Tuesday, 7 February 2012

555 / 556 Inverting Buffer (Schmitt trigger) or NOT gate

555/556 Inverting Buffer (Schmitt trigger) or NOT gate

555 buffer circuit
555 inverting buffer circuit
(a NOT gate)
NOT gate symbol
NOT gate symbol
The buffer circuit's input has a very high impedance (about 1Mohm) so it requires only a few µA, but the output can sink or source up to 200mA. This enables a high impedance signal source (such as an LDR) to switch a low impedance output transducer (such as a lamp). It is an inverting buffer or NOT gate because the output logic state (low/high) is the inverse of the input state:
  • Input low (< 1/3 Vs) makes output high, +Vs
  • Input high (> 2/3 Vs) makes output low, 0V
When the input voltage is between 1/3 and 2/3 Vs the output remains in its present state. This intermediate input region is a deadspace where there is no response, a property called hysteresis, it is like backlash in a mechanical linkage. This type of circuit is called a Schmitt trigger. If high sensitivity is required the hysteresis is a problem, but in many circuits it is a helpful property. It gives the input a high immunity to noise because once the circuit output has switched high or low the input must change back by at least 1/3 Vs to make the output switch back.

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