0clock timer

CIRCUIT IDEAS 96 • FEBRUARY 2007 • ELECTRONICS FOR YOU WWW.EFYMAG.COM W ith this simple clock- controlled timer, you will never again miss your favourite TV or radio programme. The TV or radio will switch on automatically at the time preset by you and will remain ‘on’ until the power supply fails or is disconnected. The circuit uses the AC signals gen- erated at the buzzer terminals of an alarm clock. The AC signals are amplified by transistors T1 and T2 and the amplified output from the emitter of T2 is fed to the inverting input of negative-voltage comparator IC LM311 (IC1). The non-inverting input of IC1 gets a presettable voltage through preset VR1. The inverting and non-inverting inputs of LM311 are different from other op-amps and it outputs sink current through pin 7 or source current through pin 1. When pin 3 of IC1 is at a higher voltage than pin 2, its output sinks as indicated by LED1. This gives a short negative pulse to the monostable wired around timer NE555. Resistor R5 keeps trigger pin 2 of IC2 high. The short-interval monostable outputs a high signal for a brief period to the gate of SCR1 (BT169) and relay RL1 energises. The latching action of SCR1 keeps the relay pulled even when the output of the monostable turns low. The relay can be de-energised by dis- connecting the supply to the circuit D. MOHAN KUMAR CLOCK TIMER S.C. DWIVEDI via switch S1. The circuit works off a 9V battery. Assemble it on a general-pur- pose PCB and enclose in a suitable cabinet. Provide an AC outlet in the cabinet to switch on the appliance us- ing the circuit. As mentioned earlier, the input signal is obtained from the buzzer terminals of the clock. Remove the small buzzer of the clock and connect point ‘A’ to the positive terminal and point ‘B’ to the negative terminal of the buzzer. Connect the mains AC terminal outlet to the nor- mally-opened (N/O) contact of relay RL1. So when the relay energises, 230V AC operates the connected appliance. Set the desired time in the clock by adjusting the alarm set-up and switch on the circuit. When the set time reaches, the appliance will switch on automatically. The circuit can also be connected to digital clocks.

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CIRCUIT

IDEAS

9 6 • F E B R U A R Y 2 0 0 7 • E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M

W ith this simple clock-controlled timer, you willnever again miss your

favourite TV or radio programme. TheTV or radio will switch on automati-cally at the time preset by you andwill remain ‘on’ until the power sup-ply fails or is disconnected.

The circuit uses the AC signals gen-erated at the buzzer terminals of analarm clock. The AC signals are am-plified by transistors T1 and T2 andthe amplified output from the emitterof T2 is fed to the inverting input ofnegative-voltage comparator IC LM311(IC1). The non-inverting input of IC1gets a presettable voltage through pre-

set VR1. The inverting and non-invert-ing inputs of LM311 are different fromother op-amps and it outputs sink cur-rent through pin 7 or source currentthrough pin 1.

When pin 3 of IC1 is at a highervoltage than pin 2, its output sinksas indicated by LED1. This gives ashort negative pulse to the monostablewired around timer NE555. ResistorR5 keeps trigger pin 2 of IC2 high.The short-interval monostable outputsa high signal for a brief period to thegate of SCR1 (BT169) and relay RL1energises. The latching action of SCR1keeps the relay pulled even when theoutput of the monostable turns low.The relay can be de-energised by dis-connecting the supply to the circuit

� D. MOHAN KUMAR

CLOCK TIMERS.C.

DWIVEDI

via switch S1.The circuit works off a 9V bat-

tery. Assemble it on a general-pur-pose PCB and enclose in a suitablecabinet. Provide an AC outlet in thecabinet to switch on the appliance us-ing the circuit. As mentioned earlier,the input signal is obtained from thebuzzer terminals of the clock. Removethe small buzzer of the clock and con-nect point ‘A’ to the positive termi-nal and point ‘B’ to the negative ter-minal of the buzzer. Connect themains AC terminal outlet to the nor-mally-opened (N/O) contact of relayRL1. So when the relay energises,230V AC operates the connected ap-pliance.

Set the desired time in the clock byadjusting the alarm set-up and switchon the circuit. When the set timereaches, the appliance will switch onautomatically. The circuit can also beconnected to digital clocks. �

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