marți, 25 octombrie 2016

Light-sensitive fire alarm

Circuit diagram of the light-sensitive fire alarm is shown in Fig. 1. It is built around L14F1 photo transistor (T1), BC547 transistor (T2) and NE555 timer (IC1).



T1 can sense both infrared and visible light. NE555 based a stable multi vibrator is used for audible warning. Resistors R3 and R4 and capacitor C1 decide the frequency. Here, frequency is around 272Hz, which can be changed by changing values of R3, R4 or C1.



When T1 senses light, T2 stops conducting. As a result, reset pin 4 of IC1 is held high. This makes the multivibrator to function and an alarm to sound in loudspeaker LS1.

An actual-size, single-side PCB of the light-sensitive fire alarm is shown in Fig. 2 and its component layout in Fig. 3. After assembling the circuit on a PCB, enclose it in a suitable plastic case.




Make a small hole in the plastic case for the photo transistor (L14F1) so that fire flame is detected/sensed easily through it. T1 sensor should not face directly towards normal light sources like sunlight, bulbs or tubelights to avoid a false alarm.

The circuit works off a 9V battery.




Build your own programmable power supply with TL431

TL431 and LM431 are relatively low-noise, stable and low-cost shunt regulators. These can be used to build all sorts of power supplies including programmable power supplies, advantages of which are enormous. You can program the output voltage with simple switches. You can also program the output voltage with digital codes coming from a microcontroller (MCU) or from the printer port of a PC. You can adjust any output voltage individually with resistors or trimmer potentiometers to the required values.

This article presents a programmable power supply built around TL431 (IC1) and two bipolar transistors BD139 and TIP31 (T1 and T2).

The circuit also includes an inverter 7406 (IC2), nine diodes 1N4007 (D1 through D9), a 12V regulator 7812 (IC3), a 5V regulator 7805 (IC4) and a few other components.

Using this circuit you can obtain around 18V, 2A unregulated output and 3V to 15V, 1A variable regulated power supply based on digitally programmable input as shown in Table I. You can also obtain around 12V and 5V, 1A regulated output across connectors CON3 and CON4, respectively.


Circuit diagram of the programmable power supply is shown in Fig. 1. The mains power supply is applied to transformer X1, which is stepped down to 18V AC, 2A and is given to bridge rectifier BR1. Fuse F1, resistor R15 and capacitor C14 protect X1 from external surge voltage. C6 is the main filtering capacitor, which should be at least 2200µF.




Voltage produced by TL431 is adjusted with potentiometers VRx (VR1 through VR6) and can be calculated with the simplified formula given below:


Vout = 2.5V×(1+R12/(R13+VRx))

The maximum output voltage (Voutmax) is thus calculated as:


Voutmax = 2.5V×(1+10k/2k) = 2.5V×6 = 15V

Because in this case we have R12 = 10-kilo-ohm and R13 = 2-kilo-ohm. The minimum output voltage (Voutmin) is calculated as:


Voutmin = 2.5V×(1+10k/(2k+47k) = 2.5V×1.2=3V

Because in this case we have R12 = 10-kilo-ohm, R13 = 2-kilo-ohm and VRx = 47-kilo-ohm.

Table 1 shows how output voltages can be set with digital codes C0 through C5.

Maximum output current is set with switches S1, S2 and S3 as shown in Table 2.




An actual-size, single-side PCB for the programmable power supply is shown in Fig. 2 and its component layout in Fig. 3. After assembling all components, enclose the PCB in a box such that 230V AC mains can be connected to the circuit easily. CON2, CON3 and CON4 can be mounted on the rear side of the cabinet. J1 through J3 shown in the PCB are simply jumper wires. Fix VR1 through VR6 on the front panel for setting output voltages.




Verify the test points given in Table III before using the circuit. After completing the circuit, connect CON5 to the MCU or printer port of the PC for setting various output voltages.
T1 and T2 should be put on a common heat-sink with thermal reliance below 2°C/W.



Maximum power dissipation of T2 can go up to 30W if we have minimum output voltage of 2.5V and maximum output current of 2A.




We can replace T1 and T2 with a power Darlington transistor with power dissipation of at least 75W. Size of the heat-sink will be reduced significantly if we use 12V cooling fan connected to the output of the auxiliary voltage regulator 7812.

Regulators 7812 and 7805 should be mounted on heat-sinks with thermal resistance below 20°C/W. Also, we can set maximum output current using a combination of three open and closed switches S1, S2 and S3.

Before using the circuit, set supply voltage around 18V-20V from a DC source with short-circuit protection. Use VR1 and set output voltage to any appropriate voltage, for example, 12V. Now, use VR2 and set another output voltage, say, 10.8V.

Similarly, use the remaining potentiometers for required output voltages. Set the output current-limiting function by connecting S1, S2 and S3 by referring to Table II. Programmed output voltage is available at CON2.

Source: electronicsforu.com

Dimming LCD Backlight

Presented here is an LCD power-saving backlight-control circuit. An LCD is used in many projects for indication or information display since it consumes less power and, hence, saves energy. But power consumption in an LCD backlight draws a lot of attention.

Pin diagram of a 16×2 alphanumeric LCD is shown in Fig. below:




Pins 15 and 16 are internally connected to the backlight LED. Normally, backlight LED+ is connected to +5V supply via a current-limiting resistor (it is not included in the circuit here for simplicity). But this LED stays on, all the time. So if you want to dim it, you need a solution so that during day time, or when ambient light is enough to make the LCD characters visible, the backlight gets dimmed and energy can be saved. For this, we have two options: one is microcontroller (MCU) based and the other is discrete-component based.

Circuit using an LCD is generally driven by an MCU. If the MCU has capture-compare pulse (CCP) width modulation module, our task becomes very easy. One such solution using a transistor is shown in the next figure. 




Here, CCP pin needs to be connected to the base resistor so that the LED can be dimmed. Then, we can use a light-dependent resistor (LDR) to sense the ambient light and command the MCU to dim the backlight accordingly.

If CCP pin is busy, or the MCU does not have a CCP pin, then an alternative circuit shown in the figure below will ensure that the backlight is dimmed accordingly.



Note that control pin 5 is not grounded through a capacitor in the usual way, but is connected to a voltage-divider network formed by resistor R5 and VR1. This configuration makes it work as a pulse width modulator, which changes the width of output pulse at pin 3 based on the voltage at pin 5.

Potmeter VR1 can be replaced with an LDR so that the LCD automatically gets dimmed during day time. At night, or when ambient light is less, backlight intensity (current) increases. Since LDR resistance changes as per ambient light, voltage at pin 5 of NE555 also varies accordingly. This variation in voltage at pin 5 changes the duty cycle of the output waveform, giving higher duty cycle when ambient light is not present, and vice versa. In this way it can be used to dim the LCD backlight for power saving.

Source: electronicsforyou.com