Twinkle Twinkle X'mas Star

Twinkle Twinkle X'mas Star

T.A. Babu

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Christmas just would not be Christmas if you do not put a flashing star on your Christmas tree. Here is the circuit of such a flashing star.

Fig. 1: Twinkle-twinkle X’mas star

Fig. 1 shows the circuit of the flashing X’mas star. At the heart of the circuit is a diac that controls charging and discharging of an electrolytic capacitor and thereby flashing of the star. The rest of the circuit functions as a solidstate AC relay for lighting the bulb fitted inside the Christmas star.

When switch S1 is in ‘on’ position, mains voltage is rectified by diode D1 and capacitor C1 charges through resistor R1. When the voltage across C1 exceeds the breakdown potential of the diac, the diac conducts and the capacitor discharges through LED1, resistor R2 and the internal LED of optocoupler MOC3041 (IC1). This discharge of energy results in a brief flash of light.

Fig. 2: Pin configurations of MOC3041 and TRIAC BT136

Use of a zero-crossing optocoupler (IC1) to drive triac BT136 virtually eliminates radio frequency interference. Every time the optocoupler receives a pulse, the triac fires and the bulb glows.

This circuit operates directly from mains, so be careful to avoid lethal shock. Use of a low-leakage, good-quality capacitor rated more than 63V is recommended. Changing the capacitor’s value alters the speed of flashes.

Before assembling the circuit on a general-puspose PCB, refer Fig. 2 for the pin configurations of MOC3041 and triac BT136. Use IC base for MOC3041.

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Liquid-Level Alarm

Liquid-Level Alarm

                                 

Pradeep G.                                                                  

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In water-level controllers for tanks, a DC current is passed through the metallic probes fitted in the water tank to sense the water level. This causes electrolysis and corrosion of probes, inhibiting the conduction of current and degrading its performance. As a consequence, probes have to be replaced regularly to maintain proper current flow.

The liquid-level alarm given here overcomes this problem. A 1kHz AC signal is passed through the probes, so there will be no electrolysis and therefore the probes last longer.

Fig. 1: Block diagram of liquid level alarm

The block diagram for the liquid-level alarm is shown in Fig. 1. The signal generator sends the generated signal to the first metallic probe. The second metallic probe is connected to the sensing circuit followed by the alarm circuit.

The complete circuit for the liquid-level alarm is shown in Fig. 2. The astable multibrator built around IC 555 (IC1) generates 1kHz square wave signal, which is fed to one of the probes via a DC blocking capacitor.

Fig. 2: Liquid level alarm

Fig. 3: Pin configuration of UM66

When the water tank is empty, pnp transistor T1 does not get negative base bias. But as water fills up in the tank, it receives 1kHz signal from IC1 via the probes immersed in water and conducts during the negative half cycle of 1kHz signals. Due to the presence of capacitor C7 (2.2µF), npn transistor T2 continues to get base bias and conducts to provide 3.3V DC to melody generator IC UM66 (IC2). Pin configuration of IC UM66 is shown in Fig. 3. Preset VR1 acts as the output loudness controller. It can be varied to set the alarm sound from the speaker at the desired level.

The circuit works off 12V unregulated power and can be used to detect any conductive liquid.

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