CD4520 Step generator

[odoardo]

Hi all,
I have a circuit that use an IC CD4520 (Binary up counter) to generate 7 steps as the diagram in the link (CD4520 can generate till 15 steps connecting other two resistors on pin 6 and connecting the operational input to the joint of these resistors).
http://www.grix.it/UserFiles/odoardo...e%20CD4520.jpg
I need that the circuit generate only one step or two steps or three till 10 steps selecting them with a switch.
Someone can help me to a right modification of the circuit.
Thanks
Odo

[mik3]

Originally Posted by odoardo

Hi all,
I have a circuit that use an IC CD4520 (Binary up counter) to generate 7 steps as the diagram in the link (CD4520 can generate till 15 steps connecting other two resistors on pin 6 and connecting the operational input to the joint of these resistors).
http://www.grix.it/UserFiles/odoardo...e%20CD4520.jpg
I need that the circuit generate only one step or two steps or three till 10 steps selecting them with a switch.
Someone can help me to a right modification of the circuit.
Thanks
Odo

Take a second op-amp (use it as a comparator) and connect the one of its inputs to Vout (according to the diagram) and the other input to a potentiometer. Then connect the output of the new op-amp to the reset pin of the counter (take care of the value of the voltage needed to reset it and if you require current limiting resistors). By varying the potentiometer you will be able to choose at what voltage (step) the op-amp will reset the counter.

[odoardo]

Thank very much,
so the circuit become this:
http://www.grix.it/UserFiles/odoardo...20CD4520_m.jpg
I'll test tomorrow.

[mik3]

Originally Posted by odoardo

Thank very much,
so the circuit become this:
http://www.grix.it/UserFiles/odoardo...20CD4520_m.jpg
I'll test tomorrow.

yes thats right but why you use the 1M resistor? you dont need it. I think this resistor will cause problems like hysterysis.

[SgtWookie]

It's not recommended to run an opamp in open-loop mode continuously, as the output will be in saturation. Texas Instruments specifically recommends against this.

If you want to run open loop, then use a comparator - that's what they're designed for.

[odoardo]

I used opamp because in the circuit there is just one not used.
Really the 1M don't need, removeded.
Now the circuit works fine from one to 15 steps, by the resistor of 39K connected in series of the pot and +15V reduce to 10 steps max of course connecting the other two resistors to pin6 of CD4520).
Just to complete the information this circuit is used on a curve tracer.
Thank to all for the best suggestions
Odoardo

[mik3]

Originally Posted by SgtWookie

It's not recommended to run an opamp in open-loop mode continuously, as the output will be in saturation. Texas Instruments specifically recommends against this.

If you want to run open loop, then use a comparator - that's what they're designed for.

But i think that the 1M resistor helps the op-amp to go into deep saturation because it is placed from the output to the non-inverting input (positive feedback).