4017 based sequencer, help with VC clock inhibit - *solved*

[seaweedfactory]

This was solved by using the two op-amps in a LM358 as a comparator followed by a non-inverting amplifier.

Hello all. I've been trying to build a simple sequencer base around the 4017 and it's going quite well. Initially I had problems with the reset circuit, but that was cleared up with a circuit from Ken Stone:

http://www.cgs.synth.net/modules/gate_sequencer2.html

I used the op-amp based buffer for the clock and reset inputs and it works great! I substituted a TL072 for the LM358. Originally I had tired to use fonik's baby 10 mini sequencer schematic for the 2N3904 buffer, but I could not get the reset function to work. That schematic is available here:

http://www.modular.fonik.de/Page22.html

I have a switch which can run the sequencer in a looping mode or freeze the sequencer on the last step until a reset pulse is received. The switch connects output 8 to the clock inhibit input via a diode. I am using a 100K pull-down resistor on the clock inhibit as well. This works great and freezes on the step until reset, as it should.

I am building this sequencer into some previously drilled panel space and have an extra input left. I thought it would be great to use this input as voltage control over the clock inhibit. However, I just can't seem to find a buffer that will switch the feature on. I tried using both of the buffer circuits above to no avail. Connecting a gate signal directly to the clock inhibit does not work and produces weird behavior in the form of step skipping, which is neat but not what I am trying to produce.

Any ideas on gating the clock inhibit input on a 4017?

Thanks everybody!

[fonik]

the clock enable is ON when tied to ground, and OFF when high (inhibit). when i take a look at the datasheet then i see that the clock enable is nothing but a non-inverted input to an AND gate, where the clock is fed inverted.
the ken stone circuit shows comparators. this is good, because the 4017 need more than 1/2 of the supply voltage to clock (2/3 IIRC?).

so i cannot see why a comparator shouldn't work, all the more when you used a pulldown resistor. just use the same configuration as for the clock's comparator. (or am i missing something?)

[seaweedfactory]

Thanks for replying, I was above to give up on this.

I had used the exact same circuit for the clock and it seemed to work fine for the clock but no the inhibit input.

However, I'll give the comparator approach another try. Do you think using TL072 with the single supply may not be enough voltage to activate the AND gate within the 4017? I'll measure the output of the comparator next time through.

Is it possible that some extra buffering is needed between the output of the comparator and the inhibit input?

[Dragon's Lair]

When you use the TL072 with a single supply, you create a "virtual" ground that is half of your supply voltage, so the op amp never actually goes to ground, try using an op amp that is designed for single supply use like the LM358 and see if that helps.

[seaweedfactory]

I dug in the parts and found a LM358 to try the comparator method again. I was not able to get the switching to work for the inhibit input. I measured the output of the comparator, it was 5.22V for my 15V system. The op-amp is switching when it is sent a square wave from an LFO, but I don't think the voltage is high enough. Any ideas on increasing the voltage coming out of the comparator? Maybe I could try switching on a transistor after the comparator output or run the output through the second stage of the LM358 to provide additional gain?

[seaweedfactory]

It worked after running the output of the comparator through a non-inverting amplifier built from the other half of the LM358.

The comparator goes from 2.5V to 12V, but it seems to be good enough for switching the clock inhibit input. Any ideas on getting it down to 0V, maybe biasing? There is also a glitch in which some steps are skipped when the clock is going very quickly, but I think I can live with it.