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Heat Sensor
Nov.24, 2010 in New Circuits Leave a Comment


Here is a simple circuit which can be used as a heat sensor. In the following circuit diagram thermistor and 100 Ohms resistance is connected in series and makes a potential devider circuit . If thermistor is of N.T.C (Negative temperature Coefficient ) type then after heating the thermistor its resistance decreases so more current flows through the thermistor and 100 Ohms resistance and we get more voltage at junction of thermistor and resistance. Suppose after heating 110 ohms thermistor its resistance value become 90 Ohms.then according to potential devider circuit the voltage across one resistor equals the ratio of that resistor’s value and the sum of resistances times the voltage across the series combination. This concept is so pervasive it has a name: voltage divider. The input-output relationship for this system, found in this particular case by voltage divider, takes the form of a ratio of the output voltage to the input voltage.
This output voltage is applied to a NPN transistor through a resistance. Emitter voltage is maintain at 4.7 volt with a help of Zener diode.
This voltage we will use as compare voltage. Transistor conducts when base voltage is greater than emitter voltage. Transistor conducts as it gets more than 4.7 base Voltage and circuit is completed through buzzer and it gives Sound.

Heat Sensor Circuit Diagram
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Circuit Easy

Welcome to CIRCUIT EASY schematic design. This site for Electronics Project, Hobbyist and Educationist. Get free Circuit Diagram and make your School Project yourself.

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This circuits uses very few component and gives melody sound. It uses 3 terminal IC UM66 and can be build small enough to be placed inside a greeting card and operated off a single 3V flat button cell.
There is not much to the circuit. The UM66 is connected to its supply and its output fed to a transistor for amplification. Any common speaker can be used or a “flat” piezoelectric tweeter like the one found in alarm wrist watches. If you use the piezo, then it can be connected directly between the output pin 1 and ground pin 3 without the transistor.
The UM66 looks like a transistor with 3 terminals. It is complete miniature tone generator with a tune. Now they come with wide variety of different tunes.
For amplification we have used a NPN transistor which is BC548. Here BC548 makes a common emitter circuit. For limiting the base current we have used a resistance of 220 Ohms so that transistor will not get damaged by excess current.

Circuit Diagram of Musical Bell
PROCEDURE :
1. Draw circuit diagram on ply board and make hole with compass or broader for component pin insertion.
2. Identify emitter base collector of transistor and pin no. of IC UM66
3. Solder all parts according to the circuit. You will need soldering iron, Soldering flux and flexible wire.
4. Make sure all points are well soldered according to the Circuit Diagram and no dry solders. Wrong connection of IC may heat up and get damage.
5. After loading battery power ON the circuit. Now you can check the function of the project.

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Light-sensitive Alarm Project

A kit for this project is available from RSH Electronics.

Download PDF version of this page

The circuit detects a sudden shadow falling on the light-sensor and sounds the bleeper when this happens. The circuit will not respond to gradual changes in brightness to avoid false alarms. The bleeper sounds for only a short time to prevent the battery running flat. Normal lighting can be used, but the circuit will work best if a beam of light is arranged to fall on the light-sensor. Breaking this beam will then cause the bleeper to sound. The light sensor is an LDR (light-dependant resistor), this has a low resistance in bright light and a high resistance in dim light.

* The light-sensitivity of the circuit can be adjusted by varying the 100k preset.
* The length of bleep can be varied from 0.5 to 10 seconds using the 1M preset.

Using the 7555 low-power timer ensures that the circuit draws very little current (about 0.5mA) except for the short times when the bleeper is sounding (this uses about 7mA). If the circuit is switched on continuously an alkaline PP3 9V battery should last about a month, but for longer life (about 6 months) you can use a pack of 6 AA alkaline batteries.

This project uses an edge-triggered 555 monostable circuit.
Parts Required

* resistors: 10k, 47k, 1M ×3
* presets: 100k, 1M
* capacitors: 0.01µF, 0.1µF, 10µF 25V radial
* transistor: BC108 (or equivalent)
* 7555 low-power timer IC
* 8-pin DIL socket for IC
* LDR (light-dependant resistor) type ORP12
* bleeper 9-12V
* on/off switch
* battery clip for 9V PP3
* stripboard 12 rows × 25 holes

Stripboard Layout
Stripboard layout for light-sensitive alarm
Circuit diagram
Circuit diagram for light-sensitive alarm

Click here for RSH Electronics A kit for this project is available from RSH Electronics. If you are new to electronics buying a kit is a good way to be sure you have the correct parts for the project.
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