Description.

This is a compact electronic siren circuit based on three transistors.This circuit is suitable for in corporating with other alarm or siren projects such as burglar alarms, automatic factory sirens etc or a simple push to on alarm.

The  electronic siren circuit given here  is  based on a complementary transistor pair consisting of Q2 & Q3 (BC557 & BC 37)  wired as an astable multivibrator oscillator,which directly drives the speaker.The transistor Q1 is used to provide a full charge on capacitor C2 when power is turned ON. When push button switch S1 is pressed , the capacitor C2 slowly discharges through resistor R8.This makes the circuit to  oscillate at a low frequency that increases to a high frequency and kept indefinitely as the capacitor is fully discharged. When the switch P1 is released, the output  frequency decreases slowly as C2 is charged to the  positive voltage through resistance R6 and the Base-Emitter junction of tramsistor Q2. When C2 is fully charged to the positive battery voltage the  circuit stops oscillating.

Circuit diagram with Parts list.

Notes.

• A 12 V battery or a a well regulated 12V DC power supply can be used to power the circuit.
• Assemble the circuit on a good quality PCB or common board.
• The switch S1 can be used to activate the alarm.
• The switch S2 can be used as a power switch.
• You can experiment on the tone of alarm by using different values for C2 and R8.

Description.

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 extend,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.

Circuit diagram with Parts list.

Notes.

• The circuit can be powered of a 3V battery.
• Assemble the circuit on a good quality PCB or common board.
• The probes can be made of two insulated copper Aluminiun wires.
• Place the probes at the position where you have to sense the level.

Description.

Here is a simple and effective circuit for driving florescent lamps from a 12 V supply.The circuit is nothing but consists of an oscillator, a MOSFET switch and a step-up transformer for driving the florescent lamp.

The IC1 TLC 555 is wired as an astable multivibrator for producing the necessary oscillations.The MOSFET Q1 is used to amplify the oscillations produced by the IC1.The out put of MOSFET is connected to the primary of the step up transformer to produce a ~400 V AC for driving the florescent lamp.

Circuit diagram with Parts list.

Notes.

• C3 and C2 must be 25V ceramic capacitors.
• C2 must be electrolytic.
• For T1, use a 10W ,230V to10V step down transformer in the inverted configuration.That is 10V winding must be connected to the MOSFET side and 240V winding must be connected to florescent lamp side.
• Use a 12V lead acid battery for powering the circuit.
• It is better to fix a heat sink to Q1.
• The florescent lamp can be of a 4W one.

Description.

This circuit will produce a visual as well as audible alert whenever the connection between the charger circuit and the battery being charged.What ever be the situation whether it be a  loose contact or breakage the circuit will give you the alarm.Such a circuit can be very handy in the fail safe charging of the batteries in some applications , like as in burglar alarms etc.This circuit can be easily assebled from the components in your electronic junk box.

As long as there is a little current flows from the charger to battery the diodes D1 and D2 will conduct.This forward voltage drop of 1.4 V across the diodes make the Q2 ON.So the transistor Q1 will be OFF.When there is no at all any current flowing (when the wire is broken or there is a loose contact) there will not be any drop across D1&D2 and so Q2 will be OFF.This makes the Q1 ON,the buzzer beeps and the LED glows.

Circuit diagram with Parts list.

Notes.

• For this circuit to operate effectively the supply voltage must atleast 1V higher than the voltage required by the charged device.

• The circuit can operate safely in a range of 3-16 V.

• Assemble the circuit can be assembled on a good quality PCB or common board.

• The transistors are not very value specific.Any PNP transistors will do the job.

Description.

Here is a simple charger circuit for charging SMF(sealed maintenance free ) batteries.The charger circuit being curernt and voltage regulated,can be safely used to charge SMF batteries of 1.2 AH rating.This circuit was actually designed in response to a request from a reader.With slight modifications you can use it for charging any sort of SMF batteries.

The first part of the circuit is the power supply made of transformer T1 and diodes D1&D2.The capacitor C1 filters the rectifier output .The next part is the section comprising of IC1 and Q1 which provides the necessary volatge and current regulation.The diode D3 prevents ther reverse flow of charge from the battery .

Circuit diagram with parts list.

Notes.

• Assemble the circuit on a good quality PCB or common board.
• An ammeter can be conncted in series with the battery to monitor the charging current.
• Do not short circuit the charging terminals of the circuit.The circuit doesnot have short circuit protection.
• R4 is the component that determines the charging current.Here it is set to be 120 mA. R4= (0.6 V/Charging current).By selecting the proper value of R4 according to equation, you can charge batteries that require different charging currents.