2 contact latching switch - to 3 contacts

[DocStein99]

I'm trying to build something with this 4017 decade counter, using the push button switches I picked out. I just realized there are only 2 contacts on the push button, and there is no contact for the OFF position of the switch. It's just a switch with 2 contacts that short when the button is down in the latch position.

I'm confused, trying to figure out how to make this work. I need to send a signal through a diode back to the RESET pin of the 4017 when the button is OFF position, and disconnect that signal when the button is toggled down. Does anyone have advice?

[trevortjes]

What is it exactly what you want to make and what should the switch do? Just reset the decade counter?

[DocStein99]

Yes - to reset the decade counter. It just takes a +5v sends through a diode to the reset pin. When the switch is engaged, it will take the +5v and send that to a potentiometer.

[MervC]

The simplest way to obtain a manual reset on a CD4017 with the SPST switch you wish to employ is by applying Vdd to one side of the switch and the reset pin to the other side. Then place a 1k resistor to the junction of the switch and the reset pin to ground. Assuming the switch is of a normally open configuration, when the switch is pressed it applies Vdd to the reset pin and resets the Johnson counter of the IC. Releasing the switch would permit normal operation of the counter.

[DocStein99]

MervC:
The simplest way to obtain a manual reset on a CD4017 with the SPST switch you wish to employ is by applying Vdd to one side of the switch and the reset pin to the other side. Then place a 1k resistor to the junction of the switch and the reset pin to ground. Assuming the switch is of a normally open configuration, when the switch is pressed it applies Vdd to the reset pin and resets the Johnson counter of the IC. Releasing the switch would permit normal operation of the counter.

Ok. If I understand you correctly - the switch will toggle the reset on the 4017 by closing the circuit between Vdd and Reset pin. When the circuit is NOT closed, the Vdd pulse will just flow from that line to the rest of the circuit. Thank you for the advice.

My only concern now I guess is there would be a the same pulse sent to the hardwired circuit, when the reset line is triggered. My first thought would be add a 10uf capacitor on the circuit to mute that very short pulse.

[TomGeorge]

Hi,
Did you google 4017 reset circuits

Look in image search, there are many using a cap and NC or NO switch.

Tom...

[MervC]

I've attached a simple schematic of the manual reset ckt. I described. As you can see, the reset pin of the 4017 would shift state from low to high when the switch is depressed applying Vdd to the junction of the switch, reset pin and the 1K resistor with Vdd being dropped across the resistor. When the switch is released, Vdd would be removed from that same junction and the reset pin would return to ground potential.

There is no need for a 10uf cap at all. If you have other things going on with your circuit involving that switch, you will have to supply a schematic and/or a detailed explanation of what your expectations are.

[DocStein99]

I attached the file I'm working with to give you an example. I haven't wired it to a board yet.

I am testing the circuit on a breadboard, without the POTs just using LEDs to see the circuit run. Once I cross the line over from one of the leads to reset, there is a very short visible pulse to LED connected to the circuit before the reset picks up. It will probably go away if there is a load on the circuit - with the pot LED and diode (which I will test next).

[MervC]

I looked over your schematic, and immediately saw the first error. The CD4017 is a CMos device and it's output cannot sink or source the current demanded by those LED's. I'll include a portion of the datasheet for the counter for you to look over. You'll see that each output can only source 1ma typically with a Vdd of 5V.

You'll have to interface with a driver that can deliver the current required for whatever the TOTAL current of each leg; transistors, bus driver, etc.

I've gotta go right now to have a glass of wine with my wife, but will further review you project schematic later. Can you give me a better description of the objective of your project and what it's supposed to do?

BTW, another thing to think about/consider is never leave inputs floating like the reset input.

[MarkT]

DocStein99:
I'm trying to build something with this 4017 decade counter, using the push button switches I picked out. I just realized there are only 2 contacts on the push button, and there is no contact for the OFF position of the switch. It's just a switch with 2 contacts that short when the button is down in the latch position.

I'm confused, trying to figure out how to make this work. I need to send a signal through a diode back to the RESET pin of the 4017 when the button is OFF position, and disconnect that signal when the button is toggled down. Does anyone have advice?

You have momentary SPST switches, but want SPDT switches. Erm, perhaps get the right kind of switch?

[DocStein99]

MarkT:
You have momentary SPST switches, but want SPDT switches. Erm, perhaps get the right kind of switch?

I do not have momentary switches. They are lighted LED push button latching switches. There are 4 contacts. 2 for the LED and 2 for the toggle button. The button pressed-down is CONNECTED state for the switch.

[DocStein99]

MervC:
I looked over your schematic, and immediately saw the first error. The CD4017 is a CMos device and it's output cannot sink or source the current demanded by those LED's. I'll include a portion of the datasheet for the counter for you to look over. You'll see that each output can only source 1ma typically with a Vdd of 5V.

You'll have to interface with a driver that can deliver the current required for whatever the TOTAL current of each leg; transistors, bus driver, etc.

I've gotta go right now to have a glass of wine with my wife, but will further review you project schematic later. Can you give me a better description of the objective of your project and what it's supposed to do?

BTW, another thing to think about/consider is never leave inputs floating like the reset input.

I think what your telling me is that I must use transistors in that circuit, just to drive the LED's and carry that voltage over to the other side of the project, where it connects to 556 timer to produce a frequency output. There were no transistors mentioned in any of the project examples I found for what I'm doing. I can wire them in of course.

I am trying to build an audio sequencer (based off circuit diagram project I found). It is tied into a 555 and 556 timers. Each leg of the decade counter I tune the POT for a different frequency to make a 8 channel sequence noise-maker. The 555 counter is the timer to adjust the frequency of the decade counter. The 556 timer is the noise-maker that accepts the signal back from the 4017 counter.

At 1ma, seems impossible. The circuit I have on my solderless breadboard powered by 5 volts. LED's and 1k resistors seems to work correctly, which would exceed the 1ma of current.

[MervC]

DocStein99:
I think what your telling me is that I must use transistors in that circuit, just to drive the LED's and carry that voltage over to the other side of the project, where it connects to 556 timer to produce a frequency output. There were no transistors mentioned in any of the project examples I found for what I'm doing. I can wire them in of course.

Actually, I'm not telling you anything other than the circuit is poorly designed with little or no consideration given toward the basic operating parameters of the IC employed. IF you wish to have a trouble free build, you must stay within the design parameters of the parts used in the design; that was the point I tried to make.

Voltage is NOT carried over anything. Simply put, voltage is dropped/reduced as a result of being applied across a load, resistance, an impendence, resulting in current flow. OR in other words, V = IR in terms of the voltage dropped across whatever load, simple or complex.

I see no other chips in the schematic you supplied so there is no way to evaluate your entire project at this time.

DocStein99:
At 1ma, seems impossible. The circuit I have on my solderless breadboard powered by 5 volts. LED's and 1k resistors seems to work correctly, which would exceed the 1ma of current.

The 1ma typical @ Vdd=5V cited from the datasheet is the current the manufacturer guarantees as a source current for the outputs with criteria. It's not the ABSOLUTE limit but rather the SAFE and GUARANTEED current for every device produced WITHIN the boundaries described. You'll have to take that issue up with RCA who developed the CD 4000 series back in the late 1960's.

You say your components "seems to work correctly", but what have you DONE to evaluate their operational status? I have done a simulation of one output of your circuit with LT Spice, of which I'll attach the circuit as tested and modeled by the program using the spice models on hand along with a stack of three graphs showing the output current when the output goes high displaying the source current of that simple series circuit and the voltage at the two relevant points.

The current when the output is sourcing is ~2.21ma far in excess of the spec'ed 1ma @ 25deg C. Voh of Q0 is ~3.89V well below the spec'ed 4.6V for Vdd=5v at the spec'ed 1ma Ioh. The voltage drop across R6 is ~1.12V. See the TI 2004 datasheet page 3 for verification of that data. (http://www.ti.com/lit/ds/symlink/cd4017b.pdf)

I wasn't able to verify the part ID of the LED's on your schematic because the were far to small, but I figured from the 510 ohm limit R you were going for about 10ma for the LED's with the 5V Vdd. Because your LED's had some illumination with just over 2ma of drive is not at all surprising, but take one of those same LED's connect the anode to 5V in series with one of your 510 ohm R's to ground and compare the intensity of the LED with those driven by the 4017. You will see a big difference

Bottom line is you need to interface the counter with a transistor array, line driver or buffer that can handle the current. The cheap, easy solution is a 74HC244 octal buffer for your 8 outputs. It's about $0.50 a copy here in the States at either Mouser or Digi-Key by the unit. It is capable of driving a given line at +/- 35ma. If you go that route, be sure to deal with BOTH enables which are active LOW.

I hope this clears up some of your confusion regarding the importance of staying well within the manufacture's spec'ed criteria.

[DocStein99]

When I say the circuit "seems to work correctly", I simply mean that it's laid out on a solderless breadboard, with standard 5mm led's and 1k resistors to ground. I can visually see each LED cycle through the pattern. So that's the extent of my detailed examination - it looks like it worked, and functioned how I expected to see, from what I read about how the chip is expected to work. The project originates from:

a very old magazine article

Is RCA still in business anymore? I'm going to call their human resources department tomorrow and demand some disciplinary action be taken for the designer of this crazy chip. I can't power the metal halide runway lights for a commercial aircraft landing strip, if this lame thing will only output 1ma of current. That's just not acceptable.... (just kidding)...

Only one of these legs are active at any given time. So a transistor, or array for each leg seems like extra circuity to me. I understand what your writing. I will study this a bit further and come up with something that I can run, and hopefully not void the warranty on the chip and cause me to file complaints and class action suits for the 10 chips I paid about $1.50 total.