I have a requirement for a timer circuit that can be mass-produced
cheaply with the following functionality:

? Adjustable ?off? period from approximately 15 minutes to approximately 6 hours
? Adjustable ?on? period from approximately 30 seconds to 5 minutes
? The adjustments ideally would be via a potentiometer
? Input voltage min 6V, max 12V DC

The circuit should be capable of driving a relay that will switch
power to a solenoid off and on.  I purchased a kit based on the 555
chip that promised adjustable times via potentiometers.  I built the
kit, and it does
work but the times are too short ? only a few seconds off and on, with
a few seconds in between.  The circuit diagram that I used can be
found at http://bardos.com.au/electronics/circuit%20board.jpg

My question is whether the components of this circuit-board can be
substituted to give me the adjustable off and on times that I am
seeking?  I can build these simple circuit-boards but do not have the
theoretical background to perform the calculations on resistance etc.

403 Access forbidden on your circuit diagram, btw...

http://www.uoguelph.ca/~antoon/gadgets/555/555.html has a nice
tutorial, including talking about the Monostable mode... nearly
halfway down the page...
In the VERY simple 9a diagram, it indicates that T = 1.1 x R x C (in
seconds) Your methods and results may vary.

Try using 1 MOhm or 2 MOhm potentiometers instead of the 200 kOhm
ones. This will give you a wider range of values to adjust the speed
of the 555 timer.

There would be an accurate, but costly solution which would involve a
counting circuitry, rotary switches, and a piezoelectrical crystal,
but I don't think it'd be adapted to your needs .

I don't seem to remember how to calculate an astable 555 with two
potentiometers like in your case .

The way I'd do it is with 2 LM555s or a LM556 chip (which is basically
two 555s, but is less costly than two 555s) . One 555 would be the
astable, and you could adjust that one with a potentiometer for the
OFF time . After the OFF time, it would give a really short pulse to
trigger the other 555 which would be driven in monostable mode . This
one would also have a potentiometer to control its length, and this
would be the one controlling the relay .


If someone knows how to get the right result with only one 555, please
post the calculations ; that diagram is confusing me for some reason .

Many thanks for comments so far - the issue with using multiple
processors is one of per-unit cost.  I've purchased a couple of one
and two Ohm pots and will post the impact on times in the next day or
so. If it fails, I'll try the twin processor idea.

According to the tutorial posted in the first comment, if T (seconds)
= R x C then using a 2 MOhm pot would require a capacitor of 11,000
micro-farads to give a range of 900 to 22000 seconds (~ 15 minutes to
6 hours). Is that correct, and is a capacitor with that rating
actually available?

Seems correct .

However, you'd have to settle with normalized values for your
components, multiples of 1,0 or 1,5 ; that is, 10,000 micro-F
capacitor instead of 11,000 .

A 10,000 micro-F capacitor is a pretty big one, though . I'd suggest
going with two 5,000uF [micro-farads] . If you want accuracy, you can
add a 1,000uF . Better accuracy, but adds to the bill as well .

Keep us updated on how it goes . Are you using LEDs to test the functioning ?

By the way, if you do get your hands on big capacitors, be careful of
polarities (+,-) . These little buggers tend to jump to your face if
you plug them wrong .

In theory you can buy 100,000,000 ohm potentiometers = 100 meg ohms,
which will extend the on and off time some, perhaps much, using
capacitors of a few thousand MFD. 22 meg ohm fixed resistors are
readily available, 1/10 th watt or more. In series with the
potentiometers the timing range will be tiny, but it will make a good
test for repeatability. The very high resistance may make the
repeatability poor, but it is worth a try, if you can find the very
high resistance potentiometers at a reasonable cost. A 2 watt rating
for the potentiometer and a 12 volt rating for the capacitor should be
enough. Applying 12 volts dc to your capacitors for a day or two may
reduce the leakage of the capacitors. The leakage becomes important
with very high resistance.  Neil

Got it working with all the assistance!  Final answer was a 33uf Cap
and 4mOhm pot for the 'on' time, which gave me 1sec to 1 min.  For the
'wait' time, two 2200uf Caps in parallel with 4mOhm pot which gave 2
min to 3 hours. Used a 2.7k resistor over the 12v with a dual color
LED - green for power, red for 'on'.  Many thanks!