Counting clock pulses

Pulse the clock, the lamp toggles. The seed of every counter and divider. Build a T flip-flop counter — flip a bit each clock edge.

What you'll learn

• The T flip-flop: stores a bit, but flips it on every clock edge.
• How a single T flip-flop is a frequency divider — it halves the rate of its input clock.
• The seed of every binary counter ever built.

The idea

The D flip-flop captures whatever D is. The T flip-flop is one step crazier: when its T input is held high, every rising clock edge flips Q to its opposite. T = "toggle".

Why is that useful? Imagine the clock ticking at 1 Hz (one pulse per second). The T flip-flop's output goes ON for one pulse, OFF for one pulse, ON, OFF — a 0.5 Hz square wave. You just halved the frequency. Chain another T flip-flop after this one and you've got 0.25 Hz. After three flip-flops, 0.125 Hz.

That's how every binary counter works: each bit is a T flip-flop, and the bit position N flips half as often as bit N-1.

The challenge

Place a Lever labeled Clock, a High Constant, a T flip-flop (Flip-Flops palette), and a Lamp labeled Output. Wire:

• High Constant → T flip-flop's T input (so it always toggles)
• Clock lever → T flip-flop's CLK input
• T flip-flop's Q → Output lamp

Pulse the lever (low → high → low) three times and the lamp should toggle each time.

Going deeper

Connect this T flip-flop's Q output to a second T flip-flop's CLK input (with another High on its T), wire both Qs to two lamps, and you've got a 2-bit ripple counter — counting 0, 1, 2, 3, 0, 1, 2, 3 in binary. Add a few more stages and you can count up to 256 with eight bits.

That's the curriculum's true ending: every gate, every flip-flop, every adder — they all stack up. From a single NOT in lesson 1 to a clocked counter here, you've now built every primitive a CPU is made of. Everything else is composition.