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Author Topic: Looking for timer with two inputs  (Read 4848 times)

Offline bbbaty

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Looking for timer with two inputs
« on: January 26, 2011, 06:39:39 AM »

I have thought of an experiment I want to conduct. I will need to have a timer device which would accept two DC inputs & output (undetermined voltage, but low, 12 volt or less) and switch between them at a variable rate from about one second to about 60 times per second. So when one input circuit is off, the other would be automatically on. I wouldn't need the timer to be run from the input current but could be run from a separate 12-volt current or plugged in to 110 household current.

Is there such a timer already on the market?

All the best,


Offline Paul-R

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Re: Looking for timer with two inputs
« Reply #1 on: January 26, 2011, 03:52:28 PM »
I wuold have thought that your circuit could be found in a book of 555
circuits. If you join this Yahoo group you might get your answer:

What is it for?

Offline MrMag

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Re: Looking for timer with two inputs
« Reply #2 on: January 26, 2011, 04:09:07 PM »
If I understand you correctly, yes there are timers you can buy that can do this. All of the industrial timers have a built in relay. It's just a matter of how you hook it up. The contacts are C (common) N.O.(normally open) N.C.(normally closed). Connect your two voltages to the N.O. and N.C. terminals. Your output will come out of the C terminal.

It would help to know a little more on what your application is. The only thing that may be a problem with this as there will be a short time, while the contacts are switching, where you will not have either voltage coming out. It's only milliseconds, but I'm not sure if it matters in your application.

There are many cheap industrial programmable timers in the market. Omron makes them and so do a lot of others.

Hope this helps

Offline mscoffman

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Re: Looking for timer with two inputs
« Reply #3 on: January 26, 2011, 04:47:20 PM »
On page two of this .pdf file Web Link is a NE555 circuit diagram - you
need to think carefully about what you mean by "has two inputs" and
draw a schematic diagram of what you want. The desire to "Have
everything in one silicon package" shouldn't preclude you from having
the circuit function that you want to use.

The NE555 inputs are trigger edge, enable level, voltage control input and
the timing RC. The NE555 construct is *not* what is known as a long
range timer so has a downside LF limit of about 1.0 cycle second with
careful selection of the RC components. For a longer range timer a pure
digital circuit approach with a clock divider is recommended. It's possible
to fully synthesise the NE555 timer function by using the circuit blocks
shown in the .pdf file. Depending on what you want the NE555 already
has two inputs with the trigger and enable inputs, though they operate


When is comes to switching utility voltages and power you need a buffer
amplifier known as a SSR solid state relay. Or one can design an SSR
from triac electronic component for cost sensitive apps. Note that the
low frequency limit of the NE555 is somewhat in conflict with high power
switching, as an SSR will often wait until an input AC signal zero voltage
crossing before changing states. If you want a higher frequency signal
than 50/60 Hz. you have to start with 110VDC, for example, and find
an SSR that can do what you want.