0clock timer

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. �