#692 Push-button Latch

Testing a simple push-button latched switch circuit for on/off control using the 555 timer.

Here’s a quick demo..

Notes

This is a simple latching circuit for push-button on/off control using the 555 timer.

How it works:

• by default:
  • the push-button is not pressed
  • the R1/R2 voltage divider pulls the 555 threshold and trigger pins to the mid-level
  • thus neither THRESHOLD(reset) or TRIGGER(set) inputs to the 555 are enabled and
  • the 555 output state is unchanged
• when the push-button is pressed:
  • if the current output is LOW,
    • the R1/R2 voltage divider mid-point will be pulled LOW
    • TRIGGER(set) falls bellow the 1/3 set-point, causing the 555 to set and output to go HIGH
    • the capacitor C1 holds the button input LOW for long enough for the button to be released
  • if the current output is HIGH,
    • the R1/R2 voltage divider mid-point will be pulled HIGH
    • THRESHOLD(reset) rises above the 2/3 set-point, causing the 555 to reset and output to go LOW
    • the capacitor C1 holds the button input high for long enough for the button to be released

Circuit Design

Testing on a breadbord:

Protoboard Build

Transfering the design to a small protoboard so that it can be used as a latched switch input.

Credits and References

• LM555 Datasheet
• Simple Latching Circuit using 555 timer
• Build a 555 Timer IC based Simple Push-on Push-off Circuit - dodgy schematic and poor description
• 555 LATCH Circuit - another 555 “latch” circuit but this is different - uses an external latching circuit to enabled/disable a 555 astable.