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Ripple counters

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axro

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I can't remember where I read it but I remember seeing that to make sure a ripple counter starts at 0 you put a really small cap the reset pin to V+. And then a high ohm resistor to ground.

When you take power away from the circuit how does the cap drain?
 

ericgibbs

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Most Helpful Member
I can't remember where I read it but I remember seeing that to make sure a ripple counter starts at 0 you put a really small cap the reset pin to V+. And then a high ohm resistor to ground.

When you take power away from the circuit how does the cap drain?

hi,
As the +V falls to 0V the series resistor discharges the cap.

If you need to speed up the discharge add a small diode across the resistor.
 
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axro

New Member
hi,
As the +V falls to 0V the series resistor discharges the cap.

If you need to speed up the discharge add a small diode across the resistor.

Any chance you could post a small schematic? I'm trying a similiar thing in a circuit simulator and it's not working.
 

ericgibbs

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Most Helpful Member
Any chance you could post a small schematic? I'm trying a similiar thing in a circuit simulator and it's not working.

hi,
Which sim are you using.?

If its LTspice post your asc file.
 

ericgibbs

Well-Known Member
Most Helpful Member
hi,
Look at this example.:)

EDIT: the 2nd image is with a diode across the resistor, this reduces the discharge time also the negative swing at the input to the following device.
 

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ericgibbs

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I'm using this: Circuit Simulator Applet

I will post an image of what I was doing.

hi,
Viewed the link, OK

The important point to note, is that when testing this type of R/C circuit is that its actually a simple differentiater and so the applied test pulse voltage must have a 'fast' rise/fall time, if you want to see the RESET waveform.
 

axro

New Member
hi,
Viewed the link, OK

The important point to note, is that when testing this type of R/C circuit is that its actually a simple differentiater and so the applied test pulse voltage must have a 'fast' rise/fall time, if you want to see the RESET waveform.

Well I'm more worried about the discharge aspect. I'm not really going to use this for a ripple counter, it was just the example I remembered. I was going to use a capacitor as the momentery switch needed for a monostable 555 timer.

Here is my circuit, it's not the circuit i want, it's just a way of testing the discharge:

diode-jpg.34239
 

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ericgibbs

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axro;799189]Well I'm more worried about the discharge aspect. I'm not really going to use this for a ripple counter, it was just the example I remembered. I was going to use a capacitor as the momentery switch needed for a monostable 555 timer.

Here is my circuit, it's not the circuit i want, it's just a way of testing the discharge:


Hi
If that RED dot on your circuit is a n/o push switch.?? then as you push the switch the cap will charge, BUT as the switch opens the only discharge path for the cap is via the reversed biassed diode.

So it will [ with an ideal diode] take a very long time to self discharge.

Your test circuit is not a true analog of what happens in an actual circuit.

Try place a resistor in parallel with the cap, so it has a discharge path when the battery voltage is switched off.
 
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axro

New Member
The red dot is an LED :)

If I put a resistor in parallel with the cap, when the cap fills won't the resistor continue to power the circuit? In this case the LED?
 

ericgibbs

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The red dot is an LED :)

If I put a resistor in parallel with the cap, when the cap fills won't the resistor continue to power the circuit? In this case the LED?

Hi,
I would like to help but I cannot determine what you are trying to do.??

You did ask about ic counter reset pulses, derived from a cap and resistor across the +V and 0V rails,
which I answered by showing the charge and discharge pulses.

Now you are comparing this with a loop circuit with capacitor, battery and LED in circuit.???

What is the question you are asking.?:)
 
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axro

New Member
Ok sorry. Here is what I'm trying to do.

When a switch is turned on I want it to trigger a monostable 555 timer. Thats really all there is to it. I only want it to run once.

I was thinking of using a cap to provide the "pulse". But I must have been mistaken because to trigger a monostable you need to make the trigger go low.

I only want the monostable triggered once when the initial switch is turned.

So what would be the best way of doing this?
 

ericgibbs

Well-Known Member
Most Helpful Member
Ok sorry. Here is what I'm trying to do.

When a switch is turned on I want it to trigger a monostable 555 timer. Thats really all there is to it. I only want it to run once.

I was thinking of using a cap to provide the "pulse". But I must have been mistaken because to trigger a monostable you need to make the trigger go low.

I only want the monostable triggered once when the initial switch is turned.

So what would be the best way of doing this?

hi,
If you want a low going pulse, then swop over the cap and resistor, junction to the mono.

At power up the cap will hold the mono inp low while its charges up.

So its a resistor to +v and the cap to 0V. OK:)

EDIT: what type of mono is it, some are edge triggered and some level triggred.???
 
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axro

New Member
Level Trigger I believe. So in the example you gave me(which works in sims btw) how does the cap discharge after the main switch is turned off?
 

ericgibbs

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Level Trigger I believe. So in the example you gave me(which works in sims btw) how does the cap discharge after the main switch is turned off?

It discharges back thru the power rail and connected devices, dont forget your mono draws current from the rail.:)
 
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