Rain Alarm : Introduction
Rain Alarm is a device which is used to give the information is the rain is occurring as it gives sound when rain falls between the two aluminium rods. As the rain drop comes between the two rods the circuit gets completed and a sound is produced from the speaker indicating the falling of rain. It can be used in factories, open godowns where the goods need to be protected from rain. It can also be used as a water level indicator by fixing the two rods at desired level when water will reach that level it will automatically produce sound.
Working of Rain Alarm :
The aluminium tubes (rain sensors) are fixed on plastic plate with a small distance between them. The plastic plate is immersed in water taken in a beaker. Presence of water between the aluminium tubes effects circuit adjustment.
When switch P is closed current flows through the speaker and an audible note is produced. The alarm is on.
When water is removed from the beaker or the tubes are taken out of water, alarm becomes dead.
When a rain alarm is set up according to circuit diagram and is connected to input A-C source then its working starts.
Since when there is no water droplet complete and speaker doesn’t produce sound. But as soon as rain starts water drop comes between the two rods the circuit gets completed and speaker starts giving sound.
Construction of Rain Alarm :
It consists of various components named below :
(i) n-p-n transistor (BC 147)
(ii) p-n-p transistor (2N 6107)
(iii) Resistors (1.5 kW, 330 kW)—0.25 W
(iv) Capacitor ceramic (0.01 mF)
(v) Two rain sensors (aluminium tube of diameter 5 mm anbd length 6 to 7 cm)
(vi) Speaker (5 to 8 W)
(vii) Battery (6 V)
(viii) Switch.
Value of Resistances and their percentage accuracy are indicated on carbon resistance by a colour code printed on them.
| Colour | Number | Multiplier | Colour | Tolerance |
| Black | 0 | 100 | ||
| Gold | 8% | |||
| Brown | 1 | 101 | ||
| Silver | 10% | |||
| Red | 2 | 102 | ||
| No Colour | 20% | |||
| Orange | 3 | 103 | ||
| Yellow | 4 | 104 | ||
| Green | 5 | 105 | ||
| Blue | 6 | 106 | ||
| Violet | 7 | 107 | ||
| Grey | 8 | 108 | ||
| White | 9 | 109 |
In Series R = R1 + R2 + R3 .........
In Parallel : 1/R = 1/R1 + 1/R2 + 1/R3 .........