11-05-2012, 03:44 PM
DIGITAL THERMOMETER
DIGITAL THERMOMETER.docx (Size: 261.39 KB / Downloads: 95)
Digital thermometer can measure temperatures up to 150c with an accuracy of +1c.temparature is read on a 1volt full scale deflection moving coil voltmeter or digital voltmeter.
Working
Operational amplifier IC741(IC3) provides a constant flow of current through the base emitter junction of npn transistor BC 108(T1). The voltage across base emitter junction of the transistor Is proportional to its temperature. the transistor used this way makes a low cost sensor. Silicon diode can be used instead of the transistor. The small variation in voltage across the base emitter junction is amplified by the second operational amplifier (IC4), before the temperature is displayed on the meter. Preset VR1 is used to set zero reading on the meter and preset VR2 Is used to set the range of temperature measurements.
Operational amplifies IC3 &IC4 operate off regulated +5v power supply, which is derived from three terminal positive voltage regulator IC 7805 (IC1) and negative low drop out regulator IC 7660(IC2).the entire circuit works off a 9v battery.
Block diagram
A VERY SIMPLE POWER FAILURE LIGHT
This is a very basic power failure lighting circuit based around a relay. This simple circuit has many uses, from lighting up rooms and walkways in the case of a power failure, to monitoring and security uses. There are many different power failure circuits out there based on 555timers or transistors but they all have different problems including limited input voltage, price and complexity, and poor backup power. This unit has been designed to work with mains power all the way down to 5 volts, and power 3 LEDs to provide light for a hallway or a child’s room in the event of power failure. The PCB includes many simple add-ons and modifications too.
Specifications
Input Voltage Max: 240 Volts AC
Input Voltage Min: 5 Volts DC
Approximant Power Consumption: 450 mW
WATER LEVEL ALARM CIRCUIT
Here is a simple water level alarm circuit that will produce an audible alarm when the water level reaches a preset level. The circuit can be powered of a 3V Battery and is very handy to use.
The circuit is based on an astable multivibrator wired around IC1 (NE 555).The operating frequency of the astable multivibrator here will depend on capacitor C1, Resistances R1, R2 and the resistance across the probes A&B.When there is no water up to the probes, they will be open and so the multivibrator will not produce oscillations and the buzzer will not beep. When there is water up to the level of probes, some current will pass through the water; the circuit will be closed to some extent, and the IC will start producing oscillations in a frequency proportional to the value of C1, R1, R2 and the resistance of water across the probes. The buzzer will beep to indicate the presence of water up to the level of the sensing probes.