Home > Circuits > Heads or Tails
Modified:20:52, 22 October 2013
INTRODUCTION
There are times when you just want to know a simple "shall I" or "shan't I" or a toss up between heads or tails. If you have no coin then this could be the answer.

Just press the button and the LEDs flash alternately then slow down to stop on one LED when the button is released.

CIRCUIT DESCRIPTION
The circuit can be broken into 3 sections, shown right.

The astable is based around a 555 with the supply for the charging capacitor controlled by an RC network which via Q1 provides a gradually diminishing voltage after the button is released. This causes the astable to gradually slow down to a stop.

BLOCK DIAGRAM

The D-type flip-flop divides the clock by switching on Q and Not Q alternately. These outpus are then sent to the second flip-flop wired as a S/R latch. The reason this is done is so that when driving an LEd the outputs can drop below acceptable logic levels - thus this may confuse the Data input on the first flip-flop. So the first one divides and the second one drives.

CIRCUIT DIAGRAM

CIRCUIT BOARD
The PCB measure 53mm x 38mm - which happens to be the size of a 3xAAA battery holder. There is a 3mm hole drilled directly in the centre of the PCB to allow it to be fixed to underside of the battery holder, thus making a compact unit.

The power switch is a miniature PCB mounted slide switch and the "Spin" button is a PCB mounted click type.

LEDs were 5mm high brightness green and red. The output from the 74HC74 dual D-type flip-flop is quite low - around 5mA but lights the LEDs quite brightly.

Power switch is a PCB mounted miniature slide switch.

Keyboard Switch - PCB mount click switch but in green

The holder used was one with long PCB connections and these were bent so they could be soldered directly to the underside of the PCB. In the prototype AA batteries and holder were used.

DOWNLOADS

PCB Layout - (PCB Wizard)

Circuit Diagram - PDF

Circuit Diagram - JPG

Designed and Written by Phil Townshend 2008
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