Yes-No Indicator Has Zero Standby Current

This circuit produces a
random “Yes” or “No” with a single button press – indicated by the
illumination of a red or green LED. The
circuit has two advantages over similar circuits. First, it uses just a
single momentary contact pushbutton, so no on-off switch is required.
When the pushbutton is pressed, an oscillator comprising the 10nF
capacitor and 22kΩ resistor at pins 1 & 2 is almost immediately
stopped by FET Q1, which pulls the
oscillator’s timing capacitor to the positive rail. However, the 220nF
capacitor and 470kΩ resistor in the gate circuit of Q1 introduce a tenth
of a second’s delay, so that about 250 oscillations take place before
the clock is stopped.

Due to variations in charge on the circuit’s capacitors, as well as
voltage and temperature variations, and the unpredictability of when the
pushbutton will be pressed, randomness is assured. The circuit has a
high degree of randomness because it takes advantage of a near-perfect
complementary square waveform at pins 10 and 11 of the 4047 IC. The
oscillator frequency (available at pin 13) is passed through an internal
divide-by-2 circuit in the 4047. This appears at pin 10 (Q), and is
inverted at pin 11 (Q-bar), thus assuring a near perfect 50:50 duty
cycle for the two LEDs.

Circuit diagram:

Yes No Indicator Circuit

Yes-No Indicator Circuit Diagram


However, that the “impartiality” of the circuit is partly contingent
on the value of the 10nF capacitor and on a reasonably equal current
flow through both LEDs. Over five trials, the
Yes-No Indicator scored 142 Yes, 158 No, with Yes falling behind No in
the fourth trial. Because the circuit only works while switch S1 is
pressed, standby current is zero, therefore a miniature 12V battery may
be used to power it. In this case the circuit could be used thousands of
times before the battery would run flat. The circuit has a further
potential use. If the LEDs are omitted and a
piezo (capacitive) sounder is wired directly to pins 10 and 11, it will
produce a loud beep when equipment is turned on, and will continue to
draw less than 0.5mA until it is switched off. The frequency of the beep
may be changed by altering the value of the 10nF capacitor and its
duration by altering the value of the 220nF capacitor.

Author: Thomas Scarborough – Copyright: Silicon Chip Electronics

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