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555 Schmitt Trigger Logic Inverter Level Translator

Versatile 555 Schmitt Trigger /Logic Inverter /Level Translator

More undocumented applications for the 555 ‘oscillator:’ Not all applications oscillate as we will see. Most are knowledgeable about common CMOS logic inverters and Schmitt triggers such as the 74HC04 and 74HC14 respectively that come six in a DIP-14 package. Often, some sections are left over and can be used for future enhancements, etc. However, what do you do when only one additional inverter or increased output drive or signal level translation is required? This is where the ubiquitous 555 once again comes to the rescue.

Schematics

555 Schmitt Trigger Logic Inverter Level Translator

(555 Schmitt Trigger Logic Inverter Level Translator)

555 Level Translator Test Circuit

(555 Level Translator Test Circuit)

Inverter

The 555 is inherently an inverter. If the threshold inputs (pins 2 & 6) are tied together, they may be used collectively as the input with pin 3 being the output.

Schmitt trigger

Since the two thresholds (1/3 Vcc & 2/3 Vcc) are widely separated, they make a very good Schmitt trigger. You may recall that Schmitt triggers are often good at cleaning up noisy AC signals by separating the desired signal from the lower amplitude noise content. It is also extremely useful in converting a slow changing input voltage signal into a truly digital output (1 or 0), and without troublesome oscillation at the thresholds.

Input thresholds

1/3 Vcc & 2/3 Vcc are not always compatible with TTL logic levels. One clever way of making it compatible is to adjust the voltage at pin 5. For instance, setting pin 5 to 2.5V sets the upper threshed (“1” level) to 2.5V, and the low threshold (“0” level) to 1.25V thus making it TTL compatible. This may be accomplished via a simple voltage divider. By making its Thevenin resistance much lower than that of the internal divider (inside the 555), manufacturing tolerances are swamped thus making the voltage repeatable from device to device.

Input impedance

Input impedance is high and lends itself well to a simple R-C input filter that helps reject noise and prevent against potential ESD damage in applications where the input wiring is exposed to the real world environment.

Output drive capability

CMOS logic is hard put to source a 5mA output –It does better at sinking, but pales compared to the 100mA source /sink capability of the 555.

Range of power supply voltage

Vcc for the 74HC series is generally rated for 3 to 5V. The 555 can support up to 15V and can be mixed with 4000 series CMOS devices.

Open collector level translator

Pin 7 is an open collector output that can be applied as an interface to either higher or lower voltage logic devices –all that is required is the addition of a pull-up resistor that is tied to the secondary Vcc. Note that this is VERY undocumented and may not always work as expected, so please test and use common sense before going off the deep end in any design –especially, since the addition of a discrete MOSFET such as the 2N7000 will always do the job (with the addition of yet another inverter because the common source connection itself inverts the signal). Note that all of the 555 devices that I tested worked OK, including the CMOS TLC555. To test this feature, I constructed a simple test circuit and took oscillographs of the signals.

Pin 7 output discontinuity

While one device, an old SG555 worked acceptably, the oscilloscope displayed a discontinuity in the negative transition –See oscillograph. All the others were clean. This type of discontinuity or bounce in the output signal can raise havoc in high speed digital logic. In this case, the CD4013 D Latch is quite slow and forgiving.

Will 555 applications ever be exhausted? NO!

Oscillographs

Photographs

Undocumented words and idioms (for our ESL friends)

real world –idiom –the harsh EMI conditions that exist outside the shielded enclosure

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