in brightness of the LED with varying input current. R6 ensures that C3 discharges in a reasonable time when there no current. When the light from the LED is shone on the LDR the automatic volume control loop is complete. High input signals to the amplifier cause the LED to shine brightly and cause the resistance of the LDR to drop to low values. This increased the amount of negative feedback and reduces the gain of the amplifier. Some experimentation will the values of R5 and R6 may be required depending on the application and personal preference. R5 can be increased up to about 2.2k and R6 can be varied between about 47k and 1Meg. All the 1uf capacitors in the circuit should be low leakage ceramic or film types. If electrolytic capacitors were used instead the high leakage currents would badly affect the operation of the circuit.
Constructing the LED-LDR photo-coupler
There are a number of options for constructing the LED-LDR photo-coupler. I soldered two wires to a white light 0805 SMD LED and then stuck the LED onto the surface of a 5mm LDR using superglue. I put the assembly into a short piece of plastic tube and sealed one end with tissue paper and superglue. The tissue paper got very hot and formed a very hard substance. Use ventilation as fumes are evolved.
Next I melted opaque crayon wax and covered the LED-LDR assembly with it. The LED and LDR were completely encased in opaque crayon wax. Finally I sealed the other end of the tube with tissue paper and superglue. It might be easier to use a larger 1206 SMD LED or you could file flat the top of an ordinary through lead LED. Using a white light LED is not absolutely necessary, a red or green LED should work as almost as effectively.
This automatic volume control is a great addition for many other electronic circuits. It can greatly improve homemade regenerative or direct conversion receivers that have a very large range of audio output. In particular it can protect your hearing and improve comfort when listening to weak signals and suddenly very loud impulse noise comes along or some other loud signal bursts in. It can also be used effectively with intercom, voice recording, and transmitter voice compression circuits.