diagram circuit

The operation of the circuit is the transistor Q1 is wired an emitter follower to provide sufficient current gain and input impedance. The second transistor is used to voltage amplify the signal in. The network of resistance and capacitors connected between emitter of Q1 and base of Q2 is used to control the tone. Variation in the value of these components varies the audio response of the system. This circuit can provide a maximum attenuation and boost of 10decibel on 10 KHz and 60 Hz frequency ranges. The input and output must be connected with respect the ground. POT R5 can be used to control bass. POT R8 can be used to control treble.

A very simple encoder circuit for a dc motor can be constructed using this circuit diagram . As you can see in the circuit diagram , the system shown consists of the HA-2542, a small 12-Vdc motor, and a position encoder. During operation, the encoder causes a series of constant-width" pulses to charge CI. The integrated pulses develop a reference voltage, which is proporţional to motor speed and is applied to the inverting input of HA-2542, The noninverting input is held at a constant voltage, which represents the desired motor speed. A difference between these two inputs will send a corrected drive signal to the motor, which completes the speed control system loop.
As you can see the circuit requires few external components , but because of the encoder wheel it has a limitations of use .If you put a pulley under the encoder wheel you can command the speed of other device , by connecting the (motor and other device ) using a belt