Description.

This is just another 20W audio amplifier circuit , but this time based on the LM1875 audio amplifier IC from National Semiconductors. With a 25V dual power supply LM1875 can deliver 20W of audio power into a 4 ohm speaker. The LM1875 requires very less external components and has very low distortion. The IC is also packed with a lot good features like fast slew rate, wide supply voltage range, high output current, high output voltage swing, thermal protection etc. The IC is available in TO-220 plastic power package and is well suitable for a variety of applications like audio systems, servo amplifiers, home theatre systems etc.

Circuit diagram.

Notes.

• Assemble the circuit on a good quality PCB.
• Use +/-25V DC dual supply for powering the circuit.
• K1 can be 4 ohm, 20W speaker.
• A proper heat sink is necessary for the IC.
• F1 and F2 are 2A fuses.

Description.
Here is the circuit diagram of a simple subwoofer filter that can be operated from a 12V DC supply. Such a circuit is very useful in automobile subwoofer applications. The circuit is nothing but a low pass filter whose pass frequency can be adjusted between 60 to 160 Hz.
The circuit is designed around the TL072 dual BIFET opamp IC. Out of the two opamps inside the chip, IC1A is wired as a buffer. The left and right audio inputs after mixing is fed to the input of the IC1A using the DPDT switch S1. Switch S1 is the phase control switch which can be used to make the subwoofer in phase with other speakers. When S1 is in position 2, 180 degree phase shift will be induced.POT R7 can be used for controlling the level. IC1B forms the low pass filter whose pass frequency can be controlled by adjusting the dual gang POT R13.

Circuit diagram .

Notes.

• Assemble the circuit on a good quality PCB.
• The circuit can be powered from 12V DC.
• IC1 must be mounted on a holder.
• S1 is a DPDT switch.
• R13 should be a dual gang linear POT.
• C5 and C6 must be polyester capacitors.
• POT R13 can be used for adjusting the pass frequency.
• POT R7 can be used for adjusting the level.

Description.

Here is a simple circuit that can be used as a tachometer. The circuit is basically a frequency to current converter which converts the incoming signal into a proportional current to drive the meter. The deflection on the ammeter will be proportional to the frequency of the incoming signal. For using this circuit as an automobile tachometer, the input terminal A should be connected to the spark plug cable and terminal B should be connected to the vehicles ground.

For calibrating the circuit, set R2 at 25K and R4 at 5K.Power up the circuit and feed the input terminal with a 60Hz square wave form your function generator. Adjust R2 so that the meter shows 0.36 mA (equal to 3600rpm). Now disconnect the input signal and adjust R3 so that the meter shows 0mA.Now connect the 60Hz signal again and if the meter does not show 0.36mA adjust R4.A completely calibrated circuit will show 0mA at 0Hz and 0.36mA at 60Hz.

Circuit diagram.

Notes.

• The circuit can be assembled on a Vero board.
• I have not tested this circuit on any automobile. So use this circuit on your automobile at your own responsibility.
• Anyway, the test using the signals from my function generator was satisfactory.
• The ignition voltage from the spark plug terminal is in the Kilo volt rage. The engine must be OFF while making connections and you must be very careful to avoid shock hazards. Try this circuit on your automobile only if you have sufficient knowledge and experience on automobile electricals. I have no responsibility on any mishap.
• The circuit can be powered from 12V DC.
• The bridge D3 can be made using four 1N4007 diodes.
• M1 can be a 1mA FSD ammeter.

Description.
Here is a very simple circuit diagram of a frequency to voltage (F to V) converter. Such a circuit finds numerous applications in projects like digital frequency meters, tachometers etc. The circuit is mainly based on a LM555 timer IC. The IC is wired in mono shot mode to convert the input frequency into a fixed pulse width, variable frequency PWM signal. Resistors R4 and capacitor C2 provides the necessary timing for the circuit. The transistor T1 forms a discharge path parallel to C2 which is necessary for re triggering the IC. Capacitor C1 acts as an input DC decoupler.
Circuit diagram.

Notes.

• The circuit can be assembled on a Vero board.
• Use 12V DC for powering the circuit.
• LM555 must be mounted on a holder.
• The output of the circuit is not a pure DC but a PWM waveform. Additional circuitry is required to convert this PWM waveform to pure DC.

Description.
Here is the circuit diagram of a 10W audio amplifier using the popular TDA2003 IC from SGS Thomson. The IC can easily deliver 10W to a 4 Ohms load at 18V DC supply voltage. The IC can be also operated from 12V and that makes it applicable in car audio systems. The useful features of TDA2003 includes short circuit protection between all pins, thermal overload protection, low harmonic distortion, low cross over distortion etc.

The circuit given here is designed as per the datasheet from the manufacturer and found to be working fine. Capacitor C7 performs the job of input DC decoupling.R2 and R3 is used to set the gain of the amplifier.C3 and R1 determines the upper cut off frequency.C6 and R4 and meant for increasing stability at high frequencies. Capacitor C5 couples the output to the speaker.

Circuit diagram with Parts list.

Notes.

• It is better to use a PCB for assembling this circuit.
• PCB layout for this circuit can be found on the datasheet of TDA2003.
• Use 18V DC for powering this circuit.
• Input ground and output ground must be properly decoupled.
• Speaker K1 can be a 4 ohm one.
• TDA2003 must be fitted with a heat sink.