Push Off Push On


The ubiquitous 555 has
yet another airing with this bistable using a simple push-button to
provide a push-on, push-off action. It uses the same principle of the
stored charge in a capacitor taking a Schmitt trigger through its
dead-band. Whereas the Schmitt trigger in that reference was made from
discrete components, the in-built dead-band arising from the two
comparators, resistor chain and bistable within the 555 is used instead.
The circuit demonstrates a stand-by switch, the state of which is
indicated by illumination of either an orange or red LED,
exclusively driven by the bipolar output of pin 3. Open-collector
output (pin 7) pulls-in a 100-mA relay to drive the application circuit;
obviously if an ON status LED is provided elsewhere, then the relay, two LEDs and two resistors can be omitted, with pin 3 being used to drive the application circuit, either directly or via a transistor.

Circuit diagram:

Push Off Push On Circuit

Push Off Push On Circuit Diagram

The original NE555 (non-CMOS) can source
or sink 200 mA from / into pin 3. Component values are not critical; the
‘dead-band’ at input pins 2 and 6 is between 1/3 and 2/3 of the supply
voltage. When the pushbutton is open-circuit, the input is clamped
within this zone (at half the supply voltage) by two equal-value
resistors, Rb. To prevent the circuit powering-up into an unknown
condition, a power-up reset may be applied with a resistor from supply
to pin 4 and capacitor to ground. A capacitor and high-value resistor
(Rt) provide a memory of the output state just prior to pushing the
button and creates a dead time, during which button contact bounce will
not cause any further change. When the button is pressed, the stored
charge is sufficient to flip the output to the opposite state before the
charge is dissipated and clamped back into the neutral zone by
resistors Rb. A minimum of 0.1 µF will work, but it is safer to allow
for button contact-bounce or hand tremble; 10 µF with 220 k gives
approximately a 2-second response.

Author: Trevor Skeggs – Copyright: Elektor July-August 2004


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