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Looking for a voltage ramp generator

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nre-the-8th

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Im looking for some help in designing a voltage ramp based around a 555 timer ic. The circuit is to be used for driving two or three cascaded LM3914 ic's in bar mode. I need the led's to light up sequentially and then, after a preset time, reset and start over again. Any help will be appreciated.
 
An LM3914 has equal-voltage steps. The capacitor in an ordinary 555 monostable charges in a curved ramp so the two do not go together well.
You could add a constant current source transistor to charge the capacitor in a straight ramp.
 
I've been out of the electronics world for a few years now and forgot more than I knew. In either case is the output still taken from the output pin of the 555 timer and can it also be used in the astable mode??
 
Another option would be to use a microcontroller.
 
Cough there it is, microcontrollers are pretty versitile and a single micro could do what the OP described.
But if you want to stick with analog then a xr2204 should be able to produce a saw wave.

edit oops I meant XR2206
 
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In astable mode, the ramp will run from Vcc/3 to 2*Vcc/3. If you use monostable mode, the ramp will run from 0V to 2*Vcc/3, but you will need another 555 (or use a 556, which is a dual 555) to trigger it. The ramp will be linear if you use a current source, as Audioguru pointed out. In either case, the ramp is on the timing capacitor, not on pin 3.
You can also use a 555 as the oscillator, and have it drive a separate switching transistor to reset the ramp. The advantage of this is you can get peak output of more or less than 2*Vcc/3. If you can spell out what you want in detail, someone here will probably design it for you. :D
 
Thank you for you replies. I am trying to cascade up to 10 LM3914 ic's together, in bar mode, to have a row of 100 led's light up in a slow running sequence. What I am having a problem with is a circuit that will generate the rising voltage needed to drive the LM3914's. Not too up to date on pic microprocessors as yet so wont be using them.
 
The datasheet for the LM555 shows a linear ramp generator circuit.
 
Hi Nre-the-8th,

I would like to suggest an alternative, a binary counter and a R2R
ladder network. Such a circuit could generate a staircase ramp which
should work equally well as a linear ramp. This circuit was based on
your first post when you needed only 30 steps. But if you add four
resistors you can expand the circuit to generate an (up to) 128 step
ramp if you also use Q4 and Q5. (Q11 is not available !)

on1aag.
 

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You might have just solved my problem. Thanks for that. I will have a go with that circuit and see if it does what I want it to.
 
If you want the ramp to be monotonic (constantly increasing), your resistors will need to be matched to around 0.5%. The absolute value isn't particularly important, but the matching is.
If not for the fact that you would need a current limiting resistor for each LED, I think the easiest solution is a shift register made up of 74HC164's. You get 8 bits per package, so it would take 13 of them to get 100 outputs, but you wouldn't need a ramp generator or the LM3914's. The advantages of the 3914 are built-in current limiting, and 10 outputs per package vs 8 for the HC164.
 
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