Description.
The TV transmitter given here uses UK standard 1 FM modulation for sound and PAL for video modulation. The audio signal to be modulated is pre-amplified using the transistor Q1 and associated components. The transistor Q2 has two jobs: production of carrier frequency and modulation. The pre-amplified audio signal is fed to the base of transistor Q2 for modulation. Capacitor C5 and inductor L1 forms the tank circuit which is responsible for producing the carrier frequency. The video signal is fed to the emitter of transistor Q2 via POT R7 for modulation. The modulated composite signal (audio+video) is transmitted by the antenna A1.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a good quality PCB.
• Inductor L1 can be made by making 4 turns of 24SWG enameled copper wire on a 6mm dia: plastic former.
• T1 can be a radio frequency transformer with built in capacitor. (Can be found on old transistor radio boards).
• Antenna A1 can be a 1M long copper wire. (Experiment with the length to get optimum performance).
• This transmitter is working in VHF band somewhat between 50 – 210MHz.
• This transmitter is compatible only with PAL B and PAL G systems.

Description.
This simple circuit produces a beeping sound that lasts for around 3 seconds whenever you make a whistle. The CMOS Hex inverter CD4049 is the heart of this circuit. Out of the six inverters in CD4049, U1a is wired as an audio amplifier which amplifies the signal picked up by the microphone M1.The U1b is wired as a band pass filter with center frequency around 2KHz.The filter is necessary in order to pass the frequency corresponding to whistling sound and suppress all other frequencies .If the filter is not there, the circuit could easily get false triggered.U1d is wired as a 3S delay monostable multivibrator.The output U1d drives the astable multivibrator formed by U1e and U1f.The astable multivibrator is operating around 4Hz.The combined effect is a intermittent beeping sound that lasts for around 3S.Transistor Q1 is used to drive the buzzer B1.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a good quality PCB.
• The circuit can be powered from a 3V battery.
• IC U1 is a CMOS CD4049 Hex inverter.
• M1 can be an electret microphone.
• B1 can be a 3V piezo buzzer.
• Mount the IC on a holder.
• The duration of beeping can be adjusted by varying the components C4 and R9.

Description.
Here is the circuit diagram of a simple 24W mono amplifier using IC TDA1516.The TDA1516 is an integrated class B power amplifier in a 13 pin SIL package. The IC has many useful features such as short circuit protection, load dump protection, thermal protection, reverse polarity protection etc. Here the IC is wired in BTL mode to deliver 24W of power into a 4 ohm speaker. This amplifier can be operated from a 12V DC supply and this makes it suitable for car audio applications.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a good quality PCB.
• The amplifier can be powered using a 12V DC supply.
• Capacitor C2 must be rated at least 25V.
• The IC must be fitted with a heat sink.
• Switch S1 is the stand by switch.

Description.
This circuit is designed as per a request made by Mr Vinoth from India. His requirement was a 12V/5A DC fan motor controller. I think this circuit is sufficient for this purpose. Quad 2 input Schmitt trigger IC CD4093 is the heart of this circuit. Out of the four Schmitt triggers inside the 4093, U1a is wired as an oscillator with adjustable duty cycle. The U1b, U1c, U1d buffers the output of the oscillator to drive the switching MOSFET Q1.The MOSFET drives the DC motor according to the switching pulse obtained from the oscillator. When R1 is varied the duty cycle varies and so do the speed of the motor. Diode D3 acts as a freewheeling diode.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a good quality PCB.
• IC U1 should be mounted on a holder.
• U1a, U1b, U1c, U1d are part of the same IC CD4093; so power supply is shown connected only once.
• The12V power supply for this circuit must be able to handle at least 5A.
• A heat sink is recommended for Q1.

Description.
The circuit given here is of a regulated dual power supply that provides +12V and -12V from the AC mains. A power supply like this is a very essential tool on the work bench of an electronic hobbyist.
The transformer T1 steps down the AC mains voltage and diodes D1, D2, D3 and D4 does the job of rectification. Capacitors C1 and C2 does the job of filtering.C3, C4, C7and C8 are decoupling capacitors. IC 7812 and 7912 are used for the purpose of voltage regulation in which the former is a positive 12V regulator and later is a negative 12V regulator. The output of 7812 will be +12V and that of 7912 will be -12V.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a good quality PCB.
• Transformer T1 can be a 230V primary; 15-0-15 V, 1A secondary step-down transformer.
• Fuse F1 can be a 500mA fuse.
• Capacitor C1,C2,C5 and C6 must be rated at least 50V.

Description.
Given below, is a very simple circuit that can be used for charging car batteries. In this circuit there is facility for monitoring the charging current and voltage.

The circuit is based on the IC MC78T12ABT from Freescale.The IC is nothing but a 7812 in TO-3 package with 3A capacity. The transformer T1 steps the mains voltage to 15V AC and diodes D1&D2 does the job of rectification. Capacitor C1 does the filtering and C2 acts as a decoupling capacitor. The ground terminal of IC1 is lifted to 2.1V using the diodes D3 , D4 and D5 . So the output from the IC1 will be a regulated 14.1V (12+2.1).The battery is charged via diode D6.The D6 blocks reverse flow of current from battery to charging circuit when the mains power is not available. Meter M1 shows the charging current and M2 shows the charging voltage.

Circuit diagram with Parts list.

Notes.

• The transformer T1 can be a 230V primary; 15-0-15V, 3A secondary step down transformer.
• The meter M1 can be a 3A ammeter.
• Meter M2 can be a 20V volt meter.
• Fuse F1 can be a 1A fuse.

Description.

This is a simple and easy to construct digital dice circuit. The circuit is based on a single IC, CD4060B.The dice consists of six LEDs marked D1 to D6.The number of LEDs glowing indicates the numeral.

The heart of this circuit is 14 stage binary ripple counter IC CD4060B.The IC also has a built-in oscillator. The oscillator output (here 2 KHz) is used to clock the binary ripple counter. The counter increments by one in its natural count sequence each time it is clocked. The oscillator in initially inhibited as long as the pushbutton switch S2 is not pressed. The counter outputs will be in logic zero state and all the six LEDs will be ON.As the push button S2 is pressed, oscillator is enabled and the counter starts counting. The counter outputs (pin 4, 5 & 7) changes from 000 to 101 and then resets to 000 to repeat the sequence. After 101 the counter does not advances to 110 because of R3, D7 & D8.When the counter just advances from 101 to 110 the diodes D7 & D8 become reverse biased and makes the reset pin (pin 12) high to reset the counter.

The counter counts as long as the push button switch S2 is pressed. Also the micro buzzer will sound as long as the IC is counting. When the push button switch S2 is released, the counting is stopped and holds the existing state to represent the random number.

Circuit diagram with Parts list.

Notes.

• Switch S1 is the ON/OFF switch.
• Switch S2 can be a push button switch.
• Buzzer K1 is a piezo buzzer.
• The circuit can be powered from a 9V PP3 battery.
• The IC must be mounted on a holder.

Description.
This is a very simple lamp flasher circuit that uses only three components (a capacitor, relay and one resistor) other than the lamp. The working of the circuit is very straight forward. When the power is switched ON the capacitor C1 charges through the resistor. When the voltage across the capacitor is sufficient, the relay switches ON and the lamp connected via the normally open contact of the relay glows. The relay remains energized until the capacitor discharges and then the lamp extinguishes. The charging and discharging cycle of the capacitor gives a flashing effect to the lamp.

Circuit diagram with Parts list.

Notes.

• The relay L1 can be a 12V SPDT relay.
• The lamp L2can be a 12V, 5W lamp.
• The frequency of flashing can be varied by changing the value of C1 and R1.

Description.

This is a simple and easy to construct circuit that can be used to provide a bidirectional drive to a DC motor. The circuit operation is straight forward. Output of an astable mutivibrator based on IC1 (NE555) is used to control the relay RL1 driving the motor. The motor is connected between the two poles of the relay contacts. The relay contacts are so wired as to reverse the DC supply to the motor when the contacts changeover.

The astable multivibrator produces a square wave at the output with its high time given by 0.69(R1+R3+R5)C1 and low time given by 0.69(R1+R2+R4)C1.The high and low times can be varied by varying potentiometers R4 and R5.For the given values the high and low times can be adjusted between 1S and 8S separately. When the IC1 output is low, the relay is de energised and the relay contacts are in position 1-1 with the result that A terminal of the motor is positive and motor runs in one direction. The IC1 output is high the relay is energised and the contacts changeover to position 2-2.Now the terminal B of the motor becomes positive and motor runs in the opposite direction. The transistor Q1 is used to drive the relay according to the output from IC1.The diode D4 acts as freewheeling diode.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a good quality PCB.
• The circuit can be powered from a 12V DC power supply.
• The IC1 must be mounted on a holder.
• The capacitor C1 must be rated at least 15V.
• The relay RL1 can be a 12V DPDT relay.

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