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