13-11-2012, 12:36 PM
Rain Detector
[attachment=39207]
Abstract
This rain detector will give you a heads-up the instant it starts to rain, hopefully giving you time to close windows and bring in possessions. The battery-powered circuit draws virtually no current when the sensor is dry and the current consumption is low when the buzzer is activated so a couple of AA cells will last a long time. Alternately, a molded power supply with a simple voltage regulator to drop the voltage to 3 volts could be used. The circuit is basically a handy flasher circuit that operates well on only 3 volts using ordinary silicon transistors. When the circuit is triggered, the buzzer is pulsed about once per second for a very short time, giving it a "dripping water" sound which seems appropriate. A slower, longer beep may be had by increasing the 1 uF capacitor. The 10 k resistor may be increased for a longer beep time without decreasing the beep rate but at some point the circuit will cease to function properly, depending on the gain of the transistors.
Parts Considerations:
• Just about any transistors will work but if you choose to use older, low gain transistors in metal cans just because they are so good looking (like I did), it might be best to try a 4.7 k and 2.2 uF in place of the 10 k and 1 uF. (I had no trouble with a 2N2222 and 2N2906 with the values shown but those are modern types.)
• The resistors are not critical at all and any type or size should work fine and reasonable close values are OK although the values shown are quite common.
• The 1uF capacitor may be just about any type, with a 16 volt aluminum electrolytic being the most likely choice. The capacitor will see about 1/2 volt reverse bias at times so a 10 volt or higher tantalum capacitor is a better choice; they can handle about 10% of their rating in reverse. A non-polar ceramic capacitor is also a good choice but don't hesitate to use a cheap electrolytic since there is a large resistor in series limiting the reverse current and this isn't exactly a deep space probe!
• The 0.1uF is not critical at all and my circuit works just fine without it. It is there in case the buzzer's noise tended to retrigger the circuit.
• The switch is any single-pole, single-throw type. My switch, grabbed from the surplus bin, has an unnecessary spring-loaded momentary position, too. I wired it to supply power to the circuit in both positions with the idea that the "test" position is for determining if the sensor has dried sufficiently to turn the circuit back on. Just an excuse to use the switch!
• The batteries are just AA alkaline cells with wires directly soldered to the ends for connections. Directly soldering to a battery is a delicate process and I recommend a battery holder for the less experienced solderer. You must make the joint very quickly or the battery will be damaged! Don't dwell on it!
• The buzzer is a 1.5 to 3 volt, 15 mA "mini buzzer" purchased at Radio Shack.
• The copper-plated nails are available at most hardware stores. Mine are 1.4 mm x 19 mm (3/4").
[attachment=39207]
Abstract
This rain detector will give you a heads-up the instant it starts to rain, hopefully giving you time to close windows and bring in possessions. The battery-powered circuit draws virtually no current when the sensor is dry and the current consumption is low when the buzzer is activated so a couple of AA cells will last a long time. Alternately, a molded power supply with a simple voltage regulator to drop the voltage to 3 volts could be used. The circuit is basically a handy flasher circuit that operates well on only 3 volts using ordinary silicon transistors. When the circuit is triggered, the buzzer is pulsed about once per second for a very short time, giving it a "dripping water" sound which seems appropriate. A slower, longer beep may be had by increasing the 1 uF capacitor. The 10 k resistor may be increased for a longer beep time without decreasing the beep rate but at some point the circuit will cease to function properly, depending on the gain of the transistors.
Parts Considerations:
• Just about any transistors will work but if you choose to use older, low gain transistors in metal cans just because they are so good looking (like I did), it might be best to try a 4.7 k and 2.2 uF in place of the 10 k and 1 uF. (I had no trouble with a 2N2222 and 2N2906 with the values shown but those are modern types.)
• The resistors are not critical at all and any type or size should work fine and reasonable close values are OK although the values shown are quite common.
• The 1uF capacitor may be just about any type, with a 16 volt aluminum electrolytic being the most likely choice. The capacitor will see about 1/2 volt reverse bias at times so a 10 volt or higher tantalum capacitor is a better choice; they can handle about 10% of their rating in reverse. A non-polar ceramic capacitor is also a good choice but don't hesitate to use a cheap electrolytic since there is a large resistor in series limiting the reverse current and this isn't exactly a deep space probe!
• The 0.1uF is not critical at all and my circuit works just fine without it. It is there in case the buzzer's noise tended to retrigger the circuit.
• The switch is any single-pole, single-throw type. My switch, grabbed from the surplus bin, has an unnecessary spring-loaded momentary position, too. I wired it to supply power to the circuit in both positions with the idea that the "test" position is for determining if the sensor has dried sufficiently to turn the circuit back on. Just an excuse to use the switch!
• The batteries are just AA alkaline cells with wires directly soldered to the ends for connections. Directly soldering to a battery is a delicate process and I recommend a battery holder for the less experienced solderer. You must make the joint very quickly or the battery will be damaged! Don't dwell on it!
• The buzzer is a 1.5 to 3 volt, 15 mA "mini buzzer" purchased at Radio Shack.
• The copper-plated nails are available at most hardware stores. Mine are 1.4 mm x 19 mm (3/4").