Description.
The circuit diagram shown here is of a Hall Effect switch. Hall Effect sensor IC DN6848 from Panasonic is the heart of the circuit. The DN6848 has a built-in Hall Effect sensor, Schmitt trigger circuit, power supply regulator and temperature compensation circuits integrated to a single chip. High sensitivity, low drift and excellent temperature stability of the DN6848 makes it well suitable for a variety of position, rotation and speed sensing applications.
When the south pole of a magnet is brought near to the front face (face with type number marking) of the IC its output pin (pin 3) goes low (zero voltage).The pin 3 is connected to the base of PNP transistor Q1 and the low stage at pin3 makes the transistor ON and makes the relay activated. In a nut shell the relay will get switched ON whenever the south pole of a magnet is brought close to the Hall Effect sensor IC DN6848. Resistor R1 will always pull the base of Q1 to a very slight positive voltage and thereby prevents accidental triggering. LED D1 glows whenever the relay is activated and resistor R2 limits the current through the D1. Diode D2 is a freewheeling diode and it protects the transistor Q1 from high voltage spikes which are induced when the relay is switched.
Circuit diagram.

Notes.

• The circuit can be assembled on a perf board.
• The relay K1 can be a 5V SPDT relay.
• Use 6V DC for powering the circuit.
• The wattage of the device that can be switched depends on the capacity of the relay.

Description.

Here is the circuit diagram of a fuse that has an automatic status indicator. This circuit can be added to circuit that operates from 12V DC. As long as the fuse is intact, the LED D3 will glow continuously and when the fuse blows off the LED will start and continue blinking.
The first part of the circuit includes an astable multivibrator built around transistors Q1 and Q2.The output of the multivibrator is coupled to the base of Q3 via diode D2.When the fuse is intact the base of Q3 will be pulled to a positive voltage by the resistor R5. The transistor will be ON and the LED D3 remains glowing. When the fuse of blown off, the base of Q3 will no longer be pulled to positive voltage and now the only biasing available at the base of Q3 will be the output from the astable multivibrator. Now the transistor Q3 will start switching in the frequency of astable multivibrator and the LED will blink in accordance.

Circuit diagram with Parts list.

Notes.

• The circuit can be assembled on a Vero board.
• This fuse circuit can be incorporated with any circuit that operates from 12V DC.
• The type no of the transistors are not very significant and almost any NPN transistors will do the job.
• The fuse F1 can be a metal wire fuse with your required rating.

Description.
A simple touch switch circuit using CD4011 is given here. The IC CD4011 is wires as a flip flop here. The 9, 13 pins of the IC works as the set and reset contacts respectively.CMOS ICs like 4011 require requires a very low current for controlling its gates. Since the pins 9 and 13 are connected to the positive via resistors R1 and R2, the logic gates of the ICs will be in high state. When we touch through the points A, B the gates of the IC will be closed and the output becomes low. This switches ON the transistor Q1 and the relay gets activated. When we touch through the points C, D the gates again becomes high and switches the transistor OFF. This makes the relay OFF. Thus by touching through the contact points A,B and C,D the appliance connected through the relay can be switched On and OFF.

Circuit diagram with Parts list.

Notes.

• The circuit can be assembled on a general purpose PCB.
• The circuit can be powered from 12V DC.
• The IC must be mounted on a holder.
• The relay can be a 12 V, 200 Ohm SPDT relay.

Description.
This is a very simple yet effective circuit in which the equipments connected at the so called trailing sockets will run only if the equipment connected at the control socket is switched on. For example, let’s connect a motor is connected to the control socket and a lamp is connected at the trailing socket. The lamp will glow only when the motor is running.

When the load connected at the control circuit is switched on, the load current flows through the diodes and as a result there will a voltage drop across the diodes. This voltage drop is sufficient enough to switch on the sensitive triac T1 and the equipments connected at the trailing sockets gets power supply. The components R2 and C1 forms a snubber circuit which protects the triac from transient fluctuations.

Almost all equipments like motors, drills, blenders, fan, old TV, radio, amplifiers etc can be connected at the control socket. In case of modern TV, computers, amplifiers, etc the power switch does not completely isolate the equipment and the equipment will draw a small amount of current in the standby mode which is sufficient enough to trigger the triac . Such equipments cannot be used on the control socket because it makes the trailing equipments ON even if the control equipment is OFF.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a good quality PCB.
• The triac T1 must be a 600V, 8A, high sensitive gate type like TIC 225M.
• Maxim load that can be connected at the trailing socket is 1000W.
• Fit the triac with a heat sink.
• Take at most care while handling this circuit as it is connected to 230V AC.

Description.

Here given is a circuit in which the status of the output pins of a Flip Flop IC can be toggled by using sound. A condenser microphone is used for picking up the sound. The first two opamps in the IC1 LM 324 is used to amplify the sound picked by the condenser microphone. The third opamp inside LM 324 is wired as a level detector. When ever the voltage produced due to sound have a level more than that of the reference voltage at pin 5 of the third opamp, its output (pin 7) goes high, triggering the flip flop IC1 CD 4027.As a result the state of the output pins of CD 4027 ( pin 1 & pin 2) toggles for each burst of sound

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a good quality PCB.
• The circuit can be powered from three 1.5 V cells in series.
• The ICs must be mounted on holders.
• All capacitors must be rated 10V.
• The mic M1 is a condenser mic.
• The sensitivity of the circuit can be adjusted by varying the preset R9.