Hello Folks,

I am using this IC with a 555 timer to trigger a series of solid state relays.
I tested my circuit on a seperate breadboard and everything worked fine: i.e. the solenoids fired at preset sequence. However, when I assembled everthing on the final board for installing into the control, I nothing works properly. The problem is, upon powerup, all the solenoids fire at once and now nothing fires in sequence. Do I have to tie the unused pins of the 4017 to ground?

 

A 4017 is a CMOS part. Floating CMOS inputs wreak havoc with deterministic behaviour. It would be a good place to start. Could you also post a schematic if that doesn't work.

 

I see 2 obvious possibilities: 1) you made a wiring error or 2) you blew out the cmos part with an electro-static discharge. I'd bet on #1, if I was a betting man.

it is a good idea to tie unused inputs to something (high or low depends - consult the datasheet) but if you didn't do it on the breadboard, it likely won't be an issue on the final assembly.

 

Did you
1. connect diodes across the solenoid coils to protect against the "back EMF"?

2. connect bypass capacitors across the supply pins of the 4017 and 555?

__________________
Len

 

No i didn't connect bypass caps. Are they caonnected in series or from ground to the supply input pins?

 

I can't agree with this. Floating CMOS inputs are totally unpredictable. You can literally wave your hand over a chip with floating input(s) and cause it to change state. They also can cause relatively high power dissipation by floating at a level that makes the pullup and the pulldown be on simultaneously.

__________________
Ron

 

well, we shall see when he solves it. My point was that grossly different behavior is likely to be a wiring error or blown chip. If he had said "I wave my hand and it behaves differently", we would be in agreement. notice also, I did say it's a good idea to tie them.

 

Philba, I didn't intend to start an argument. All I meant was "but if you didn't do it on the breadboard, it likely won't be an issue on the final assembly" sounds to me like he shouldn't worry about it. He definitely should.

__________________
Ron

 

They are connected across the Vcc and Gnd pins of the IC.

I agree with RonH, with CMOS devices, inputs should not be left open.

__________________
Len

 

no argument but it's a good idea to quote in context - it seems like you were saying that I said don't bother tying them. I did say it was a good idea. I still suspect wiring error.

 

Hi Guys,

I've been trying to find any wiring errors and as of yet have not found anything wrong. i suspect that the problem lies with the fact that i didn't use a cap on the ICs power source terminals. One thing that I should mention is that the circuit also contains 6 one shot devices, and a hex inverter before the drivers to the SSRs. I don't have any way to draw you a schematic, can anyone suggest a source for a schematic drawing program?

Mark

 

Diptrace at http://diptrace.com/ is quite good. It has a 30 day trial.

ExpressSch from http://www.expresspcb.com/ExpressPCB...c_software.htm is decent, and will get you by. Fortunatly, It is free.

EagleCad is decent also. A lot of people have it. http://www.cadsoft.de/freeware.htm. It has a freeware version that should suffice.

Hope this helps!

 

Hi Guys,

I was able to get the circuit to work by putting a capacitor across the power and ground terminals of the 4017 counter and 4069 hex inverter chips. Thanks for the advice, how does one determine the size of the capacitor to use? I had two 15 mfd electrolytic caps and they did the trick, however I know that there must be a way to properly size the cap to use.

 

The value of a supply bypass capacitor depends on the internal resistance of the power supply. If the power supply is a disposable battery then its internal resistance increases as it runs down so the bypass capacitor's value must be larger.

__________________
Uncle $crooge

 

Ah, thanks for the clarification audioguru! I had always been wondering how to choose bypass caps.