This circuit is the telephone pickup small amplifiers for To expand the voice conversation louder. or intercepted phone call. It use pickup telephone and LM358 op-amp IC as preamplifier and LM386 out to speaker and use 9V battery Low power consumption.
The working principle
The complete circuit of the The telephone pickup amplifier is shown in Figure 1 the working start with a signal from magnetic pickup will entered in to J1-Jack to amplify amplitude up by preamplifier. Which is serves by IC1/2 and IC1/1 as LM358 OP-AMP IC will get voltage biasing from input into half of power supply, to audio signal swing to be balanced, both positive and half negative half wave. By resistors R4 and R5 There is capacitors C6 pull AC signal to ground to purity voltage on the AC impedance is low.
Figure 1 The complete telephone pickup amplifier.
The pickup signal will be connected through C2 to first preamplifier IC. Which R3-resistor is set an input impedance of circuit about 100K ohms. This preamplifier serves as amplify weak signal into higher signal, And is also a sound filter, By sound frequency of telephone there are in range of 300 Hertz to 3 KHz.
The working of filter circuit wil be feedback on RC form. Whic include capacitors-C3 and R1-resistors. The cut off frequency of circuit can be calculated from relations.
Frequencies (f) = 1/(6.28xRC) Hz….(1)
By R = resistance (ohm), and C = capacitance (F) see in circuit we use 220pF capacitor and 270K resistor, then calculated will be:
the Cut off frequency (f)
= 2680 Hertz
The pickup signal will is amplified by the preamplifier set of IC2/1 and IC1/1, both section to high gain. The output signal from preamplifier will be send through C7 capacitor to amplify out to speaker by LM386 are well known in as amplifier and controlled level audio by VR1-volumes of a horseshoe.
How to build this projects
The PCB layout is shown in Figure 2 Building, same as a general Kit Figure 3 is put low component before such as a jumper wire. Then, put resistors, capacitors and ICs. Reasons the device to short before, due to if the device is fully advanced, to make the device smaller later difficulties. After placing all the equipment already.Check the terminals of the capacitor or the ICs properly. If correct, it is sure to test the circuit.
Figure 2 the PCB layout of The telephone pickup small amplifiers
Figure 3 the components layout of the telephone pickup small amplifiers
Tested and applied.
We connected a battery to this circuit. Which can use voltage from 6 volts up to 9 volts. On the prototype uses battery 9V rectangular. Start with adjust VR1 to the middle position, then magnetic pickup in the input jack and take it to attach the speaker phone. Next, picking up the phone will need hear a test tone through the speakers. On call or chat. It must be heard by the speaker.
If place the speakers of this projects near the microphone of the phone too much. It can cause feedback whining up. We modified by adjusting VR1, a lowered voice. Placement of the phone is important.
Because the pickup has a very high sensitivity. May induce unwanted noise out to extend it.
So, where to place the phone and this project should be placed away from other magnetic field sources. such as Transformers, television monitors, computer or wiring 220 volts. To prevent 50 Hz hum induced into the circuit.
The components list.
¼ W + 5% Resistors
R2, R4, R5, R6——10K
C1, C9—————-100uF 16V___Electrolytic
C3, C10————–100pF 50V____Ceramic
C4, C5, C7———-0.1uF 50V____Ceramic
C6, C8—————10uF 16V___Electrolytic
IC1——————-LM358__DUAL DIFFERENTIAL INPUT OPERATIONAL AMPLIFIERS.
IC2——————-LM386 Low Voltage Audio Power Amplifier
B1 —– Battery 9V with connector
SPK1-small speaker 8ohm 0.25W.