Description.
For circuits using TTL ICs the supply voltage is a great concern and a slight increase in supply from the rated 5V may damage the IC. Using fuses alone does not solve the problem because a fuse may take several milliseconds to blow off and that’s enough time for the IC to get damaged.
In this circuit a crowbar scheme is used in which a triac short circuits the power supply and burns the fuse. The burning time of the fuse is not a concern because the power supply is already shorted by the triac and the output voltage will be zero. When the output voltage exceeds 5.6 volts the zener diode D2 conducts and switches ON the triac T1.Now T1 acts as a closed switch, shorting the circuit. The output voltage drops to zero and fuse gets burned off. Since the switching of triac takes place within few micro seconds there will be no damage to the TTL ICs or any other such voltage sensitive components in the load circuit.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a general purpose PCB.
• If 1A Bridge is not available, make one using four 1N4007 diodes.
• The trip voltage can be varied by varying the values of D2 and R2.
• All capacitors must be rated at least 25V.
• The transformer T1 can be a 230 V AC primary, 12v secondary, 2A step-down transformer.

Description.
Here is the circuit diagram of an adjustable voltage regulator using IC L200. L200 is a monolithic integrated adjustable voltage regulator IC having features like current limiting, thermal shut down, power limiting, input over voltage protection etc. Here the regulator is designed to produce an output adjustable between 2.85V to 15V at 1A.The resistors R1 and R2 determines the output voltage. The resistor R3 determines the limiting value of output current, here 1A. Capacitors C1 and C2 does filtering.Do not give more than 40V to the input on L200.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a general purpose PCB.
• Power the circuit with 18V DC
• Always the input voltage must be few volts higher than the max: regulator output.
• Fit the IC1 with a heat sink.
• Output voltage can be varied by adjusting POT R2.

Design formulas.
V out = 2.77 [1+(R2/R1)]
Current limit (I sc)= 0.45/R3

Description.
Here is a simple scheme for adding a blown fuse indicator to your existing power supply circuit. This is done by just adding a resistor and LED to the existing circuit. The LED and resistor are connected in series and this combination is connected in parallel to the existing fuse. When the fuse is intact, it offers a low resistance and so the voltage drop across it will be not sufficient enough to glow the LED. When the fuse is blown off, it is equivalent to infinite resistance and so the entire power supply will drop across the resistor LED series combination and makes the LED glow. The resistance R1 is used to limit the current through LED.
The component values of other components are not given in the diagram because it depends on your specific power supply requirements. Any way for a conventional 12V power supply the component values are as follows: 1N4007 for D1,D2,D3 and D4; 230V primary,12V secondary ,2A step-down transformer for T1, 2A fuse and a 1000uF/25V capacitor for C1.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a general purpose PCB.
• The circuit will not work if the load is not connected.
• A highly resistive load may also impart the working of the circuit.
• The components for this circuit can easily obtained from your electronic junk box.

Description.
This is a simple electronic alarm circuit that imitates the chirping of a canary. The circuit is nothing but a Hartley oscillator with few more passive components added. As the capacitor C1 charges through the resistor R1 and the transistor Q1 is driven to cut off. This makes the oscillations to stop. As the capacitor discharges through the Resistor R1 and base emitter junction of the transistor the oscillation start again. Actually the R1 and C1 are the components that make the characteristic chirping sound.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a general purpose PCB.
• The circuit can be powered from a 9V PP3 battery.
• The transformer T1 can be a audio output transformer like LT700.
• If LT700 is not available, try the audio output transformer used on you old transistor radio board.
• The speaker can be an 8 ohm tweeter.
• Switch S1 can be a push button switch.
• The chirping sound can be altered by changing the value of R1 and C1.

Description.
Here is the circuit diagram of a simple 3 band graphic equalizer circuit using a single IC and few components. The IC used here is LF 351 which is a wide bandwidth single JFET operational amplifier. The high input impedance of the IC makes this circuit compatible with most of the audio signal sources. The opamp is wired as an inverting amplifier. The input signal is fed to the inverting input of the opamp via the filter network. The filter network can produce a +/- 20 dB enhancement or cut on the three frequency bands 50Hz,1KHz and 10KHz.POTs R1, R2 and R3 can be used for adjusting the gain of the different bands.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a general purpose PCB.
• The Circuit can be powered from anything between 6 to 30V DC.
• I am using 12V here.
• Increasing supply voltage will of course increase the gain.
• The IC must be mounted on a holder.
• The electrolytic capacitors must be rated higher than the supply voltage.