Thursday, October 2, 2014
Simple IC 555 Touch Activated Switch Circuit
- An LED with a suitable series resistance may be included in the circuit and placed in the centre 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.
- In the circuit, some special precautions have been taken to avoid interference pick-up, to which TTL ICs are especially prone.
- To minimise stray pick-up, the interconnections should be as short as possible.
- Capacitor C2 is rather large compared to the conventional value (0.0lp.F) used in this position. This modification has proved quite effective.
- The power supply shown provides an appropriate DC voltage to drive a conventional relay.
- 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 l.8M to IOM. Rl may be excluded altogether.
- If the unitis situated away from the touch sensor, a shielded wire should be used for the connection.
- Tl acts as an inverter and converts a rising edge into a falling edge. Transistor T2 is used to provide sufficient drive capability to drive the relay. The external load is connected across the g relay contacts.
- An additional decoupling capacitor ((0.0lp.F) may be added as close as possible to the supply terminals of IC 7493. The prototype was constructed on a conventional bread- board.
- The circuit basically comprises a touch activated mono- stable vibrator using an NE555 IC, which activates the ITL s 7493 IC used as a bistable. , .
- Another modification is the addition of Cl-Rl parallel combination.
- 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.
- This causes a time delay before the load is actually activated. To avoid this, Tl is introduced.
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