Thursday, September 25, 2014

Simple Touch Operated Switch Circuit Diagram Using IC 555

  1. The touch operated switch circuit basically comprises a touch activated mono- stable vibrator using an NE555 IC, which activates the TFL 7493 IC used as a bistable.
  1. Transistor T2 is used to provide sufficient drive capability to drive the relay.
  2. The power supply shown provides an appropriate DC voltage to drive a conventional relay. In the circuit, some special precautions have been taken to avoid interference pick-up, to which I`l`L ICs are especially prone. Capacitor C2 is rather large compared to the conventional value (0.01].tF) used in this position.
  3. The extremal load is connected across the relay contacts.
  4. This modification has proved quite effective. Another modification is the addition of Cl-RI parallel combination. The value of Rl is best chosen by trail and error, and is a compromise between sensitivity and immunity from stray pick·up, Its value lies in the range 1.8M to 1OM. R1 may be excluded altogether. An additional decoupling capacitor (0.01uF) may be added as close as possible to the supply terminals of IC 7493.
  5. An LED with a suitable series resistance may be included in the circuit and placed in the center of the touch plate to  indicate when the relay is activated. In cases of severe stray pick-up, the unit should be enclosed in a metal casing that should be grounded.
  6. The prototype was constructed on a conventional bread board. To minimize stray pick-up, the interconnections should be as short as possible. If the unit is situated away from the touch sensor, a shielded wire should be used for the connection.
  7. This causes a time delay before the load is actually activated. To avoid this, TI is introduced.
  8. TI acts as an inverter and converts a rising edge into a falling edge.
  9. The monostable is touch activated and it provides the triggering input to the bistable. However, the bistable triggers only on the falling edge of the monostable output pulse. 
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