completely. In this way,
I can turn the timer off
or on at will.
Note that the timer
is based on a four-hour
cycle. This can be
changed in the program code to whatever
you wish. Later, I will
explain in more detail
how to do this.
Let’s take a look at
the circuit. Having only
one control keeps the
number of parts to a
minimum. Most of the components are actually used in the
In order to keep the construction details simple, I will look
at each sub-system individually. You can make you own decisions based on the components you have in your junk box.
Other than the knob itself, I only needed a single LED
to display the state of the control relay.
Any LED will work. I also needed a resistor, used to
drop the 5 volt regulator, down to an acceptable level, as
shown in Schematic 3.
Schematic 1 shows the power system. I used a very
small low-power 300 mA transformer to supply both the
power to the microcontroller circuit and to the controller
relay. It’s important that you use a 12 volt transformer so
that you can power the controller relay from a tap off the
rectifier bridge. If the relay you decide to use draws more
than 250 mA, you will need to use a larger transformer.
U1 is a simple 1 amp 7805 regulator. You can use just
about any 5 volt regulator to power the logic circuit. The
two 470 uF capacitors are very important; if you have larger capacitors in your junk box, you can use those.
I used the Nemesis microcontroller because it’s small,
cheap, and you don’t need an expensive programmer to
program it. The Nemesis has seven A-to-D ports that I
used to read the position of the knob, and plenty of ports
for driving the indicator and control system. Other
than that, I only needed a single .1 uF and 10K resistor for
Schematic 2 shows how I connected R2, the control
knob. Any potentiometer from 1K to 100K can be used.
Just make sure it is not an audio taper.
In order to program the Nemesis, I also needed a
RS232 interface. You can build one with a MAX232 chip or
purchase one for less than $10 from the Kronos Robotics
website ( www.kronosrobotics.com). The software for
programming the Nemesis is free and can also be downloaded from the website.
The control system consists of a TIP 41 Darlington