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The circuit uses a 555 timer wired as an astable oscillator and powered by the emitter current of the BC109C. Under dry conditions, the transistor will have no bias current and be fully off. As the probes get wet, a small current flows between base and emitter and the transistor switches on. A larger current flows in the collector circuit enabling the 555 osillator to sound. Simple Water Activated Alarm  Circuit diagram : An On/Off switch is provided and remember to use a non-reactive metal for the probe contacts. Gold or silver plated contacts from an old relay may be used, however a cheap alternative is to wire alternate copper strips from a piece of veroboard. These will eventually oxidize over but as very little current is flowing in the base circuit, the higher impedance caused by oxidization is not important. No base resistor is necessary as the transistor is in emitter follower, current limit being the impedance at the emitter (the oscillator circuit).

This simple Sound-Activated Lamp circuit shown int the schematic diagram actives the switch using sound. We can use this circuit for various applications, such as automatic (sound-controlled) disco light or car’s LED light show.  The Q1 amplify the audio from mic. The R1 is used to adjust the peak of signal to greater than about 0.7 volts, act as sensitivity adjuster. A certain level, the signal coming from microphone, after amplification by Q1, will trigger the SCR and light lamp I1. If we change the lamp with a relay, then we can get a sound-activated relay/switch, which can be used to control more powerful / high wattage high voltage lamps. If we use a relay, place a 1N4007 diode in parallel with the relay coil to prevent the back-emf from  relay coil destroying the SCR.  Sound-Activated Lamp Circuit Diagram

Description This device allows one or more lamps to illuminate at sunset and turn off at dawn.Q1 and Q2 form a trigger device for the SCR, providing short pulses at 100Hz frequency. Pulse duration is set by R2 and C1.When the light hits R1, the photo resistor assumes a very low resistance value, almost shorting C1 and preventing circuit operation. When R1 is in the dark, its resistance value becomes very high thus enabling circuit operation.  Circuit Diagram: Parts R1 = LDR R2 = 100K R3 = 200K R4 = 470R R5 = 12K R6 = 1K R7 = 470R C1 = 10nF-63V D1 = TIC106D D2 = 1N4007 D3 = 1N4007 D4 = 1N4007 D5 = 1N4007 Q1 = BD327 Q2 = BD337 SK1 = Female Mains Socket   Notes: R3 allows fine setting of operating threshold and R2 value can be raised to 150K maximum. Several lamps wired in parallel can be connected to the circuit, provided total power dissipation of the load does not exceed about 300 - 500W. PL1 can be omitted and the input mains supply wires connected in parallel to any swi...

Compact circuit, Can be wired in parallel to existing switches This device allows one or more lamps to illuminate at sunset and turn off at dawn.Q1 and Q2 form a trigger device for the SCR, providing short pulses at 100Hz frequency. Pulse duration is set by R2 and C1.When the light hits R1, the photo resistor assumes a very low resistance value, almost shorting C1 and preventing circuit operation. When R1 is in the dark, its resistance value becomes very high thus enabling circuit operation.   Circuit Diagram: Dark Activated 220 volt AC Lamp Circuit Diagram Parts: R1 = LDR R2 = 100K R3 = 200K R4 = 470R R5 = 12KR6 = 1KR7 = 470R C1 = 10nF-63V D1 = TIC106D D2 = 1N4007D3 = 1N4007D4 = 1N4007 D5 = 1N4007 Q1 = BD327 Q2 = BD337 SK1 = Female Mains Socket   Notes: R3 allows fine setting of operating threshold and R2 value can be raised to 150K maximum. Several lamps wired in parallel can be connected to the circuit, provided total power dissipation of the load does not exceed about 30...

The circuit can be used for switching OFF a particular lamp or group of lamps in response to the varying ambient light levels. The unit once built can be used for switching OFF a lamp when dawn breaks and switching it ON when dusk sets in. The power supply is a standard transformer, bridge, capacitor network, which supplies a clean DC to the circuit for executing the proposed actions. Automatic light activated switch circuit schematic The LDR must be placed outside the box, meaning its sensing surface should be exposed toward the ambient area from where the light level is required to be sensed. The circuit can be used as an automatic street light controller system or a simple light activated switch.

Compact circuit, Can be wired in parallel to existing switches This device allows one or more lamps to illuminate at sunset and turn off at dawn.Q1 and Q2 form a trigger device for the SCR, providing short pulses at 100Hz frequency. Pulse duration is set by R2 and C1.When the light hits R1, the photo resistor assumes a very low resistance value, almost shorting C1 and preventing circuit operation. When R1 is in the dark, its resistance value becomes very high thus enabling circuit operation. Circuit Diagram: Dark Activated 220 volt AC Lamp Circuit Diagram   Parts: R1 = LDR R2 = 100K R3 = 200K R4 = 470R R5 = 12K R6 = 1K R7 = 470R C1 = 10nF-63V D1 = TIC106D D2 = 1N4007 D3 = 1N4007 D4 = 1N4007 D5 = 1N4007 Q1 = BD327 Q2 = BD337 SK1 = Female Mains Socket Notes: R3 allows fine setting of operating threshold and R2 value can be raised to 150K maximum. Several lamps wired in parallel can be connected to the circuit, provided total power dissipation of the load does not exceed about 300 - ...