Description.
Here is the schematic of a very simple electronic thermostat using IC LM56. LM56 is a very accurate dual output low power thermostat from National Semiconductors.LM56 has various useful features like internal temperature sensor, two internal voltage comparators, internal voltage reference etc. Two stable temperature trip points (VT1 and VT2) are created by dividing the LM56’s 1.250V internal voltage reference using 3 external resistors (R1, R2 and R3). There are two digital outputs for LM56 .Output1 becomes LOW when the temperature increases above T1 and goes HIGH when the temperature decreases below (T1±Hysteresis Temperature). In the same way, Output2 becomes LOW when the temperature goes above T2 and goes HIGH when the temperature goes below (T2±Hysteresis Temperature).
By connecting a cooler as the load for relay L1 and a heater as the load for relay L2, a very simple and useful temperature control system can be constructed.

The values of R1, R2 and R3 for the required trip points VT1 and VT2 can be determined using the following equations.
VT1 = 1.250V x (R1)/ (R1 + R2 + R3)
VT2 = 1.250V x (R1 + R2)/ (R1 + R2 + R3)
where:
(R1 + R2 + R3) = 27 k Ohms and
VT1 or T2 = [6.20 mV/degree Celsius x T] = 395 mV therefore:
R1 = VT1/ (1.25V) x 27 k Ohms
R2 = (VT2/ (1.25V) x 27 k Ohms) – R1
R3 = 27 k Ohms – R1 – R2

Circuit diagram with Parts list.

Notes.

• The circuit can be assembled on a Vero board.
• Use 5V DC for powering the circuit.
• L1 and L2 can be 5V relays.Their current ratings must be selected according to the load they switch.
• Do not give more than 10V to LM56.

Description.

Just about any power transistor can be used in  this megaphone.Its suitable for boats,playing fields etc.the transistors Q1  & Q2 are the HEP-230 type which are easily available in the market.The transistors are parallely connected to handle the required power and speaker matching.The microphone is a carbon type like that used in telephone hand sets.If  a regular carbon mike is used the push-to-talk (PTT) switch can be connected in place of S1 to provide PTT operation.There will be no warm up or “capacitor charge time.

Circuit diagram with Parts list.

Notes. 

• The circuit can be powered from a 12V battery or 12V DC power supply.
•  The POT R1 can be adjusted to obtain maximum volume with minimum distortion.
• It will be always better to mount the transistors on heat sink.

Description.

This is a compact electronic siren circuit based on three transistors.This circuit is suitable for in corporating with other alarm or siren projects such as burglar alarms, automatic factory sirens etc or a simple push to on alarm.

The  electronic siren circuit given here  is  based on a complementary transistor pair consisting of Q2 & Q3 (BC557 & BC 37)  wired as an astable multivibrator oscillator,which directly drives the speaker.The transistor Q1 is used to provide a full charge on capacitor C2 when power is turned ON. When push button switch S1 is pressed , the capacitor C2 slowly discharges through resistor R8.This makes the circuit to  oscillate at a low frequency that increases to a high frequency and kept indefinitely as the capacitor is fully discharged. When the switch P1 is released, the output  frequency decreases slowly as C2 is charged to the  positive voltage through resistance R6 and the Base-Emitter junction of tramsistor Q2. When C2 is fully charged to the positive battery voltage the  circuit stops oscillating.

Circuit diagram with Parts list.

Notes.

• A 12 V battery or a a well regulated 12V DC power supply can be used to power the circuit.
• Assemble the circuit on a good quality PCB or common board.
• The switch S1 can be used to activate the alarm.
• The switch S2 can be used as a power switch.
• You can experiment on the tone of alarm by using different values for C2 and R8.

Description.

This circuit can give a visual indication of the rate of air flow.It can be also used to check whether there is air flow in a given space.

The filament of a incandescent bulb is the sensing part of the circuit.When there is no air flow the resistance of the filament will be low.When there is air flow the resistance drops , because the moving air will remove some of the heat generated in the filament.This variations in the resistance will produce variation of voltage across the filament.These variations will be picked up by the opamp (LM339) and the brightness of the LED at its output will be varied proportionally to the airflow.

Circuit diagram with Parts list.

Notes.

• The filament L1 can be made by removing  the glass of a 40W incandescent bulb.
• The circuit can be powered from a 12 V DC power supply.

Description.

Here is the circuit diagram of a magnetic proximity switch that finds a lot of applications in many fields.The circuit is based on a magnetic reed switch(S1) as the proximity sensor. A monostable multivibrator based on NE555 (IC1) and a toggle flip flop based on CD4013 (IC2) does the rest of the circuit.

When a magnet is reached in proximity of S1 it closes to give a negative trigger at pin 2 of IC1.The output of IC1 goes high for a time determines by R2 and C2.This clocks the IC2 wired as a toggle flip flop.The output (pin 1 ) of IC2 goes high and the transistor Q1 is biased to ON.The relay is activated and so do the equipment connected to the relay.The LED D1 glows when IC1 is triggered.

Circuit diagram with Parts list.

Notes.

• Switch S1 can be a general purpose magnetic reed switch.
• The equipment to control can be connected using NC,NO and C points of the relay according to the application.
• Use a 12 regulated power supply for powering the circuit.