Time delays up to half an hour can be achieved with this project, which can be used to turn off a TV, audio equipment, lamps, and many other home appliances.
The basic project is powered directly from a 12 V or a 6 V battery, but modifications can be made to power it from the ac power line as shown in Fig. 1.
We recommend a DPDT mini relay (Radio Shack 275-249) that can be mounted on a solderless board or a universal printed circuit board, but other sensitive relays with coils rated for 6 or 12 V can be used.
The relay,s coils should have resistances in the range between 200 to 500 ohm (12 V) or 100 to 250 ohm (6 V).
Circuit operation is the same as described in other tiomers using the 4093 described in this site.
The only difference is the output stage With a transistor to drive the relay.
A schematic diagram of the Auto Turn-Off Relay ís shown in Fig. 2.
Proper positioning of the polarized components (diode, electrolytic capacitors, and power supply) must be observed. Wiring to the load must be rated according the amount of current drain.
Capacitor C1 is chosen according the desired delay range. Using a 1,000 µF capacitor and a 4.7 M ohm potentiometer, time delays up to half an hour can be obtained.
To operate the unit, connect the load to the relay contacts and adjust R2 to the desired time delay. Press S1 to start. At this time, the load receives power, and LED1 glows.
After the adjusted time delay, the unit automatically turns off, along with the controlled load. To use the unit again, you have to press S2 to discharge C1 before a new start.
As the 4093 has a very high input impedance, all projects with large values of timer capacitors should have a parallel SPST momentary switch to discharge them after each cycle of operation, except in the versions that use automatic discharge networks.
IC1 - 4093 CMOS integrated circuit
Q1 - 2N2222 NPN general purpose silicon transistor
D1 - 1N4148 or equivalent general purpose silicon diode
K1 -12 Vdc, 43 mA, 280 Q mini DPDT relay, contacts rated to 1 A (Radio Shack 275-249) or a 6 V unit (see text)
LED1 - Red common LED
Sl, S2 - Mini SPST momentary switch normally open, Radio Shack 275-1547
R1 -100,000 ohm, 1/4 W, 5% resistor
R2 2,200,000 ohm or 4,700,000 ohm potentiometer
R3 - 4,00 ohm, 1/4 W, 5% resistor
R4 - 680 ohm, 1/4 W, 5% resistor (6 V) - 1,000 ohm, 1/4 W, 5% resistor (12 V)
C1 - 10 µF to 1,000 µF, 12 WVDC electrolytic capacitor (see text)
