Simple Room Temperature Monitor Circuit

A very simple yet highly precise air temperature sensor gauge circuit has been presented here. The use of the highly versatile and accurate IC LM 308 makes the circuit respond and react superbly to the smallest temperature changes happening over its surrounding atmosphere. Diode 1N4148 is used as an active ambient temperature sensor here.

The electronic air temperature sensor gauge circuit presented here is very accurate in its function, categorically due to its minimum level of hysteresis. Complete circuit description and construction clues included herein.

Circuit Description

The present circuit of an electronic air temperature sensor gauge circuit is outstandingly accurate and can be very effectively used to monitor the atmospheric temperature variations. Let’s briefly study its circuit functioning:

Here as usual we use the very versatile “garden diode” 1N4148 as the sensor due to its typical drawback (or rather an advantage for the present case) of changing its conduction characteristic in the influence of a varying ambient temperature. The diode 1N4148 is comfortably able to produce a linear and an exponential voltage drop across itself in response to a corresponding increase in the ambient temperature. This voltage drop is around 2mV for every degree rise in temperature. This particular feature of 1N4148 is extensively exploited in many low range temperature sensor circuits.

Referring to the  figure we see that, IC1 is wired as an inverting amplifier and forms the heart of the circuit.

Its non inverting pin # 3 is held at a particular fixed reference voltage with the help of Z1, R4, P1 and R6.

Transistor T1 and T2 are used as a constant current source and helps in maintaining higher accuracy of the circuit.

The inverting input of the IC is connected to the sensor and monitors even the slightest change in the voltage variation across the sensor diode D1. These voltage variations as explained, is directly proportional to the changes in the ambient temperature.

The sensed temperature variation is instantly amplified into a corresponding voltage level by the IC and is received at its output pin #6.

The relevant readings are directly translated into degree Celsius through a 0-1V FSD moving coil type meter.

Parts List

R1, R4 = 12K,

R2 = 100E,

R3 = 1M,

R5 = 91K,

R6 = 510K,

P1 = 10K PRESET,

P2 = 100K PRESET,

C1 = 33pF,

C2, C3 = 0.0033uF,

T1, T2 = BC 557,

Z1= 4.7 V, 400mW,

D1 = 1N4148,

IC1 = LM308,

General Purpose Board as per size.

B1 and B2 = 9V PP3 battery.

M1 = 0 – 1 V, FSD moving coil type voltmeter

Setting Up the Circuit

The procedure is a bit critical and requires special attention. To complete the procedure you will need two accurately known temperature sources (hot and cold) and an accurate mercury-in-glass thermometer.

The calibration may be completed through the following points:

Initially keep the presets set at their midways. Connect a voltmeter (1 V FSD) at the output of the circuit.

For the cold temperature source, water at about room temperature is used here.

Dip the sensor and the glass thermometer into the water and record the temperature in the glass thermometer and the equivalent voltage outcome in the voltmeter.

Take a bowl of oil, heat it to about 100 degrees Celsius and wait until its temperature stabilizes down to about 80 degrees Celsius.

As above, immerse the two sensors and compare them with the above result. The voltage reading should be equal to the temperature change in the glass thermometer times 10 mill volt. Didn’t get it? Well, let’s read the following example.

Suppose, the cold temperature source water is at 25 degrees Celsius (room temperature), the hot source, as we know is at 80 degrees Celsius. Thus, the difference or the temperature change between them is equal to 55 degrees Celsius. Therefore the difference in the voltage readings should be 55 multiplied by 10 = 550 mill volts, or 0.55 volts.

If you don’t quite get the criterion satisfied, adjust P2 and continue to repeat the steps, until finally you achieve it.

Once the above rate of change (10 mV per 1 degree Celsius) is set, just adjust P1 so that the meter shows 0.25 volts at 25 degrees (sensor held in water at room temperature).

Night alert

    Idea of this circuit came to me at midnight when my pet dog started barking continuously on sensing a moving shadow, perhaps that of an intruder. Dogs have a night adaptation capability to maximize the sensitivity of vision in low light. They are well adapted to see moving objects rather than stationary ones in darkness. 

    This circuit turns a lamp ‘on’ for a short duration when the dog barks, giving an impression that the occupants have been alerted. 

The condenser microphone fitted in the dog’s cage senses barking sound and generates AC signals, which pass through DC blocking capacitor C1 to the base of transistor BC549 (T1). Transistor T1 along with transistor T2 amplifies the sound signals and provides current pulses from the collector of T2. 

The input trigger pulse is applied to the collector of transistor T3 and coupled by capacitor C3 to the base of transistor T4 causing T4 to cut off. The collector voltage of transistor T4 forward biases transistor T3 via resistor R8. Transistor T1 conducts and capacitor C3 discharges to keep transistor T4 cut-off. Transistor T4 remains cut-off until capacitor C3 charges enough to enable it to conduct. 

    When transistor T4 conducts, its collector voltage goes low to drive transistor T3 into cut-off state. Resistor R9 andcapacitor C3 are timing components. When fully charged, capacitor C3 takes about two minutes to discharge. So when sound is produced in front of the condenser mic, TRiAC1 (BT136) fires and the bulb (B1) glows for about two minutes. 

    Assemble the circuit on a general-purpose PCB and enclose in a plastic cabinet. Power to the circuit can be derived from a 12V, 500mA step-down transformer with rectifier and smoothing capacitor. Solder the triac ensuring sufficient spacing between the pins to avoid short circuit. Fix the unit in the dog’s cage, with the lamp inside or outside as desired. Connect the microphone to the circuit using a short length of shielded wire. 

Enclose the microphone in a tube to increase its sensitivity. 

    Caution. Since the circuit uses 230V AC, many of its points are at AC mains voltage. it could give you lethal shock if you are not careful. So if you don’t know much about working with line voltages, do not attempt to construct this circuit. EFY will not be responsible for any kind of resulting loss or damage. 

Car Anti-Theft Guard

 

Fig. 1: Circuit of car anti-theft guard

Fig. 2: Wiring diagram for door switch (S1) 

    Here is an easy-to-build car anti-theft guard. The circuit, shown in Fig. 1, is simple and easy to understand.When key-operated switch S2 of the car is turned on, 12V DC supply from the car battery is extended to the entire circuit through polarity-guard diode D5. Blinking LED1 flashes to indicate that the guard circuit is enabled. It works off 12V power supply along with current-limiting resistor R4 in series. 

    When the car door is closed, door switch S1 is in ‘on’ position and 12V power supply is available across

resistor R1, which prevents transistor T1 from conducting. In this position, anti theft guard circuit is in sleep mode. When someone opens the car door, switch S1 becomes ‘off’ as shown in Fig. 2. As a result, transistor T1 conducts to fire relay -driver SCR1 (BT169) after a short delay introduced by capacitor C1. Electromagnetic relay RL1 energizes and its N/O contact connects the power supply to piezobuzzer PZ1, which starts sounding to indicate that someone is trying to steal your car. To reset the circuit, turn off switch S2 using car key. This will cutoff the power supply to the circuit and stop the buzzer sound. 

    Assemble the circuit on a general-purpose PCB and house in a small box. Connect switch S1 to the car door and keep piezobuzzer PZ1 at an appropriate place in the car. • 

AUTOMATIC LOW-POWER EMERGENCY LIGHT

Here is a white-LED-based emergency light that offers the following advantages: 

1. It is highly bright due to the use of white LEDs. 

2. The light turns on automatically when mains supply fails, and turns off when mains power resumes. 

3. It has its own battery charger. 

When the battery is fully charged, charging stops automatically. The circuit comprises two sections: charger power supply and LED driver.The charger power supply section is built around 3-terminal adjustable regulator IC LM317 (IC1), while the LED driver section is built around transistor BD140 (T2). 

In the charger power supply section, input AC mains is stepped down by transformer X1 to deliver 9V, 500 

mA to the bridge rectifier, which comprises diodes D1 through D4. Filter capacitor C1 eliminates ripples. Unregulated DC voltage is fed to input pin 3 of IC1 and provides charging current through diode D5 and limiting resistor R16. By adjusting preset VR1, the output voltage can be adjusted to deliver the required charging current. When the battery gets charged to 6.8V, zener diode ZD1 conducts and charging current from regulator IC1 finds a path through transistor T1 to ground and it stops charging of the battery. The LED driver section uses a total of twelve 10mm white LEDs. All the LEDs are connected in parallel with a 100-ohm resistor in series with each. The common-anode junction of all the twelve LEDs is connected to the collector of pnp transistor T2 and the emitter of transistor T2 is directly connected to the positive terminal of 

6V battery. The unregulated DC voltage, produced at the cathode junction of diodes D1 and D3, is fed to the base of transistor T2 through a 1kilo-ohm resistor. 

When mains power is available, the base of transistor T2 remains high and T2 does not conduct. Thus LEDs are off. On the other hand, when mains fails, the base of transistor T2 becomes low and it conducts. This makes all the LEDs (LED1 through LED12) glow. The mains power supply, when available, charges the battery and keeps the LEDs off as transistor T2 remains cut-off. During mains failure, the charging section

stops working and the battery supply makes the LEDs glow. Assemble the circuit on a general-purpose PCB and enclose in a cabinet with enough space for battery and switches. Mount the LEDs on the cabinet such that they light up the room. A hole in the cabinet should be drilled to connect 230V AC input for the primary of the transformer. 

50 Watt Small Homemade Inverter

A 50 watt inverter might look quite trivial, but it can serve some useful purposes to you. When outdoors, this small power house can be used for operating small electronic gadgets, soldering iron, table top radios, incandescent lights, fans etc.

Let’s learn how to build this homemade 50 watt inverter unit, beginning with a brief description regarding the circuit diagram and its functioning:

Circuit Description

The circuit may be understood with the following points:

Referring to the figure, transistors T1 and T2 along with the other R1, R2, R3 R4, C1 and C2 together form a simple astable multivibrator (AMV) circuit. A multivibrator circuit basically is composed of two symmetrical half stages, here its formed by the left and the right hand side transistor stages which conduct in tandem or in simple words the left and the right stages conduct alternately in a kind of a perpetual “motion”, generating a continuous flip flop action.

The above action is responsible of creating the required oscillations for our inverter circuit. The frequency of the oscillation is directly proportional to the values of the capacitors or/and the resistors at the base of each transistor.

Lowering the values of the capacitors increases the frequency while increasing the values of the resistors decreases the frequency and vice versa. Here the values are chosen so as to produce a stable frequency of 50 Hz.

Readers, who wish to alter the frequency to 60 Hz, may easily do it by just changing the capacitor values appropriately.

Transistors T2 and T3 are placed at the two output arms of the AMV circuit. These are high gain; high current Darlington paired transistors, used as the output devices for the present configuration.

The frequency from the AMV is fed to the base of T2 and T3 alternately which in turn switch the transformer secondary winding, dumping the entire battery power in the transformer winding.

This results in a fast magnetic induction switching across the transformer windings, resulting the required the mains voltage at the output of the transformer.

Parts Required

You will require the following components for making this 50 watt homemade inverter circuit:

R1, R2 = 100K,

R3, R4 = 330 Ohms,

R5, R6 = 470 Ohms, 2 Watt,
R7, R8 = 22 Ohms, 5 Watt

C1, C2 = 0.22 uF, Ceramic Disc,
D1, D2 = 1N5402 or 1N5408

T1, T2 = 8050,

T3, T4 = BC316,
T5, T6 = 2N3055 (TO-220)

General purpose PCB = cut into the desired size, approximately 5 by 4 inches should suffice.

Battery: 12 volts, Current not less than 10 AH.

Transformer = 9 – 0 – 9 volts, 5 Amps, Output winding may be 220 V or 120 volts as per your country specifications

Sundries: Metallic box, fuse holder, connecting cords, sockets etc

Testing and Setting Up the Circuit

After you finish making the above explained inverter circuit, you may do the testing of the unit in the following manner:

Initially do not connect the transformer or battery to the circuit.

Using a small DC power supply power the circuit.

If everything is done rightly, the circuit should start oscillating at the rated frequency of 50 Hz.

You can check this by connecting the prods of a frequency meter across T3’s or T4’s collector and the ground. The positive of the prod should go to the collector of the transistor.

If you don’t own a frequency meter, never mind, you do a rough checking by connecting a headphone pin across the above explained terminals of the circuit. If you hear a loud humming sound, will prove that your circuit is generating the required frequency output.

Now it’s time to integrate the battery and the transformer to the above circuit.

Connect everything as shown in the figure.

Connect a 40 watt incandescent lamp  at the output of the transformer. And switch ON the battery to the circuit.

The bulb will immediately come ON brightly…..your homemade 50 watt inverrer is ready and may be used as desired by for powering many small appliances whenever required.

Long range FM transmitter

The power outputs of most of these circuits are very low because no power amplifier stages were incorporated. 
The transmitter circuit described here has an extra RF power amplifier stage, after the oscillator stage, to raise the power output to 200-250 milliwatts. With a good matching 50-ohm ground plane antenna or multi-element Yagi antenna, this transmitter can provide reasonably good signal strength up to a distance of about 2 kilometres.
The circuit built around transistor T1 (BF494) is a basic low-power variable-frequency VHF oscillator. A varicap diode circuit is included to change the frequency of the transmitter and to provide frequency modulation by audio signals. The output of the oscillator is about 50 milliwatts. Transistor T2 (2N3866) forms a VHF-class A power amplifier. It boosts the oscillator signals� power four to five times. Thus, 200-250 milliwatts of power is generated at the collector of transistor T2.
For better results, assemble the circuit on a good-quality glass epoxy board and house the transmitter inside an aluminium case. Shield the oscillator stage using an aluminium sheet.
Coil winding details are given below:
L1 - 4 turns of 20 SWG wire close wound over 8mm diameter plastic former.
L2 - 2 turns of 24 SWG wire near top end of L1.
(Note: No core (i.e. air core) is used for the above coils)
L3 - 7 turns of 24 SWG wire close wound with 4mm diameter air core.
L4 - 7 turns of 24 SWG wire-wound on a ferrite bead (as choke)
Potentiometer VR1 is used to vary the fundamental frequency whereas potentiometer VR2 is used as power control. For hum-free operation, operate the transmitter on a 12V rechargeable battery pack of 10 x 1.2-volt Ni-Cd cells. Transistor T2 must be mounted on a heat sink. Do not switch on the transmitter without a matching antenna. Adjust both trimmers (VC1 and VC2) for maximum transmission power. Adjust potentiometer VR1 to set the fundamental frequency near 100 MHz.
This transmitter should only be used for educational purposes. Regular transmission using such a transmitter without a licence is illegal in India