74ls192 circuit with relays resets display help

Thread Starter

rmitch

Joined May 24, 2008
24
I'll explain best I can. I have a bandsaw mill I built and I'm working on a project where I can select a measurement by 1/8th of an inch and full inches up or down. I select a measurement and push an up or down button which runs a 12v motor to my selected measurement and stops. Hope that makes sense.
So my measurements are set by push buttons. Which are sent to a picaxe chip. With each push of a button the picaxe does some calculations, and sends pulses to the 74194 counter, 7447 decoder then the 7 seg displays, displaying my desired length I wish to move. No problems yet, this works fine. I then push a button for up or down, which goes to the picaxe then out to a small relay, one relay for up, one for down. Still no problems yet. Works good on my proto board. Where I do have a problem is when I hook it up to the 12volt reversing relay of the drive motor. It all works good. Except for when the larger reversing relay activates my displays reset to 0. Some reason they reset on activation going up, and reset on deactivation going down. I do have flybacks the relays on the circuit board, and I have triedflybacks on the reversing relay. I bought a cheap oscilloscope to find spikes on the 74192 reset pin and I was unsuccessful. Not sure if I was using the unit right but I could find the pulses on the count pin of the 74192. I'm Iot sure what more to do. IMG_20190605_194852338.jpg IMG_20190605_194726340.jpg
 

dl324

Joined Mar 30, 2015
11,861
How much capacitance do you have on the input and output of the 5V regulator? The 5V relays didn't cause any problems?

I noticed you're using some NTE. Unless you're getting them for free, they're way overpriced.
 

Sensacell

Joined Jun 19, 2012
2,624
Are you switching the relay while current is flowing in the motor?

Look at where the ground currents flow, if the relays draw high current, consider a 'star' ground for the return path.
 

JohnInTX

Joined Jun 26, 2012
4,341
It looks like your are pulling CLR (pins 14) down with the resistor (220?). CLR is active high. With TTL, active high inputs have much less noise margin than active low (like LOAD/). It doesn't take much noise to get into the grey input voltage area (which is interpreted as a high by TTL).
To verify this, can you jumper CLR to ground to make a hard low? If that fixes the problem, consider an RC filter at the CLR pin or better yet, can you use LOAD/ with inputs ABCD tied to 0 and a stiff pullup on LOAD/? Active low has much better noise margin.

The power paths are also important as pointed out above.

Good luck!
 

Thread Starter

rmitch

Joined May 24, 2008
24
The capacitors for the 7805 are what the data sheet showed to use.

I only have the problem when the board is hooked to the 12 v reversing relay.

I'm not switching the relays while current is flowing to the motor. ( I did try flybacks diodes across the rev relay coils. 1n4001s.)

The small relays switch the 12+ sides of the reversing relay. I have a designated 12 wire from the battery for this.

I will jumper the reset pin to ground. And try that. Could this be the wrong size resistor to hold this pin low?
 

JohnInTX

Joined Jun 26, 2012
4,341
I will jumper the reset pin to ground. And try that. Could this be the wrong size resistor to hold this pin low?
For a 'LS input you have to sink -.4mA at no more than .8V to guarantee a low input.
.8V/.4mA = 2000 ohms. So your resistor should be considerably less than that to make a solid LOW, the lower the better consistent with what's driving the pin.

You should have dedicated 12V and ground to the relays, separate from any digital power. Twisting the wires to the relay coils can help with inductive effects as will physical separation of those wires from the digital section.
 

dendad

Joined Feb 20, 2016
3,611
Also, a 100nF cap across the power pins of each logis IC is a good idea.
I'd probably add a 470uF electro on the main supply too.
 

Thread Starter

rmitch

Joined May 24, 2008
24
The resister holding the reset pin low is an area i did have in question. i know what you mention is basic electronics, and it looks like i made an error there somewhere. Making a mistake is where i learn the best. i will replace the resistor with a 2k or less if i have one and see how it works. I will also review the datasheet and get a better understanding of what is needed.

i did install caps at the power to the IC's except for the 7447 IC's. I just didnt do that yet. It did help with unreliable operation if the picaxe chip. I also came to the conclusion that the added 470 cap would help. I just wasnt sure. It should it be on the regulated 5v side right?
 
Last edited:

dendad

Joined Feb 20, 2016
3,611
If you can, wire the caps from the 5V pin of each IC to the 0V pin of thr same IC. Put them under the board with cap leads as short as you can.
 

Thread Starter

rmitch

Joined May 24, 2008
24
I found where I came up with 220 ohm resistor. I didn't have enough zeros in my calculation. Regardless of that. I need to clarify something. Datasheet says .8v to hold pin low. Does that mean .8v and lower? Or between .8v and input high voltage.
 

dl324

Joined Mar 30, 2015
11,861
I found where I came up with 220 ohm resistor. I didn't have enough zeros in my calculation. Regardless of that. I need to clarify something. Datasheet says .8v to hold pin low. Does that mean .8v and lower? Or between .8v and input high voltage.
0.8V or, preferably, lower. That's what the VIL max spec means:
upload_2019-6-6_7-39-1.png
 

Thread Starter

rmitch

Joined May 24, 2008
24
That's what I thought. So isn't my 220 ohm resistor doing the job it needs to? Or am I mistakenly somehow. Please explain
 

marcf

Joined Dec 29, 2014
260
Is it possible for you to connect an inverter to the input and provide a active pull down? IMHO trying to pull down TTL with a resistor is a 'fools errand'
 

Thread Starter

rmitch

Joined May 24, 2008
24
i will look into the active pull down option. I goofed up and forgot to disconnect a hot wire from the battery yesterday. i relized this was a problem when it sparked across my board. Today i found out my picaxe chip is shot. Bad day! I had to walk away for a bit. I got new chips ordered, with a few extras even.

My list of things to try are:
1 - installing a 470 cap on the regulated side of the voltage regular.
2 - im going to remove any pulses to the reset pin in the program, and tie the pin directly to ground to see if that helps
3 - look into the active pull down option. I think i will have to use a transistor for this.
4. see if i can figure out why it resets when the motor "starts" on the up movement. and resets when the motor "stops" on the down movement.
there has to me something to this.
 

Thread Starter

rmitch

Joined May 24, 2008
24
so i installed the 470 cap and tied the reset pin directly to ground. It helped the resetting problem, but it just moved the problem, and makes the counter count now when its not suposed to.
I can barely see voltage spikes and drops in my cheapy oscilliscope. Would it be better to use schottky diodes instead of regular diodes for the flyback
 

eetech00

Joined Jun 8, 2013
2,037
Hi

Is the dc motor and your circuit board sharing the same power source?

Can you draw a diagram showing how the motor is connected to the board?

Im wondering if the sudden current draw by the motor is causing the board to reset.

eT
 

Thread Starter

rmitch

Joined May 24, 2008
24
Hi

Is the dc motor and your circuit board sharing the same power source?

Can you draw a diagram showing how the motor is connected to the board?

Im wondering if the sudden current draw by the motor is causing the board to reset.

eT
They do share the same 12v battery. The sudden current draw could be the problem. I can see a voltage drop once In a while with my cheepy scope On the reset pin of the 74ls192. The 5v supply seems stable tho. The motor I'm driving draws about 25 amps. Ill try isolating the motor and circuit board with different batteries and see if that resolves the problem. I think we're on to something here. Not sure why I didn't think of that.
I think the reversing relay I have also has some sort of breaking ability. The motor stops suddenly when it shuts off. Verses a plain relay I tried which seem to let the motor coast to a stop. Which is not desirable in my situation. I have the reversing relay wired as you would normally for push buttons, Except I use relays on the top right of the board for the 12 volt signals to the reversing relay. I borrow the 12 signal from just before the 7805 regulator on the board.
 
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