AUTOMATIC OFF TIMER FOR DVD PLAYERS

      Are you in the habit of falling asleep while listening to music? If yes, you’ll love this circuit. It will automatically start functioning when you switch off your bedroom light and shall turn your CD player ‘off’ after a predetermined time. In the presence of ambient light, or when you switch on light of the room in the morning, the CD player will again start playing. Unlike the usual timers, you don’t have to set this timer before sleeping.

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      The circuit derives its power directly from the bridge rectifiers. When ‘on’/‘off’ switch S1 is closed, LED1 glows to indicate that the circuit is powered ‘on.’

      In the presence of light, the resistance of the light-dependent resistor (LDR1) is low, so transistor T1 conducts to drive transistor T2 into cutoff state and the timer circuit remains inactive.

     The collector of transistor T2 is connected to reset pin 12 of IC CD4060 (IC1) via signal diode D5. IC CD4060 is a 14-stage ripple counter with a built-in oscillator. The time period of oscillations (t) is determined by capacitor C3 and resistor R8 connected to pins 9 and 10 of IC1, respectively, as

follows:

t=2.3RC

where ‘R’ is the value of resistor R8 and ‘C’ is the value of capacitor C3.

     When transistor T2 is cut-off, its collector voltage is high. So pin 12 of IC1 is high and IC1 is in reset condition.

     When light is switched off, the resistance of LDR1 increases, driving transistor T1 into cut-off state. The collector voltage of transistor T1 goes high to light up LED2 (indicating that the timer circuit is enabled) and transistor T2 starts conducting. As the collector voltage of transistor T2 goes low to around 0.2V, ground potential becomes available at reset pin 12 of IC1. The low state at pin 12 enables the oscillator and it starts counting. LED3 at pin 7 of IC1 starts blinking. Its blinking frequency depends on the R-C components connected between its pins 9 and 10.

     The status of LED2 and LED3 in the circuit with light falling and not falling on LDR1 is given below:

LDR1	Timer LED2	Reset pin 12	Count LED3
Light	Off	High	Off
Dark	On	Low	Blink

     During counting, in case the power fails momentarily, capacitor C2 (1000μF) will provide the necessary power backup for IC1. That is, during the period, pin 3 of IC1 is low. When output pin 3 of IC1 goes high, the relay is energised through transistors T3 and T4 and, at the same time, counting is disabled by the feedback from pins 3 through 11 (clock input) of IC1 via signal diode D7. That is, due to the feedback, output pin 3 remains high unless another high-to-low pulse is received at its reset pin 12.

     After the relay is energised, there will be no AC power in the socket. The glowing of LED5 indicates that your CD player has been switched off.

     The desired ‘off’ time period for the timer circuit can be set by choosing proper values of resistor R8 and capacitor C3. If R8 is 680 kilo-ohms and C3 is 0.22 μF, the ‘off’ time period is around 45 minutes.

     The glowing of LED4 gives the warning that your CD player is going to be switched off shortly. In case you want to extend the timer setting for another round, just press reset switch S2 momentarily. LED4 stops glowing and counting starts again from the initial stage.

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Push Off Push On

The ubiquitous 555 has yet another airing with this bistable using a simple push-button to provide a push-on, push-off action. It uses the same principle of the stored charge in a capacitor taking a Schmitt trigger through its dead-band. Whereas the Schmitt trigger in that reference was made from discrete components, the in-built dead-band arising from the two comparators, resistor chain and bistable within the 555 is used instead. The circuit demonstrates a stand-by switch, the state of which is indicated by illumination of either an orange or red LED, exclusively driven by the bipolar output of pin 3. Open-collector output (pin 7) pulls-in a 100mA relay to drive the application circuit; obviously if an ON status LED is provided elsewhere, then the relay, two LEDs and two resistors can be omitted, with pin 3 being used to drive the application circuit, either directly or via a transistor.

The original NE555 (non-CMOS) can source or sink 200mA from / into pin 3. Component values are not critical; the ‘dead-band’ at input pins 2 and 6 is between 1/3 and 2/3 of the supply voltage. When the pushbutton is open-circuit, the input is clamped within this zone (at half the supply voltage) by two equal-value resistors, Rb. To prevent the circuit powering-up into an unknown condition, a power-up reset may be applied with a resistor from supply to pin 4 and capacitor to ground. A capacitor and high-value resistor (Rt) provide a memory of the output state just prior to pushing the button and creates a dead time, during which button contact bounce will not cause any further change. When the button is pressed, the stored charge is sufficient to flip the output to the opposite state before the charge is dissipated and clamped back into the neutral zone by resistors Rb. A minimum of 0.1 µF will work, but it is safer to allow for button contact-bounce or hand tremble; 10 µF with 220 k gives approximately a 2-second response.

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High and Low Mains Voltage Cut Off Circuit

dThis simple circuit can do a lot and immediately break the power from the appliance if there is any increase or decrease in the Mains voltage. It similar to the working of A Stabilizer to sense high and low voltage fluctuations. The trick is simple and the circuit costs only Rs.100.

The idea behind the circuit is that any voltage increase or decrease in the 230 volt mains lines, will change the voltage in the Primary of the transformer. A corresponding voltage change also appears in the secondary also. This is used in the circuit to operate the relay.

When the voltage level is normal, say 230 volts, relay will be energized since T2 conducting. This is set by the preset VR2. Load will get power through the Common and NO contacts of the relay. When the voltage drops, T2 turns off  because of the setting of VR2 and relay de-energize to cut off power to the appliance. When the voltage increases again, relay will be activated again.

VR1 is used to set the high voltage limit. When the voltage is normal or low , Zener diode ZD will not conduct ( the limit is set by VR1). When the voltage rises above this set limit, ZD conducts and T1 switches on. This drops the voltage to VR2 and T2 switches off. Relay again deactivates.

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