Emergency Light and Alarm circuit

This is a simple and easy emergency light and alarm circuit. This circuit is permanently plugged into a mains socket and NI-CD batteries are trickle-charged. When a power outage occurs, the lamp automatically illuminates. Instead of illuminating a lamp, an alarm sounder can be chosen.

When power supply is restored, the lamp or the alarm is switched-off. A switch provides a "latch-up" function, in order to extend lamp or alarm operation even when power is restored.

Emergency Light and Alarm circuit diagram

Component parts list:

R1 = 220K
R2 = 470R
R3 = 390R
R4 = 1K5
R5 = 1R
R6 = 10K
R7 = 330K
R8 = 470R
R9 = 100R
C1 = 330nF/400V Polyester Capacitor
C2 = 10µF/63V Electrolytic Capacitor
C3 = 100nF/63V Polyester Capacitor
C4 = 10nF/63V Polyester Capacitor
D1-D5 = 1N4007
D6 = LED Green
D7 = 1N4148
Q1,Q3,Q4 = BC547
Q2,Q5 = BC327
SW1,SW2 = SPST Switches
SW3 = SPDT Switch
LP1 = 2.2V or 2.5V 250-300mA Torch Lamp Bulb
SPKR = 8 Ohm Loudspeaker
B1 = 2.5V Battery (two AA NI-CD rechargeable cells wired in series)
PL1 = Male Mains plug

Circuit Works:
Mains voltage is reduced to about 12V DC at C2's terminals, by means of the reactance of C1 and the diode bridge (D1-D4). This avoids the use of a mains transformer.

Trickle-charging current for the battery B1 is provided by the series resistor R3, D5 and the green LED D6 that also monitors the presence of mains supply and correct battery charging.
Q2 & Q3 form a self-latching pair that start operating when a power outage occurs. In this case, Q1 biasing becomes positive, so this transistor turns on the self latching pair.

If SW3 is set as shown in the circuit diagram, the lamp illuminates via SW2, which is normally closed; if set the other way, a square wave audio frequency generator formed by Q4, Q5 and related components is activated, driving the loudspeaker.

If SW1 is left open, when mains supply is restored the lamp or the alarm continue to operate. They can be disabled by opening the main on-off switch SW2.

If SW1 is closed, restoration of the mains power supply terminates lamp or alarm operation, by applying a positive bias to the Base of Q2.

  • Close SW2 after the circuit is plugged.
  • Warning! The circuit is connected to 230Vac mains, then some parts in the circuit board are subjected to lethal potential!. Avoid touching the circuit when plugged and enclose it in a plastic box.

Emergency Light and Alarm circuit source: http://www.redcircuits.com/Page45.htm

Amplifier Circuit based 4 Transistors

Amplifier Circuit based 4 Transistors diagram

Above diagram very simple and easy to build class AB audio amplifier circuit which is working with 4 transistors. In class AB operation each output device performs more than half of the input signal cycle. Up to 78% effectiveness is possible with class AB designs and cross over distortion is minimized. The circuit shown here is appropriate for small radio receivers, audio players, intercom, phone etc.

Transistor Q1 with its connected parts is wired as a pre-amplifier stage. The audio input is coupled to the base of Q1 through resistor R1 and capacitor C1. Resistor R3 delivers collector to base bias for Q1 and C3 is an AC by pass capacitor for the collector resistor R4. Collector to base biasing is a great method of biasing for circuits like this because it provides enough negative feedback, prevents thermal runway and stabilizes the operating point. The second stage will be the driver stage for the push pull pair. Q2 with its connected parts carry out this job. This stage can also be collector to base biased, and its input is coupled towards the output from the pre-amplifier stage using capacitor C2. Resistor R8 limits the collector present of Q2.The third stage is the class AB push pull section comprising of transistors Q3 and Q4. Diodes D1 and D2 provides the bias voltage for the push pull stage. The output from the amplifier is coupled to the loud speaker through the capacitor C4. C5 and C6 are power supply filter capacitors.

  1. The circuit can be assembled on a vero board.
  2. K1 may be an 8 ohm/5Wof loud speaker.
  3. C6 must be grounded near towards the Q1 and C5 should be grounded near to the loud speaker ground. This reduces noise.
  4. Use 5V DC power supply for powering the circuit.

EL34 Stereo Tube Amplifier 2x90 Watts

The following are step by step how to create a stereo tube amplifier using EL34 power tube. This stereo tube amplifier circuit is capable of producing an audio output of 90 watts for each channel.

EL34 Stereo Tube Amplifier 2x90 Watts

Download EL34 Stereo Tube Amplifier 2x90 Watts step by step guide.
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220V Light Dimmer circuit

The following diagram is the schematic diagram of 220V Light Dimmer.

220V Light Dimmer diagram:

220V Light Dimmer circuit

Parts List:
R1 = 10K
R2 = 2K2
R3 = 180K
Potentiometer 250K
C1 = 150nF/400V dipped polyester
C2 = 47nF/630V dipped polyester
C3 = 220nF/250V AC dipped polyester
Triac = 2N6075 or BT136-500D
Diac = HT-32
FUSE 2A or 3A

Download the 220V Light Dimmer circuit documentation here.