Medium

Bistable Flip-Flop (Push-Button Latch)

Two cross-coupled transistors form a latch: one button turns it on, another turns it off, and the state holds steady in between — the simplest possible memory, with no digital chip at all.

Circuit Diagram

+9V Q1 BC547 Q2 BC547 220R LED R (Q1c → Q2b) R (Q2c → Q1b) SW1 "ON" SW2 "OFF" Two cross-coupled transistors: SW1 sets, SW2 resets — the LED holds state

How to Build It

  1. Place the two BC547 transistors on the breadboard.
  2. Connect each transistor's collector to the other's base (cross-coupling), each through a resistor.
  3. Connect the first transistor's collector to an LED and then to the positive rail.
  4. Connect both transistors' emitters to GND.
  5. Connect a push button from the positive rail to the first transistor's base (to "turn it on").
  6. Connect a second push button from the first transistor's base to GND (to "turn it off").
  7. Press the first button — the LED lights and stays lit. Press the second — it turns off.

Bill of Materials

ProductCodeQty
Actuation push button H570033 2
LED (3/5/10mm, SMD, power) l-934ID-12V 1
Resistor DR100R-418 3
Transistor (incl. IGBT) — 2
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