I have already explained in detail the working of a rheostat. To know more about the component click on the link below.

TAKE A LOOK : POTENTIOMETER AND RHEOSTAT – WORKING AND COMPARISON

Although the below explained process is not applicable for any electronic circuits, you will clearly get a clear idea on how a rheostat works.

Components

1. The components needed for the connection are
2. Flashlight bulb and socket [1]
3. Dry cell lantern battery/D-cell battery [2]
4. Wire [About 15 to 17 inches and another one 2 inches]
5. Spring [1]
6. Wire Clippers [A pair]
7. A typical spring can be obtained from a widow roll up. You can even get to buy one at a cheap rate.

Procedure

• Connect the two Dry cell lantern/D-cell batteries tail-to-tail, so that the positive polarity of one battery is connected to the negative polarity of the other.
• Using a wire cutter, cut the wire in equal lengths. One wire should be at least 8 centimetres long.
• Connect the wires onto the open ends of both the batteries.
• The end of one wire must be connected to the bulb socket with the bulb in it.
• Connect the second wire to one end of the long spring.
• Connect the free end of one wire to one terminal of the light socket.
• Connect the other free wire to one end of the spring.
• Take the two inch wire and connect it to the second terminal of bulb socket.
• Connect the other end of the two inch wire onto the other end of the spring.

How to make a Rheostat

What happens?

As soon as the circuit is in closed loop, the bulb begins to glow. Although the glow intensity is less, when you move the wire through the spring onto the other end where the wire is connected, the bulb starts to glow more brightly. When both the wires are nearby the glow will be in its maximum.

The spring is mainly made of steel wire. Steel wires are not very good conductors of electricity. Thus the resistance of the circuit also increases. If the spring length is long enough you will get to see different stages of the glow. Thus you will get to see the working of a rheostat.

Description.
This is a simple LED torch circuit based on IC MAX660 from MAXIM semiconductors. The MAX 660 is a CMOS type monolithic type voltage converter IC. The IC can easily drive three extra bright white LEDs.The LEDs are connected in parallel to the output pin 8 of the IC. The circuit has good battery life. The switch S1 can be a push to ON switch.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a general purpose PCB.
• The IC must be mounted on a holder.
• The circuit can be powered from two torch cells connected in series.
• The capacitors C1 and C2 must be Tantalum type.
• The diodes D1 to D3 must be of 1N4148.

Description.

Here is the circuit diagram of an ultrasonic mosquito repeller.The circuit is based on the theory that insects like mosquito can be repelled by using sound frequencies in the ultrasonic (above 20KHz) range.The circuit is nothing but a PLL IC CMOS 4047 wired as an oscillator working at 22KHz.A complementary symmetry amplifier consisting of four transistor is used to amplify the sound.The piezo buzzer converts the output of amplifier to ultrasonic sound that can be heard by the insects.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a general purpose PCB.
• The circuit can be powered from 12V DC.
• The buzzer can be any general purpose piezo buzzer.
• The IC1 must be mounted on a holder.

Description.
Here is a simple heater circuit that can be operated from a 12V battery. The first part of the circuit is an astable multivibrator build around the two transistors Q1 and Q2 .The ON time of transistor Q2 is set to 0.5 S. The ON time of transistor Q1 can be varied by using the POT R2.The output pulses at the collector of Q2 is used to drive the Darlington power transistor Q3(TIP122).The transistor Q3 drives the heating elements L1 to L3.The net heat produced can be varied by  selecting the desired combination of heating elements at the output circuit sing switches S1 and S2.The net heat can be also varied by varying the duty cycle of the triggering pulse using POT R2.

Circuit diagram with Parts list.

Notes.

• The circuit can be assembled on a general purpose PCB.
• The transistor Q3 must be fitted with a heat sink.
• The elements L1 to L3 can be 10W heating coils.
• The switches S1 and S2 must be able to withstand at least 5A.
• The circuit can be powered from a 12V battery.
• The LED D1 gives a visual indication of the duty cycle of the circuit.

Description.

Here is a simple circuit that will give a visual indication when the soil water level inside your flower pot goes low below a certain limit.

The U1C and associated components are wired as an oscillator producing a 2KHz square wave. This square wave is given to one gate input of U1D via a variable potential divider former by R1 and R2.When the resistance across the probes A and B are low that is when soil moisture level is high, the C2 will divert the square wave to ground. The output of U1D will be high. The U1 A inverts this high state to low and so the IC U1B is blocked from producing oscillations. The LED will remain OFF. When there is no moisture across the probes, the C2 cannot bypass the 2KHz signal to the ground and it appears at the gate input of U1D.The output of U1D goes low, and it is inverted to high by U1A.The oscillator wired around U1B is activated and it starts oscillating. These oscillations are amplified by Q1 to drive the LED and LED starts pulsating as an indication of low moisture. Since square wave is used there won’t be any oxidation on the probes. The resistor R7 limits the current through LED and ensures a longer battery life.

Circuit diagram with Parts list.

Notes.

• Power the circuit from a 3V battery.
• Two metal wires 10 cm long and 5cm apart driven into the soil will do the job for probes.
• The probes are to be connected at the terminals A and B shown in circuit.
• Capacitors C1 and C2 must be polyester type.
• The IC U1 is a quad two input Schmitt NAND IC 4093.
• The sensitivity can be adjusted by varying the preset R2.
• Mount the IC on a holder.