Light alarm circuit with the special edition.

This is a light alarm circuit with the special edition, that improve this circuit a whole new to higher efficiency. At the same time that lower price. By cutting R-potentiomer (horseshoe-shaped resistor) take away and use a solar cell instead of LDR.

Which unless will has the price lower, also reduce a Difficulty of application little down because Not require any tuning.

The difference between LDR and solar cell is that LDR will has resistance electrical current is high when use in at dark. And will has lower resistance when the light drop on it. Next the solar cell that has a feature of high value resistor when in the dark. But when get light, it will produce a certain amount of electrical current. Which depends on the brightness of the light over its body.

And in the sound signal generator that will is set new form. By use dual matching pair transistors, which can provide a the signal very stronger than general. There is also an LED to show the condition of circuit as well.

How this circuit works
From circuit in Figure 1 will see that in the sound signal generator section. We use matched pair transistors number CS 9012 and CS 9013 to connected together with Resistors of 3 pcs and one of capacitors, as the sound signal generator circuit at frequency about of 1,220 Hz. And at common point of this circuit will connected to Collector of transistor-Q2, look like this, If Q2 is not conduct current will cause the sound signal generator section also not has current flow. And in this section, base of transistors-Q2 will be connected to emitter of E of Q1, which called that Darlington compound has The gain of the current is very high.

Light alarm circuit with the special edition

Figure 1 the Light alarm circuit with the special edition.

The connecting that at pin B of Q1. when compare with ground, will must have voltage to it exceeds 1.2 V so will have current flow, thus we so connect the positive of the solar cell to base of Q1 and the negative of solar cell will be connect to divide point between Resistors-10K and two diodes. At this point we connect diode in the forward bias. by use Using 2 diodes in series, the voltage drop across the diode, about 1.2 V. So while solar cells have not been lighting At pin B of Q1. No current will flow. The voltage of the diodes are not enough.

However, when the solar cells are light, It will create a certain amount of voltage to about 200-400 mV Depending on the brightness of the light. and the voltage this amount will be connected series with the diodes voltage. Therefore the voltage so equal about 1.4 – 1.6 V Which was enough to make the Q1 and Q2 conduct current and has Audio signal out to the speakers. The LED is connected to pin C of Q1 that will glow when Q1 conduct or while there is sound enough to indicate the condition of the circuit.

How to build this projects
This project is not used many components so can assemble on the universal PCB board. But can make a PCB. Then, the wiring for circuit wiring and various components can view of the example in Figure 2. as Single-sided PCB layout(Left) and The components layout (Right). You aro also must check carefully for the polarity of solar cell, the electrolytic capacitors,diodes, transistors in correctly.

Single sided PCB layout and components layout

Figure 2 Single-sided PCB layout(Left) and The components layout (Right)
This circuit use 9 volts power supply, and all resistors in 1/4 watts, the speaker size as 2 inch 0.25W 8 ohms.

The components list
Q1___C1815 or C458_______45V 100mA NPN Transistor
Q2,Q3_CS9013_____50V 800mA NPN Transistor
Q4____CS9012_____50V 800mA PNP Transistor
D1,D2__1N4148_____75V 150mA Diodes
CP_____Solar cell____see in text.
C1_____33uF 16V____Electrolytic___capacitor
R1_____10K___Resistor___1/4 watts 5%
R4_____120 ohms_”___________”___

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