Should you build this project? Hey, the answer is already given for you!
Can't make a decision?
Worried that if you make the wrong choice you'll get the blame? Well,
here's your saviour: press the button and the decision is made for you.
And if that decision turns out to be the wrong one, you can
always say to your mum / dad / teacher / partner / boss / etc, "Look, it's not my fault.
That was the decision the box made . . ."
How it works
There are two main parts to this project - an oscillator (based
on IC1) and a LED driver (based on IC2).
In a nutshell, when you press the pushbutton, power is supplied
to the circuit and the "reservoir" capacitor on the main supply rail charges to
the battery voltage (3V). At the same time, the resistor and capacitor around
one of the Schmitt NAND gates (IC1c) cause it to oscillate.
The word NAND is a contraction of NOT & AND. The "AND" part
means that both inputs to the gate need to be a logic "high" for the gate to
operate and the "NOT" means the output is opposite, or inverted, to the input.
There is a "truth table" shown in Table 1 which shows what happens to the
output, depending on what is occurring at the input.
The "Schmitt" part of the name refers to a feature of the
threshold points, or triggering, of the gate. The voltage levels at which it
triggers, either low or high, are quite precise but more importantly, are widely
separated. This makes a Schmitt trigger more immune to noisy triggering
As you can see, the two inputs to the gate are connected
together, effectively turning it into an inverter. As such, the input and output
can never be the same state - when the input is high, the output must be low and
When you press and hold the button, the IC is powered up but at
that instant the inputs are in a low state (because the 1μF capacitor is not charged).
Therefore the output is high.
The capacitor then starts to charge via the 68kΩ resistor from output to
When the capacitor voltage passes the gate's upper threshold
voltage (ie, the input goes high), the output goes low. The capacitor then
starts to discharge, the voltage eventually dropping below the gate's lower
threshold voltage. The output then goes high again.
This keeps happening as long as power is applied to the
circuit. It's called a "relaxation oscillator" and is a very easy way to make
any form of pulse generator.
The frequency at which it operates is determined by the values
of the resistor and capacitor. The formula is 1/0.55 x RC, where R is in ohms
and C is in Farads (note that - Farads, not microfarads).
Therefore if the resistor is exactly 68,000 ohms
(unlikely!) and the capacitor is exactly 1μF (even more unlikely!), the frequency
of this oscillator circuit will be 1/ 0.55 x 68,000 x 0.000001, or 1/0.0374, or
approximately 26Hz (actually 26.7Hz).
Why did we say it was unlikely that the resistor and capacitor
wouldn't be exactly what their marked value said?
If the resistor has a 1% tolerance, its actual value could be
anywhere from 99% of 68,000 ohms (67,320Ω) to 101% (68,680Ω). And capacitors normally
have a much wider tolerance - as much as 20% or more. So you can see we are not
talking exact values in a simple circuit such as this.
Kept up so far?
OK, here's a quick quiz to see if you've kept up with us so
far. If we increased the resistor to 100kΩ and decreased the capacitor to 470nF,
what would the oscillator frequency be?
If you answered about 36Hz, well done. If you had the right
digits but were out by several factors of 10, it's time to brush up on your
nanofarads, microfarads and Farads! (1nF= 0.000000001F; 1μF = 0.000001F).
So we have an oscillator running at 26Hz or thereabouts. Its
output is a square wave with a "duty cycle" of 50% - that means its "high" state
is the same length of time as its "low" state.
The square wave is fed into a second NAND gate, IC1d (also
connected as an inverter) which ensures it is nice and clean. This acts as a
"buffer", making sure that any load connected to the gate won't interfere with
the charging/discharging cycle of the capacitor in the oscillator.
Everything except the battery is mounted on the PC board. Provision is made for either a supercap or a smaller electro.
It is then fed into yet another NAND gate, IC1b, this time
wired as a true NAND. In a NAND gate, the output will be low only if both inputs
are high. If either or both inputs are low, the output will be high.
Here, one of the inputs (pin 6) is connected to the pushbutton
switch via a 4.7kΩ resistor. Normally this input is at a logic "low", courtesy of the
to earth. But when the pushbutton is pressed, it is taken to a logic "high".
When a logic "high" is also present at pin 5 (when the output of IC1d goes
high), IC1b's output will go low.
Conversely, when either input goes low (because the pushbutton
is released or when IC1d's output goes low) the output goes high.
But IC1d's output (and IC1b's pin 5 input) continues to go high
and low, courtesy of the oscillator. While that pushbutton remains pressed, IC1b
allows the pulse train through.
Finally, the pulse train is put through yet another NAND gate
(IC1a), again wired as an inverter.
To be truthful, this final pulse inversion is not necessary but
we had a spare gate in the IC anyway (it's a "quad" NAND
Into the counter
The square wave output from this series of gates is fed to a
4017 decade counter. Now you might be thinking, "how come a decade counter -
doesn't that mean ten?" And you'd be right.
But the 4017 is a clever device - it can count to one, to two,
to three . . . and so on, all the way up to ten. All you have to do is "reset"
it when it gets to the number you want it to count to.
On the circuit diagram, you will note that Q4 (pin 10) and MR
(pin 15) are connected. Q4 goes high on the fifth count (after Q0, then Q1, then
Q2, then Q3). When Q4 goes high, it tells the reset pin (15) to reset the
counter to zero and start all over again.
Those other outputs we mentioned (Q0-Q3) are each connected,
via a transistor, to a LED. As each goes high in turn, it turns the associated
transistor on, which causes its LED, between emitter and earth, to light.
And here's what it looks like assembled. This is an early prototype - some components have been moved slightly.
Because of the speed of the oscillator (26Hz, remember), the
four LEDs flash much faster than the eye can follow, so all look like they are
How fast do they flash? That's easy: 26/4 or about 6.5Hz. That
means that there are six-and-a-half cycles of the lamps each second, faster than
the eye can follow.
Incidentally, IC2 has its pin 13 input tied low and its pin 14
input used as the clock input. What this does is make the IC respond to
low-to-high logic transistions.
Now, what happens when you let go of the pushbutton?
The battery is no longer connected to the circuit. While there
is still a supply line to the counter circuit (courtesy of the charged
"reservoir" capacitor), one of IC1b's inputs is isolated from the supply by the
series diode. So the pulses stop.
But as we said, the counter section still has a supply, as do
the collectors of the four transistors. So that section of the circuit continues
working. Whatever output of IC2 that was high at the instant that the pulses
stopped remains high, holding on its particular transistor and of course LED, at
least for a short time while the capacitor discharges.
So one LED - and only one LED - remains lit. And which
particular LED is lit is completely random, depending entirely on when you
released the pushbutton.
Due to the fact that the oscillator is running at 26Hz, it is
impossible for you to let go the button to achieve a particular result. You
would have to be able to not only accurately judge periods of 40 thousandths of
a second but also release the button at the exact point in time required. The
person who can do that hasn't yet been born!
About that capacitor
We mentioned before that a "reservoir" capacitor connected to
the supply line charges when the pushbutton is pressed and discharges through
the circuit when it is released.
The assembled project, using the 3300μF electrolytic and the Jaycar box. With a supercap
DSE box is required.
Eventually, the point is reached where the charge is too low to
push enough current through the LED, so it dies. You can see this happen: the
LED doesn't suddenly go out but gradually gets dimmer.
The time it takes to go completely out depends entirely on the
size of the capacitor used to hold the charge. With a 3300μF capacitor, it lasts for a
little over a second - just long enough for you to get an answer - but it could
be longer! How?
You're probably one step ahead of us by now - with a larger
capacitor, of course.
How long? How does 30 seconds sound? We replaced the
with a so-called supercap-acitor, rated at 0.5F. Yes, that's right - half a
Farad, or 500,000μF.
These capacitors are usually used for much the same reason as
we use it here - to hold a charge for a short time in the absence of power (eg,
when there is a power supply dropout or glitch).
They're not as cheap as "ordinary" electros - probably about $4
each or so - but they really do hold a charge. Whether you want to use one of
these or go for the much cheaper 3300μF is entirely up to you - and your
There is one other "little" problem with using a supercap -
it's not so little. You may need to use a slightly larger case to fit it in. But
we'll look at this further on.
The 3300μF will normally be rated at 16V while the supercap is much
lower - 5.5V is common. But with a 3V supply rail, 5.5V is plenty.
Another thing you could do is use some superbright LEDs in
place of the standard LEDs. These are more expensive - perhaps three or four
times the price as standard LEDs - but are much more efficient at converting
current into light so they are brighter.
Same-size artwork for the PC board and front panel.
All components are mounted on a single PC board measuring 46 x
63mm and coded 08112021. With the 3300μF electro, it just fits into a small (83
x 55 x 28mm) zippy box, sitting on top of the 2 x AA battery holder, with the
pushbutton switch and four LEDs just poking through the top.
With the supercap, you'll need a larger case.
Begin construction by comparing your PC board with the
published pattern. These days, problems with commercially-made boards are very
rare but it is good practice to check every board before attacking it with your
Solder in the resistors first and use two of the resistor lead
off-cuts for the two links on the board. Then put the diode in (the right way
around!). Next are the four transistors. The transistors mount down on the board
as far as they will go.
The two power supply PC stakes, or pins, can go in now. These
actually mount upside-down to the way we normally use them - their longer length
goes on the copper side of the PC board. Don't solder the battery connections
Next, solder in the capacitors. First to go in is the 100nF
polyester, followed by the 1μF timing capacitor.
Two points to note here: first, make sure you get the electro's
polarity right (the "-" goes to the outside of the PC board) and second, leave
enough lead length so that it can lie flat on the board. Better still, bend the
leads down 90 before soldering it in.
The supply "reservoir" capacitor goes in next. If you are using
electrolytic, it goes in the same way as the 1μF timing capacitor (ie, bent over 90
If you are using a supercap, it goes straight down, as you would normally mount
a capacitor on a PC board.
When you photocopy the front panel, make two copies and you can use one as a drilling template.
Now solder in the four LEDs, taking care again with polarity.
If you are using a supercap, there needs to be a good 3-5mm between the top of
the capacitor and the top of the LEDs, so that they can poke through the case
Next solder in the two ICs. Both orient the same way (notch
towards the centre of the PC board) but of course they must go in their right
spots. When soldering their pins, make sure you don't bridge solder between
them. The pins are very close together and it's easy to do.
Finally, solder in the pushbutton switch. It goes in so that
the flat on its body runs parallel with the longer sides of the PC board. It can
easily fit the other way around but if you put it in like this, all you'll have
will be a dead short!
Apart from the battery connections, your board is now complete.
Give it a good check to make sure you haven't got any shorts, solder bridges,
dry joints, etc.
If everything checks out, solder on the battery leads (but
don't have the batteries in place when you do). The black lead is the one
closest to the corner of the board.
Checking it out
The only easy way to check it out is to use it! Pop the
batteries into their holder (the right way around). Hopefully, absolutely
nothing happens (ie, no LEDs light). If they do, you have a short somewhere.
Now press the push-button switch - all the LEDs should come on
together. So far, so good. Let the switch go and hopefully one LED is on and all
others are off. Wait a while (depending on which capacitor you've used) and the
LED should dim and die.
If so - it's finished, apart from mounting it in its
A good case
Table 1: the "truth table" for a NAND gate. Only when both inputs are high is the output low.
If you've used the 3300μF capacitor, the board should fit inside
the Jaycar HB6015 jiffy box (or similar) with the battery holder underneath.
If you've used a supercap, it's likely that it will be just a
smidgeon too high, meaning you won't be able to get the lid on!
Fortunately, there is an alternative box, the Dick Smith
Electronics H-2874, which is 40mm high (compared to 28mm high). So that will
give you all the clearance you need.
But remember that the LEDs will need to be mounted higher and
you may even need to mount the push-button switch on tiny "stilts" (resistor
pigtail offcuts are ideal).
The front panel label will fit either box - glue the label to
the lid and drill your holes to suit.
If you find the board slops around inside the case, put a small
piece of foam plastic between it and the battery holder to force it right up
against the lid.
Decision time . . .
Now, there's a decision to be made. Do I use the supercap or
Gee, I wish I had something to help me decide!
Parts List - Decision Maker
1 PC board, 46 x 63mm, coded 08112021
1 plastic utility case, either 83 x 54 x 31mm (eg, Jaycar HB6015) or 85 x 56
x 40mm (eg DSE H2874) - see text
1 SPST momentary action pushbutton switch, PC mounting (Jaycar SP-0720,
Altronics S1094 or similar)
1 2 x AA battery holder (with battery snap if required)
2 PC stakes
1 4093 quad NAND gate (IC1)
1 4017 decade counter (IC2)
4 BC548 transistors (or similar general purpose NPN) (Q1-Q4)
1 1N4001 power diode (or similar general purpose power diode) (D1)
4 red LEDs, 5mm (normal or ultrabrite - see text) (LED1-LED4)
16VW electrolytic or 1 0.5F 5.5VW supercap
1 1μF 16VW
1 100nF MKT polyester
Resistors (0.25W, 1%)